Anal. Chem. 1990, 62, 268R-303R
Mass Spectrometry A. L.Burlingame* Department of Pharmaceutical Chemistry, The Mass Spectrometry Facility and the Liver Center, University of California, San Francisco, California 94143-0446
D. S . Millington Department of Pediatrics, Duke University Medical Center, Durham, North Carolina 27710
D. L.Norwood Department of Pediatrics, Duke University Medical Center, Durham, North Carolina 27710
D. H.Russell Department of Chemistry, Texas A&M University, College Station, Texas 77843
A. OVERVIEW Over the past 40 years it has often been questioned whether the kind and amount of information obtruned in organic and natural products chemistry by using mass spectrometry was really worth the relatively large financial investment in instrumentation, not to mention operation costs, maintenance, and development, particularly in academic settings. These coet/benefit questions were never asked in the petroleum and pharmaceutical industries, nor later in environmental or clinical areas, where the power of high resolution GC/MS became the standards a ainst which other assays were validated. Such questions Eave been raised again, particularly with respect to the four-sector tandem systems and the high costs of multichannel array detection systems together with the compellin need for concerted application of electrospray and laser mettods to the same problems. We are witnessing the rapid evolution of a professional discipline, namely, biological mass spectrometry, in which the leading figures have grown roots in an extensive biolo ical constituency across virtually all of the disciplines in the fSe sciences and medicine. While less than five years old, this discipline promises to deliver the methodologies of choice for investigations at picomole levels in structural biology, bringing protein biochemistry and physiology back on a competitive footing, habeen dominated in the mantime by molecular biology. The hgh speed,accuracy, and experimental versatility of these new instruments invites investigations of virtually all classes of biological molecules and more than compensates for the initial capital cost in instrumentation. The utility of mass spectrometicbased methods for precise studies of atoms, isotopes, and small molecules was recognized clearly by Thomson (AI, A2). Its cumulative importance and present utility remain unchallenged. It is destined to fi re y a i n as the primary analytical instrumentation on the sini-Huygens mission to Titan to probe the existence and molecular nature of prebiotic or anic chemistry so glaringly absent in the V i mission to It is curious that Lowell (A3) and Thomson were contemporaries-the ’uxtaposition of one’s wildest dreams and one’s most proflound understanding. As in the seven stages of man, the development or metamorphosis of thisdiverse endeavor appears to unde o periodic rebirth: isotopes, the atomic weights, nuclear p%ysics, petroleum, gas-phase kinetics, physiological chemistry, materials science, natural products, environmental chemistry.... Recent discoveries, inventions, and new instrumentation have initiated yet another revolution providing the key to macromolecular mass spectrometry and structural biolo The breakthroughs came with methods to eject/ionize sampgi from their natural habitat, polar fluids (A4),obviating previous
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obligatory sample vaporization. While earlier methods of mass spectrometry (MS)have skirted direct studies of intact proteins, glycoconjugates, and nucleic acids, key developments are now available to pursue these ends embodied in new commercial instruments, including tandem four-sector mass spectrometers with multichannel array detection systems, electrospray, and nebulizaspectrometersand spectrometers. A coupling strategies may variety of liquid be employed with one or more of these instruments and it would appear the LCMS systems are now understood to the point that useful information can be obtained rather readily from the peptide mixtures of protein digests at the 5-30-pmol range per component. In fact, one extremely important criterion that applies to all of these methods and instruments is that they are, in general, able to produce high-quality mass spectral information with sample consumption in the low picomole range. For the first time, it is clear that these instruments and ca abilities, when integrated into a strategy including the ju$icious use of the automated Edman sequencers and amino acid analyzers,will provide the necessary and sufficient arsenal for rapid and precise characterization of proteins and recombinant proteins, including their posttranslational moclifications (A5,A6). Obtaining molecular weight maps of proteln digests is now routine. Sequences of picomole quantities of peptides and covalent1 modified eptides may be readily obtained from their higg-energy colgsion-induced dissociation spectra, and molecular weights of small- and medium-sized proteins and mixtures of proteins may be accurately determined from electrospray measurements. At this writing, it would appear that matrix-modulated laser time-of-flight methods have additional differential advantages compared to electrospray methods, particularly above mass 1oOOOO. In addition, molecular ions for intact transfer RNAs have been obtained by the new laser matrix method (A5). Indications are that high molecular weight DNA (e410 kdaltons) may be ablated from frozen aqueous films by pulsed laser techniques ( A n . While it is certainly premature to delineate the relative merits of electrospray and matrixmodulated UV laser methods, it seems clear, even with quadrupole mass resolution, that the electrospray method will open a completely new ca ability for characterization of mixtures of intact proteins. !his is particularly true for those of recombinant origin where minor components, due to ragged N or C termini, the microheterogeneity of lycosylation, or other posttranslational modifications may e present, even at the 1% level (A8-AIO). For samples that run by electrospray, whether they be intact proteins or various compo-
0003-2700/90/0362-268R$09.50~0 0 1990 American Chemical Society
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MASS SPECTROMETRY A. L. Bydwam is Rofesxx of Chemkby
and Pharmaceutical Chemistry in the Depnment 01 Pharmaceuticai WHtmistry. Universin, 01 California. San Francisco. He Is also Director 01 the NIH-supported National Blo-organic, Biomedical Mass Spectrometry Facility and of the core mass spectrometry lacilhy 01 the Liver Center at UCSF. He received his B.S. from the University of Rhode lriand and his W.D. from the MasSaChwens Instiiute of TechnolDgy in 1982 wilh K. Biemann in determination of the ~tructure01 indole alkaloas. He immediately joined the Department of Chemistry and Space Sciences Labratory of the Universih of California. Berkeley. as Asistant IVofesor 01 Chemistry. He assumed his Current rerponsibilnies in 1978. From 1964 to 1973,’he was a member of several interdisciplinary scientific t e a m and comminees entrusted wilh the planning and Conduct of the lunar science program and the preliminary examination and distributikm of lunar Samples tram the U.S. Apollo and USSR Luna sample return missions. During this time. as director of the mass rpectmmetry Unit on the Berkeley campus. he pioneered the development of real-time. high-sensitivity. high-resolution mass rpectromeny. field ionization kinetics. and deuterium difference rpechoscopy in NMR. During 1970-1972 he was awarded a J. S. Guggenheim Memaiai Fellowship. which was Spent on biochemical-biomedical applications of mass spectrometry With J. Sjiivall at the Kamiinska Insliiute. Stockhoim. His cunent interests focus an structural studies O f modified proteins. receptors. and glycOconlugales Of biomedical importance and the development of techniques and instrumentation to advance such studies 01 natural and recombinant biD polymers. DaVm s. rminnpt~nis R O M ~ K ~ IASMCW Professor in the Depnment of Pediatrics. Genetics and Metabolism Division. Duke University Medical Center. He received his BSc. and Ph.D. in organic chemism from the University of Liverpwl. and from 1989 to 1971 was a postdoctoral fellow wilh Dr. K. L. Rinehart at the University 01 Illinois. He then spent two years each at Universin, College. Cardin. and at the Tenovus Institule for Cancer Research, Cardiff, before .a. joining VG Organic Division. I n 1980 he returned lo the Uniled States as an Associate Professor in the School 01 Public Health st the Universily 01 North Carolina In Chapel iC Hill. before transferring to Duke in 1983. His research Interests have ranm from amlications 01 mass sDBCtrometrV in natural oroduct. biomedical. en& ~~~~.~ ronmeiial. and toxicalogicai research io fundamental studies 01 linked scan IUnCtionS lor metastable ion detection. Currentty. his research interests are focused MI lb devebpment of new diagnostic tests fagenetic disease using tandem mass speclrome1ry and on Stdies of the metabolism 01 amino acids and fany acids both in viho and in vivo using Stable isotopes. He has a been a member of the American Society for Mass Spectrometry Since 1975 ~
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Danm L. ~orvrmdis ~ssistatant~ e d i ~RS ai Search Rofessor in the Division 01 Genetiand Metabolism. Depamnent of Pediahics at Duke University Medical Center. He received his B.S. degree in blochemism hom Virginia PolyteChnlc lnstihlte and State Univ-
-sin, in 1977 and his Ph.D. in environmental chemism from lb UniversW of Nonh Carolina 81 Chapel Hill in 1985. His graduate work, under the direction of Rofessor RUSsell F. Christman and Dr. J. Ronald H a s . involved the structural eiucidation of natural prcdducl organic materhl derived from soil and natural waters utilizing mass spechometry and NMR. He then scent two years wo&ing 8s a research analytical chemist In me mass spemmeby f a c i l i a1 Research Triangle Institute. HIS present research interests include the s t r ~ c t ~ r aelucidation l Of metaboines derived hom calaboiism of fats and branchedchain amino acids, the development of mass spectromebic t e c h niques for the anaipis 01 ~miousbiomaiecules. and the evaluation of various high-performance iiqua ChrOmatographylmaSS Spechometry (HPLC/MS) combinations (particularly continuous-flow FAB). Dr. Norwood is a member Of the American Chemical Society and the Amedcan Socieh for Mass Smctmmetry
nents of protein digests, the accuracy of chemical molecular wei ht measurement is impressive, within 1-2 daltons. Due to tge relatively low mam resolution of the laser time-of-flight system, it seems less suited to characterization of mixtures of relatively closely related proteins and also appears less precise in measurement of accurate chemical molecular weight. The best case, it appears, is accuracy within 10 daltons; hut
D. H. R-I1 is PToIess~of Chemistry a1 Texas A8M Unlvsrsily. He received his B.S. from the University of Arkansas-Link Rock in 1974 and his Ph.D. in 1978. His graduate work, under Um S u ~ I S i o 01 n Rolessor M. L. Gross. involved studies of the ion-mlecuie reactions using ion cyciohon mass spectrometry and unimoiewler reactions 01 gas-phase ions. He spent two years at Oak Ridge Nstbnsl Labaatory 85 a research scielllist in the Analytical Chemir try Division, working an the development of tandem mass spectrometry for the study of . , photoinduced diSSoCiation reactions and gas-phase ions. His present research interests include the use of lasers in mass spechometry as both ionization sources and a means to induce dissociation 01 ionic cluster fragments 01 transition-metal carbonyls and bionwleCulesand Um use of Fourier translam mass Spctrometry (FT-MS) for investigations 01 gasghase ion-molecule r e actions nnd as an analysis method for high mdecular mas (mlr > 5000) molecules. He is a member of the American Chemical Society and Um A m erican Society for Mass Spectromehy. ~
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in the worst case, it could be as much as 1%of the mass, partly
due to the unresolved questions of adducts and fragmentation. Finally, it should he obvious that studies of minor changes in molecular weight of proteins and mixtures of proteins with their covalently modified counterparts would benefit significantly from using high mass resolution analyzers possessing a wide dynamic range. These are truly revolutionary times in macromolecular m a s spectrometry, which will quickly lead to extensive practical research benefits in structural biology. One might anticipate that after a “shakedown cruise”, where both academic and industrial “sailors” have a chance to exercise these capabilities in concert on their challenging problems, new research strategies in biological chemistry will evolve with mass spectrometry as an integral part. These endeavors will lead to a new generation of commercialized instrumentation with optimal sensitivity in the femtomole region. While the inherently necessary instruments are now available for studies in protein biochemistry, success is still highly dependent upon knowledgeable and careful sample preparation, including cleanup, handling, and the cumulative experience of the scientists or laboratories involved. The conventional protocols of any biochemistry and molecular biology research laboratory are replete with practices and problems that can thwart the best m a s spectrometry laboratory. Some of the most common are use of mixtures of oligomeric substances as detergents, rather than a single pure component, that have UV-visible ahsorption spectra virtually the same as those of protein. These can he misleading and difficult to remove from di est mixtures. Various other polymers, plasticizers, buffers, andgcontaminants are commonly found and must he eliminated. While sample volatilizihility and thermal-stability constrained much of the past five decades of development of m a s spectrometric methods, present methods have removed all these barriers. Mass spectrometric methodology may now he applied to virtually any structural biology problem and is likely to become the method of choice for many biological studies. This being the case, it is of essential importance that the biological scientists involve the mass spectrometrist in his initial experimental design strategy and get out of the habit of considering mass spectrometry as a last resort when other methods have failed or have not provided the information desired.
B. SCOPE Throughout the past two decades this review series has provided critical assessments of the major developments and opportunities in mass spectrometry. It is well recognized hy the experts, at least, that fundamental discoveries in physiochemical phenomena and technical inventions continue to make revolutionary changes in our collective ability to study challenging problems at the forefront of the atomic and molecular sciences (RZ).These fundamental discoveries usually require years of sustained development and often the fahrication of new kinds of instrumentation and computer support to bring them to fruition for applications in the chemical, materials, and biological sciences. One could point presently to the development of electrospray and matrix-assisted laser methods as the premier advances in this twrryear period. One ANALYTICAL CHEMISTRY, VOC. 62, NO. 12. JUNE 15. 1990
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can then reflect on the early contributions of Dole over the years and how, exact1 Fenn ot to the scheme that works so well at present ( B 2 f ) o rto tge many who have worked on laser-induced "desorption" prior to the excitement generated by the high mass results reported by Hillenkamp and Karas after finding the right energy-transducing matrices (B3). One might anticipate the future utilities of ion traps (B3a) Fourier transform mass spectrometers and consider what mix of interdisciplinaryingredients will eventually bring these methods into widespread use and importance. This review is again selective, in both the topics covered and the contributions used to illustrate new methods, new instruments, and new problems, but we have included sufficient reference to the original literature that the interested reader may fiid the necessary detail and documentation. This review continues the cumulative continuity noted earlier, and it is certainly worthwhile for the graduate student or beginner in the field to read previous reviews in the series to gain insight as to how various topics have grown and matured. Mass Spectrometr Reviews (B4)is in its ninth volume and has a new editor, Dr. &orris Bursey of the University of North Carolina. The field owes a debt of gratitude to Dr. Michael L. Gross for his leadership in selecting and soliciting timely topi- for exhaustive review, ranging through all aspects of gas-phase chemistry, instrumentation, and applications. The American Society for Mass Spectrometry has sponsored initiation of its own journal, Journal of the American Society for Mass Spectrometry (B5),with the first issue appearing this spring under the editorship of Dr. M. L. Gross. In addition, this journal, Analytical Chemistry, has continued its recognition of the vowing importance of mass spectrometry in analytical chemistry with the continuing a pointment of an associate editor for mass spectrometry, r. Catherine Fenselau, who succeeds Dr. Klaus Biemann. It is increasingly difficult to keep up with the extent of the interestin and important applications of mass spectrometry in virtuaby all of the atomic and molecular sciences, such as atom counting a t surfaces (B6), spatially resolved organic analysis of carbonaceous meteorites ( B 3 , on-line continuous analysis of isotopic compositions of individual organic compounds from ancient ecosystems separated in a gas chromatographic effemtogram detection of perfluorocarbon atmosfluent (B8), pheric tracers b negative ion chemical ionization techniques (B9),or "chlordbenzofuran and chlorodibenzo-p-dioxinlevels in Chilean mummies dated to about 2800 years before the present" (BIOI. We have attempted to obtain and report the uintessence of the current literature for the topics chosen. review has utilized Chemical Abstracts searching service more than in the past, but the help of a large number of colleagues is acknowledged in obtaining the overall result presented here. Reports of the 36th and 37th ASMS Conference on Mass Spectrometry and Allied Topics have been published ( B I I , B12), and the 38th through 41st Conferences will be held in Tucson, Washington DC, Nashville, and Las Vegas, respectively. Proceedings of the Eleventh International Mass Spectrometry Conference held in Bordeaux, August 29 to September 2,1988, have been published (B13). Symposium papers from the International Symposium on Mass Spectrometr in the Applied Health Sciences held in Barcelona, Septemger 28-30, 1987, have been published (B14). The Proceedings of the Second International Symposium on Mass Spectrometry in the Health and Life Sciences held in San Francisco, August 27-30, 1989, represent a comprehensive, thematic treatment of biological mass s trometry (A5). The Second International Symposium on Kss Spectrometry in the Applied Health Sciences was held in Barcelona, A ril 17-20, 1990 (B15), and the Twelfth International d s s Spectrometry Conference will be held in Amsterdam, August 26-30, 1991 (BI6). Several volumes of importance have a peared in this period, including MS j M S by Busch et al. ( B l R Mass Spectrometr by Lawson (B18),Mass S ectrometry of Biological Material by McEwen and Larsen (8;9), Biomedical Mass Spectrometry by Suelter and Watson (8201,Mass Spectrometry of Peptides and Proteins by Desiderio (B21),and Mass Spectrometry: General Techniques, Proteins, Glycoconjugates and Nucleic Acids in the Methods in Enz mology series by McCloskey (A6),Field Desorption Mass Jpectrometry by Prdkai (B22), Handbook of Static Secondary Ion Mass Spectrometry by
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Briggs et al. (B23),a volume on the analysis of sterols and steroids (B24), and a com endium of 750 mass spectra of eicosanoids by Pace-Asci& ( B E ) . Several cha ters have a peared on applications to protein structure an sequence ekcidation in Techniques in Protein Chemistry (B26),in Protein Sequencing-A Practical Approach ( B 2 3 ,Advanced Drug Delivery Reviews (B28),and Methods in Enzymology (A6). Dell has presented a comprehensive chapter on FAB MS of carbohydrates (B29). It should be noted that there are some particularly good chapters addressing the structural issues in the characterization of recombinant proteins and glycoproteins obtained from different expression systems and the need for structural characterization of protein-based therapeutic agents by the biotechnolog industry by Carr (B30),Carr et al. (B31),Vandlen et al. (5329, Stults (B33), Stults et al. (B34),Carr et al. (B35),Scoble and Martin (B36) and Burlingame and co-workers (A9). The use of stable isotopes, especially in clinical research, is increasing rapidly but is still hampered by limited availability. The Proceedings of the Third International Symposium on the Synthesis and Applications of Isotopically Labelled Compounds,edited by Baillie and Jones, has appeared (B37) and is a useful reference to the literature on stable isotope applications. The use of stable isotopes in clinical studies was reviewed by Thompson et al. (B38). In the area of drug metabolism, a comprehensive review by Harvey has appeared in the Special Periodical Reports of the Royal Chemical Society series (B39). Numerous papers on this topic can also be found in the special issue of Biomedical and Environmental Mass Spectrometry devoted to the Proceedings of the Seventh International Symposium on Mass Spectrometry in the Life Sciences (B40). The proceedings of another important symposium, the fifth in a series on LC/MS, SFC/MS and MS MS were published in a special volume of the Journal of C romatography (B41). The Proceedings of the Second International Symposium on Applied Mass Spectrometry in the Health Sciences, held in Barcelona during April, 1990, will a pear in a dedicated issue of the Journal of Chrornatograp!y, Biomedical Applications. Another symposium in the series on Mass Spectrometry in the Life Sciences is scheduled to take place in Gent, Belgium, in August, 1990, and proceedings are to be published, as usual, in a dedicated issue of Biomedical and Environmental Mass Spectrometry. Some of the traditional areas of coverage in this review have been omitted, in particular UNIMOLECULAR and BIMOLECULAR REACTIONS and some of the coverage of the fundamentals of gas-phase ion chemistry. Over the past two years these areas have been covered in specialized reviews, and in the most recent volume of Specialist Periodical Reports (B42). In this volume the fundamentals of ionization processes and ion dynamics (Lifshitz), structures and reactions of gas-phase ions (Burgers and Terlouw), chemistry of organic negative ions (OH& and Bowie) are covered, and the cover e is far more extensive than can be included in this review. Yn addition, articles published in recent issues of Muss Spectrometry Reviews cover most of the rapidly developing areas of ion chemistry. For example, reviews have appeared on gas-phase equilibrium measurements (B43),ion trap (quadrupole and ICR type traps) photodissociation studies (B44), the gas-phase chemistry of distonic radical cations (B45),the role of ion-neutral complexes in unimolecular dissociations (B46),and the chemical reactions occurring in the interfacial regions of desorption ionization (B47). Finally, the section on components of nucleic acids has been omitted, since two comprehensive chapters have appeared on nucleic acids ( B e ) and modified nucleic acids (B49).
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C. INNOVATIVE TECHNIQUES AND INSTRUMENTATION Quadrupole Ion Traps. Major progress has been made in the development of quadrupole ion traps over the past two years. Many of the develo ments in this field are discussed in a review by Nourse and 8ooks ( C l ) . This paper covers the theory of the quadrupole ion trap, the various acquisition modes of mass spectral data, tandem mass spectrometry, and chemical ionization/ion molecule reactions and photodissociation. The paper also describes external ion sources for the ion trap as well as the extension of the mass range (C2),e.g., m / z 4000 with sufficient sensitivity for MS/MS experiments
MASS SPECTROMETRY
on femtomole levels of pe tides. The papers on quadrupo e ion traps over the ast two years demonstrate a range of experiments that can e performed by using this techno lo^, ranging from the analysis of tissue extracts and isotope dilution (C3)to laser desorption of biochemical compounds (C4). Most of these experiments parallel the developments of ion cyclotron resonance over the past 10 ears. For example, so-called “self-chemicalionization” has L en used for the analysis of a range of compound types (C5), and developments of this ionization method have been evaluated in terms of general utility and instrument performance (C6,C7). Lim et al. also examined the utility of chemical ionization with the ion trap and demonstrated linear to 100 mg (C8). The wide response over the range 50 dynamic range was achievefgby varying the reaction and ionization times. The ion-molecule reactions of CHzCl+with a variety of organic molecules was examined. The CH2Cl+ ion reacts by addition of CH,Cl+ followed by elimination of HC1, resulting in the net addition of methyne to the compound (C9). The structures of the product ions can then be probed by colliiion-induced dissociation. Isomeric c 5 H ~compounds were distinguished by ion-molecule reactions with specific reagent ions; geometric isomers can be distinguished, but stereoisomers cannot be distinguished (ClO). Fourier Transform Ion Cyclotron Resonance. After 15 years of develo ment and rimarily use by physical chemists interested) in ion-motcule reaction chemistry, Fourier transform ion c clotron resonance (FT-ICR) mass spectrometry is gradudy being looked upon as a viable analytical instrument. One area where the FT-ICR instrument has demonstrated unique capabilities is as a mass analyzer for laser deso tion ionization. Numerous applications of laser desorption YT-ICR have been reported in the past five years or so. Brenna has discussed the desi and construction of a Nd:YAG-based laser microprobe R - I C R and presented results on polymers such as polyimide, poly(ethy1ene glycol), pol (phenylene sulfide), and poly(methy1 methAlthough the hi h resolution/high mass acrylate) (&I). capabilities of FT-ICR are current$ in question (C12),Wilkins’ group has reported high-remluhon (m z SOOOO) detection of hi h-mass (mlz 5922) ions formed by aser desorption of poly flpropylene lycol)-4000, and the mass measurement accurac reportecfis quite impressive, ca. 5-10 pm (C13). Clear&, these results demonstrate that high resorution/hi h mass measurements are not fundamentally limited by t e technology or dynamics of ion trapping. It is, however, important to keep in mind that laser desorption FT-ICR of neutral polymers is dramatically different from SIMS ionization from an external ion source. S ecifically, laser desorption produces a high abundance o secondary ions and neutrals-for several hundred milliseconds following the laser the ion cell is sufficiently of the ions by ion-neutral ion yield from laser desorption is sufficientlyhigh to saturate the cell with ions, and thus ion-ion relaxation mechanism may be operative. Lam et al. used laser desorption FT-ICR to robe the structure of common pol saccharides (C14). AltKough the utilit of laser desorption h - I C R for polysaccharide analysis ) , showed that unique, was Jemonstrated previously ( c I ~am structurally informative fr ent ions are observed and these ions are not observed i n E h abundance by li uid-SIMS ionization. Thus, the laser desorption FT-IC8 data are complementary to existing methods. Hsu and Marshall have reported on laser desorption FT-ICR data for a sensitive and accurate (in terms of chemical formula from precise mass measurements) method for identif dyes in solid pol ers (Cl6). Laser desorption FT-ICR E b e e n used to s t u r t h e carbon clusters formed from both soot and coal samples (Cl7, CIS). Chiarelli and Gross evaluated the utility of laser desor tion FT-ICR for the analysis of mixtures of amino acids anftripeptides (C19). Nuwaysir examined FT-ICR photodissociation (308nm) of laser desorbed ions (e.g., porphyrins, metalloporph ins, and alkaloids) as a means of structural characterization g 2 0 ) . In each case the photofr ent ion yield is enhanced by the presence of a metal ( F e , r n , or Cr) atom. The metal atom serves as the absorbing chromophore (i.e., nonphotodissociating ions that do not contain a metal, readily undergo photodissociation if a metal is added). In the systems examined,
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those systems containing com lex ligands, specifically chain-type ligands containin o ar functional groups, fragment by rupture of the li and fonds, vis-&vis rupture of the metal-ligand bond, and t us information on the structure of the ligand is obtained from the mass spectral data. Amster et al. (C20a)used laser desorption to generate neutral molecules, e.g., thymidine, that are subsequently ionized by roton transfer (CH6+and CzH6+)ion-molecule reactions. Alt ough this experiment has not been fully exploited, there are many advantages for analysis and structural characterization: (i) specific reagent ions can be used to tailor the types of ions produced, e.g., by controlling the thermochemistry of the reaction intact molecular ions or fragment ions can be formed exclusively; (ii) a degree of selectivity can be achieved in terms of functional groups or compound type. Development of FT-ICR hardware and ex erimental sequences continues to progress at a rapid rate. and Jones have introduced a personal computer-based FT-ICR instrument that has impressive performance specifications (C21). All of the ion excitation and detection steps are carried out on the computer by using extension boards, Le., external frequency synthesizers and waveform digitizers are not required. Williams and Marshall have discussed Hartley transforms (HT) for conversion of time-domain ICR signals; the FT and HT yield the same spectra, but the HT method is twice as fast as the FT (C22). Rempel et al. introduced a multichannel heterodyne detection for high resolution/accurate mass measurements. The experiment is analogous to peak-matching commonly used in magnetic sector instruments (C23). The method is compared to ‘undersampling” or “foldover”,which is also used to sample widely separated (in The two methods are comparable, terms of frequency) sy$. but the results show t t foldover” is more sensitive to noise than are heterodyne techniques. The multi le-foldover experiment is discussed in detail by Wang andbarshall (C24). One of the advantages of FT-ICR is that tandem mass spectrometry experiments can be performed and, in principle, it is possible to do many stages of MS/MS in a single exeriment. As with all forms of tandem mass spectrometry, owever, the central issue still remains: what is the best method to activate the ions? (This issue is addressed further in the section on ION ACTIVATION.) In addition, in FTICR there are still questions concerning the method used for accelerating the ions to the desired velocity for collisional activation. Recently, McIver reported on the use of impulse excitation for both ion detection and ion activation (C25). Although impulse excitation appears to be superior for detection of high mass ions and presumably would be advantageous for acceleration of high mass ions for CID, the excitation is not mass selective and cannot be used for selectively ejecting or exciting specific ions. Nonetheless, it is feasible to mass-select the ion for CID by usin8 radio frequency (rf) excitation and then excite for CID by wmg impulse excitation. Kerley and Russell have demonstrated mass- and energy-selective ion partitioning in a two-section ion cell (C26). The method described is superior for mass-selected ion chemistry over single-section ion cell experiments, especial1 those experiments that r uire different neutral reagents. Jwifi~ally, only those ions a j e c t e d to collision-induceddissociation or photodissociation are partitioned to the analyzer region, and thus interferences are minimized and the environment in which CID is performed is clean. The mass- and ener lective partitioning experiment was taken one step furtg;; include phase-specific ion excitation (or deexcitation) (C27). The phase-specific ion excitation can be used for high-resolution mass selection and as a diagnostic probe of the ion trap. McLafferty cites several examples of MS/MS ex eriments inof biomolecules (mlz 2000-3000) performed on F!-ICR struments (CB),and Cody has discussed accurate mass measurements of CID product ions ((229). Improvements in ion detection are still a focal point of much research in FT-ICR. Eyler’s group has develo ed a method for computer simulation of ion trajectories for I8R (C30),and Russell’s group has used similar computer-simulated ion trajectories to address issues related to high-resolution detection of high-mass ions. Because desorption ionization methods yield secondary ions having a large distribution of kinetic energies, the usual assumptions (a near-thermal kinetic energy ions formed in the center of the ion cell) involved in modeling ion detection are no longer valid. The computer-
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simulated ion trajectories that examine the effect of initial radial velocity on ion detection are consistent with the experimentally observed loss in mass resolution and sensitivity for detection of high-mass ions (C31). Ions trapped with nonthermal kinetic energies are not accelerated uniformly by the applied rf field, e.g., ions that are moving out-of-phasewith respect to the ap lied rf field must undergo phase advancement before accereration occurs. The effects of phase angle between the nonthermal ion and the applied rf field were modeled by computer-simulated ion trajectories (C32). Again, the results of the computer simulations are consistent with experiments, viz., the coherence of the ion packet decreases as the mlz of the ion increases, and the applied rf acceleration field causes a shift in the center of the cyclotron orbit (influencing both sensitivity and m/z resolution);this effect also increases as the m / z of the ion increases, and off-resonance excitation of the ion ensemble yields better resolution and sensitivity (presumably because the off-resonant applied field phase synchronizes the ion ensemble). Experiments to test these concepts directly have not yet been performed, but the observations are consistent with the theory. Clearly, more work designed to understand the dynamics of the ion trap are in order; such studies must go beyond the traditional level, e.g., ions with near thermal kinetic energies and located at the center of the ion trap. Many of the experiments now performed on FT-ICR instruments do not deal with idealized conditions. The effects of magnetic field inhomogeneities on ion motion (in a two-section ion cell or ions injected from an external ion source) have been examined by using SIMION computer simulations (C33). The work clearly shows the pitfalls associated with the use of construction materials having small magnetic susceptibilities, e.g., 316 stainless steel. The ma netic susceptibilities of the stainless steel are sufficiently L g e to influence ion trapping and can cause acceleration of the ions (conversion of z-axismotion to x-y-plane motion), both factors ultimately influence ion detection and m / z resolution. New cell desi ns have been introduced by Marshall (C34, C35) and Russefl (C36). In each case the cell is designed to minimize the aberrations in the electric field lines imposed by the finite dimensions of the ion cell. Marshall introduced the ideal of “screened”electrostatic ion traps with the objective to improve the shapes of the electrostatic trapping well (C34). Although this concept improves the performance of the ion tra and permits ion trapping in a near-zero trappin well, it &es nothing to eliminate the problems introduced y the imperfections in the rf electric field radient used for ion excitation. Russell (C37) and Marshalf (C38) independently proposed ”shimmed”or “field-corrected” ion cells, similar in many respects to the omegatron cells for eliminating the effects of imperfect electrostatic field gradients on ion excitation. Although the two cells introduced by these workers differ significantly, the ob’ectives of the cell designs are similar, i.e., to make the ion ce 1 dimensions appear infinite to the ions that are trapped inside. The performance of both ion cells are far superior to cells constructed of flat metal lates. An interestin result emerges from the cell design usedgy Russell. Specificalfy,Russell’s ion cell uses a rod to d e w the ion image current, and it is shown that a cell designed of cylindrical excite plates and rod receivers has near-perfect excitation field lines in the x-y plane. Furthermore,the electrostatic trapping well and the z-axis rf excitation field lines can be greatly im roved by using concave electrostatic tra ping plates; thus a &ld-corrected ion cell can be constructe$ from cylindrical plates, rod receivers, and concave trap plates and one does not have to use a “se mented” cell or screens to design a “near-perfect“ ICR ce8. Laude has reported a relatively simple method for trapping ions from an external ion source (C39). The performance of the instrument is described, and details of the experiment are given; however, the mechanism by which the ions are trapped is as yet unknown. Ta lor has described an rf plasma source compatible with FT-I6R (C40),and Cody has described the use of a two-section ion cell to se arate the preparation of CI reagent ions from sample introluction ( ~ 4 1 ) . Time-of-Flight Mass Spectrometry. Time-of-flight (TOF) mass spectrometry is the method of choice for many types of experiments, especially those experiments involving pulsed ionization methods (e.g., laser desorption or photodissociation) and low ion count rates (plasma desorption and
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static SIMS). Cotter has reviewed TOF with particular emphasis on the increasing role of the method in biomolecule mass spectrometry (C42). Although the paper deals with basic principles, the most useful features of the paper are the discussion of focusing methods for improved mlz resolution and detection methods. Standing has published a review on recent advances in TOF for SIMS applications, and this paper ives ample background on the method and its capabilities C43). Standing et al. also reported on measurements of dissociation reactions of peptide ions (range of m / z 1OOO) by TOF methods (C44). These studies involve correlated measurements of neutral and charged products of dissociation reactions and photodissociationreactions of derivatized (dinitrophenyl) peptides. Photodissociation has also been performed in a reflectron TOF instrument (C45). Although this study was limited to cluster ions, the method is compatible with structural characterization of biomolecules. A similar system for studying photodissociation was described by Ueda et al. (C46). TOF laser desorption experiments using a 300-ps UV laser have been performed with a linear instrument (C47). The mlz resolution in this experiment is -1500 at m / z 400, and structurally significant fragment ions are observed for the cationized ion of sucrose. UV (193 and 248 nm) laser desorption of several polymers was studied by TOF (C48).The ejected products of the laser pulse are examined in terms of monomer units and intact units of the polymer. New TOF experiments and continued instrument development open novel and interesting research areas. For example, a fast pulsed laser system was used to roduce a fast ulsed electron impact ion source for TOF. $he device can e! used for performing medium-resolution E1 spectra on a conventional TOF system (C49). Hues et al. have developed a pulsed alkali-metal-ion gun for TOF-SIMS that produces primary ion pulses of loo00 can be achieved by a linear TOF (C52). A versatile TOF instrument for performing both desorption ionization and electron impact ionization studies of solid surfaces was designed by Eldridge (C53),and a PC-based data acquisition system for TOF was described by Della-Negra and Le Beyec (C54). Conzemius et al. have also described a new instrument control/data acquisition system for TOF instruments. The instrument is designed around the CAMAC crate and operates at a re etition rate of up to 1.5 kHz with time resolution of 5 ns ($55). Laser Desorption/Multiphoton Ionization/Photodissociation. Lasers now play an important role in fundamental and applied mass spectrometry. Although spectroscopy of gas-phase ionic species is a growing and important area of ion chemistry and basic physical chemistry, the coverage of laser/mass spectrometry will be limited to methods and techniques that are directly applicable to structural characterization. In these applications the laser is used as an energy source for sputtering/ionization of sample and/or activation by resonant absorption of a photon. In nearly all cases the spectroscopic issues are of secondary im ortance. Instead, the sputtered ion or photofragment ion yiefd is mass analyzed. In both instances the desired result is either molecular weight or structural characterization. In many respects, laser desorption ionization is more attractive as an analytical method than secondary ion mass spectrometry methods because a large array of experimental variables can be controlled, e.g., wavelength (energy) of the incident photon beam, photon beam fluence (product of photon beam density and pulse duration), focal properties of the incident beam, etc. However, until recently the success of laser desorption for the analysis of large biomolecules was quite limited. Karas and Hillenkamp published a series of papers recently that demonstrate that laser desorption can be successfully used to anal ze proteins exceeding 10000 daltons (C56). Earlier work gy Tanaka et al. (C57) demone proteins, up to a mass of ca. 34OOO strated that ions from daltons, could be formed y laser des0 tion ( ulsed N2laser) from a glycerol matrix containing a Tnely gspersed metal
7
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MASS SPECTROMETRY
powder. Karas and Hillenkamp report abundant molecular ion yields for laser desorption from an organic matrix that is strongly absorbing at the incident laser wavelen h. It is pro sed that the matrix serves to control energy eposition, a n f i h e high abundance of the molecular ions, presumably [M + HI+, suggests that the method is a “soft” ionization rocess (not intended to imply that the ions are “cold”). ieveral papers on the ”matrix-assisted UV laser desorption/ionization mass spectrometry” have now appeared that demonstrate the method (C58), and the authors have addressed issues related to sensitivity of the method (C59) as well as the utility of the method for molecular microprobing (C60). Considerable progress has also been made in the combined use of laser desorption and multiphoton ionization of complex molecules. Grotemeyer and co-workers have reported on the use of laser desorption for producing a seeded molecular beam that is cooled in an expansion nozzle and ionized by multiphoton ionization (C61). The principal advantage of the approach described by Grotemeyer is that both molecular weight and structure information can be obtained; the extent of fragmentation can be varied by changin the fluence and wavelength of the MPI laser. The methodtas been demonstrated on a variety of classes of biologically important molecules (C62) (pe tides, glycosides, and steroids), and detailed studies have ieen reported on small peptides (35unprotected di- and tripeptide4 (C63). Another important feature of the experimental apparatus used by Grotemeyer is the mass resolution; the instrument is based on a reflectron-time-of-flight and mass resolution of several 1OOO can be obtained. Lubman’s research group has also made significant progress in the development of laser desorption/MPI/molecular beam mass spectrometry (C641, and Lubman has reviewed both the theory and instrumentation (C65) and a plications (C66) of the technique. Lubman’s reviews inclu1e the theory of MPI, molecular beam technology including velocity distributions produced by different expansion conditions, and the mass s ectrometry instrumentation. In the applications paper Lu man re orts mass spectral data for simple model compounds and fata for one complex peptide (viz. angiotensin I) that provide sufficient fragmentation for sequence determination. In addition, he discusses supercritical fluid chromatography/MPI and atmospheric pressure/MPI. Photodissociation of ions, formed by a variety of ionization methods, is still very much in a developmental stage. For example, many of the photodissociation studies reported over this review period are aimed toward understanding the photochemistry/ photophysics of small model compounds, e.g., C&+ formed from different isomeric precursor neutrals (C67) or the competitive production of C7H and C7H8+ions from as well as &e influence of internal n-butylbenzene ions (C68), energies of the ions prior to photoexcitation (C69). Van der Hart has reviewed photodissociation of tra ped ions (C70), and Watson et al. have compared photoiissociation and collision-induced dissociation of a series of N-alkylpyridinium cations ((771). There are relatively few examples of analytical ap lications of photodissociation methods, and the analytical ut&y of the data is not yet clear1 established. Tecklenburg et al. have reported on the laser &bible and low-energy W; 514-454 nm) hotdissociation of model amino acids and peptides ionized y! Xeo liquid SIMS (C72). The molecules investigated in this study are derivatized (dinitrophenyl (DNP) roup added to the N terminus) with a chromophore to yielf a visible/UVabsorbing ion. The paper comparw the yield of photofrapent ions to the metastable and collision-induced dissociation product ions and the degree of fragmentation of the chromophore roup to the peptide portion of the molecule. Structural& distinct fragmentation reactions are reported for DNP-leucine and DNP-isoleucine, and fragmentation reactions to yield distonic ions are observed for many of the photoexcited [M + H]+ ions. Last, the rate of dissociation of photoexcited ions of DNP-peptides is shown to decrease with increasing molecular weights (e.g., degrees of freedom). Segar and Johnston have examined the mechanism of fra ment ion formation for model derivatized benzyloxycar%onyl (Cbz) dipeptide ions formed by MPI (C73). The experiments (performed b using 266-nm photons) address issues related to controlleJiionization to produce intact molecular ions or at high laser powers the production of struc-
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turally significant fragment ions. The results are compared to mass spectral data obtained by usin more conventional ionization methods, and factors relatetf to structure determination are discussed. The unique capabilities to exploit spectroscopic differencesfor distinguishing different isomeric species are demonstrated for Cbz-leu-ala-OCH, and Cbz-ileala-OCH,. Desorption Ionization. Development of new ionization methods and detailed experimentation of established ionization methods continue to occupy the efforts of numerous research groups. In recent years particle-induced des0 tion ionization methods have become the most widely use? but field ionization (FI) and field desorption (FD) methods are still used in some areas. Lattimer and Schulten discuss FI and FD in a recent review, covering both principles and applications, for both analysis and molecular weight determination (C74). Derrick and co-workers have examined the FD ion yield by using laser heatin versus resistive heatin of the emitter; similar spectra are ogserved for each case ( 75). A more recent introduction (or “reintroduction“, the original work having been performed over 20 years ago) (C76) to the array of ionization methods suitable for thermally labile molecules is referred to as electrospray ionization (C77). Ions are produced by electros raying ions from dilute solutions, and samples with molec& weights in the range 50Oo-50OOO daltons have been successfully ionized. In all cases the ions are formed with little fragmentation and with multiple (ashi h as 45) char es. Smith and co-workers have examined t i e collision-intuced dissociation of large multiply charged polypeptides and proteins ionized by electrospray (C78). Although the CID spectra are quite complex, identification of the charge state of the CID products ions is difficult and assignment of the fra ment ion (in terms of mechanism of formation or structure7 is not straightforward, it is clear that abundant CID product ions are produced and that further developmental work may yield useful structural information. It was not so lon ago that the detractors of tandem mass spectrometry male similar statements concerning the use of MS/MS for sequencing peptide ions and structural characterization of complex biomolecules. Smith and co-workers have also examined the effects of internal energy of the large polyatomic ions, formed by electrospray, on the CID s ectra (C79). So-called “cold” ions are quite resistant to CID, w1ereas “hot” ions readily undergo CID at the low energies (multiple collisions) of the triple quadrupole experiment. General mechanistic questions concerning the ionization of large molecules have been addressed in several studies. Haakansson and Sundqvist have examined the applications of electronic sputtering of biomolecules, s ecifically measurements on thin films of biomolecules a i o r b e d onto nitrocellulose materials (C80). The initial axial velocity distributions from biomolecules of different sample thickness have also been examined (C81). A strong dependence for the axial energies on sample thickness is observed only for H+ and H2+,whereas the polyatomic ions are inde endent of the sample thickness. The measurements provi8 insight about the ionization process, but more importantly the results have direct implication for mass calibration of the time-of-flight spectra. The mechanism of ion formation has been investigated by using directional correlation between the primary particle and ejected molecular ions (C82). On the basis of the measured incident ion direction and ionized cylindrical reeons produced b the fast ion, it is proposed that the large ions are formed gy direct radial momentum transfer. Fales and co-workers have made direct comparisons of mass spectral data for monoglucosyl conjugates for fast-atom bombardment and 262Cfplasma desorption. In general, the authors feel that PDMS yields superior results for low molecular weight species because the interfering matrix signals are absent (C83). Similar statements can be made about kiloelectronvolt energy SIMS of solid-state samples (C84). The relative yields of molecular ions for the two methods have also been compared (C85). Although the yield of molecular ions by both methods decreases as the sample molecular weight increases, a larger decrease was observed for the kiloelectronvolt FAB experiment. Close examination of the parameters influencing the secondary ion yields continue to shed light on the mechanism of desorption ionization. Beavis and co-workers examined the IR laser desorption of intact molecules from transparent
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matrices e.g., NaCl or ",NOS (C86). Such matrices increase the t o d ion eld as well the shot-to-shot re roducibility, whereas IR-acorbing mahces decrease the yielzof molecular ions and increase the yield of pyrolysis products. Jardine et al. have reported on the combined use of a lutathione matrix and nitrocellulose sample su port for PDMS analysis of peptides (C87). The mettod is sufficiently sensitive to permit analysis of picomole levels of peptides, but more importantly the analysis of peptide samples could not be accomplished by methods that use only nitrocellulose, e.g., growth hormone releasing factor, human C5a complement roteins, thioredoxin (Escherichiacoli),interleukin-2, and guman growth hormone. Benninghoven and co-workers examined the influence of sample primary structure and substrates on the SIMS 'eld for peptides, molecular weight range 500-1900 amu (C88Y The secondary ion yields are highest for peptides that contain basic amino acid residues. Molecular ion yields are low for peptides havin blocked N termini; formation of Ag+ cationized species is oiserved in these cases. Liquid SIMS ionization is complex because the yield for anal e ions is always in direct competition with the ion yield for t e matrix, and thus it is necessary to reduce the matrix ion yield while maintaining the analyte ion yield. Caldwell and Gross have addressed this issue from the standpoint of a nondiscriminator matrix for the quantitative analysis of fatty acids (C89). $h e ratio of matrix to analyte has been investigated by using bradykinin/water/glycerol samples; optimum ionization yields are observed for glycerol/sample ratios of 3001 (C90). 'These results are explained in terms of surface concentration of the analyte. The yield of intact ions formed by 252CfPDMS was examined as a function of sample concentration and absor tion time on nitrocellulose (C91). Ligon and Dorn examine$ the effect of added acid or base on the yield of anal ions from dilute glycerol solutions (C92). Monofunction analytes generally show enhanced signals upon addition of acid or base; however, ion yields for polyfunctionalanalytes are generally reduced upon addition of acid or base. The effects are attributed to changes in the surface activity (glycerol solubilit ) due to changes in the charge state. Matrix temperature bmperature range 20-40 "C) effects have been examined by Sunner and co-workers (C93). The yield of anal@ ions increases rapidly as the matrix temperature is increased. Also, the fragmentation of matrix ions decreases, the abundance of matrix cluster ions increases, and a decrease in the bac round signal is observed as the temperature is increased. e results are explained in terms of a p h e explosion model for desorption ionization. Sunner and co-workers have published several additional apers on the mechanism of desorption ionization from liqui matrices (C94). Chemical reactions occurring at the interfaces between solids/li uids and the vacuum of the ion source were examined by Han and Cooks (C95). Zwitterionic compounds (ammonium salts) and the monosodium salt of cytidine 5'-diphosphocholine undergo beam-induced alkyl-transfer reactions; it is proposed that the reaction occurs by an intermolecular mechanism. The roposed mechanism is consistent with the decrease in alkyy transfer observed for NH C1, ptoluenesulfonic acid, or liquid triethanolamine. % d i d anion formation (one-electron reduction) of oligonucleotides and related compounds are common processes that occur in negative ion liquid SIMS, and Laramee and co-workers showed that this proceas is in competition with proton loss (C96). The yield for ions formed by one-electron reduction is shown to correlate with the matrix electron affinity. Novel ex riments involving new developments in SIMSrelated metpds include the use of higher ener beams (e.g., 3-30-keV Cs+ ions) (C97)-from glycerol a n y thioglycerol matrices the secondary ion yield maximum at 15-20 kev-and the use polyatomic primary beams for SIMS (C98),giving secondary ion yields increases of 9-24-fold. Blam et al. have published several apers on the use of clusters as projectiles for SIMS(CW. e yield for intact molecular ions of organics (amino acids) increases by as much as a factor of 50 (C100, ClOl). Hunt's group has also investigated the desorption ionization of organic molecules by usin megaelectronvolt polyatomic ions (C102). The results of dunt's studies show that ionization efficiencies for the polyatomic ions are comarable to that for 252CfPDMS and show clprly that colEctive, nonlinear effects exist in desorption ionization
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(C1OM105).There have also been several studies on the use of pulsed liquid SIMS experiments (C106) that show both enhanced sensitivities over continuous SIMS ion beams and direct compatibility with magnetic sector tandem instruments (C107).
D. TANDEM MASS SPECTROMETRY Mass Spectrometry/Mass Spectrometry. The use of tandem mass spectrometry methods for structural characterization of complex organic and biologically important compounds continues to be one of the growth areas of applied mass spectrometry. In addition, tandem mass spectrometry has become one of the most wideiy used tools for studying the fundamental roperties of gas phase ionic species. The ionization metho& developed over the past 10 years are capable of producing a high abundance of intact "molecular ion", usually of the type [M + H]+ or [M - HI-, that can be subjected to a variety of structural probes (01-03). MS/MS experiments are now standard accessories on most commercially available instruments (04-061,and these experiments have been performed on a variety of sample t es (07)and with a variety of ionization methods (08-01 Instrumentation designed specifically for tandem mass spectrometry studies of large biomolecules has been described and evaluated (012), and improvements in the performance of first-generation four-sector instruments have been reported (013). In the latter case a four-sector instrument was modified by addition of an array detector. The authors re ort good CID spectra for renin substrate ( m / z 1758) and ACFH (m/z 2464) for 10-Wpmol samples-a 50-fold decrease in sample size over conventional detectors. Barber et al. have examined the (low-ener ) collision-induced dissociation reactions of multiply char$ ions of model pe tides (e.g., gramicidein S and an iotensin I) (014). The 2HI2+ions dissociate to yieldtoth singly and doubly charged fragment ions. In most cases the fr ment ions observed for [M + 2HI2+are the same as those %served for the [M + H]+ ion. The major difference observed for the doubly charged ions is that both fragments formed by a dissociation reaction can c y the charge, and thus somegroduct ions are enhanced in the ID spectra of the [M + 2H] ions. De Pauw has reported an interesting application of matrix effects for structural characterization of peptide ions (025).He notes that s ecific dissociation reaction channels can be enhanced by adling inorganic salts to the liquid-SIMS matrix. Although the changes in the dissociation reaction produds are attributed to cation effects and changes in the surface of the liquid matrix, it should also be noted that such effects can arise from changes in the internal energy distribution of the ion population. Thorne and Gaskell studied the processes leading to internal fragments of peptide ions, i.e., roduct ions formed by combination reactions such as B a n l Y cleavages (016). In the cases examined the internal fragment ions were formed by initial Y-type cleava e. Ion Activation Met%ods/Fundamentals. The major issues that remain to be dealt with are centered around uestions as to how to activate the mass-selected ion to affect The ob'edive of studies addressing the activation must critically emp asize ways to maximize the structural detail extracted from the relative abundance of fragment ions and, because the yield for fragment ions from high-energy collision-induced dissociation is low, it is also important to consider ways to improve the sensitivity of the experiment. (CID product ion yields are low for kiloelectronvolt energy collisions between the incident ion and atomic neutrals (e.g., He, Ar, and Xe), whereas CID product ion yields are reported to be high for low-energy CID (1-10 eV). . The differences between the two experiments, however, is related to the numbers of collisions that occur prior to dissociation and the time scale for dissociation to occur. That is, ions subjected to kiloelectronvolt ener CID undergo few collisions (1-5) (013,whereas ions s u g c t e d to low-energy CID undergo many (>lo) collisions prior to dissociation. Russell has presented data that suggest that the large energy loss values reported for peptide ions (018) arise by multiple collision processes (019).}Estimates for the efficiency for kiloelectronvolt energy CID of pe tides (in the ran e of m / z 1OOO) vary from a few per cent to fractions o per cent (022). Any discussion of fundamentals of CID of lar e ions must deal with three basic issues: (1)specifgic etails (e.g.,
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MASS SPECTROMETRY
collision cross section, excitation probabilities, competing reactions such as ion scattering or excitation of the target neutral) of the ener -transfer process accompan ‘ng the bimolecular ion-neutrr collision, (2) the dissociation (as well as other modes of deexcitation) of the collisionally activated ion, and (3) the inherent discrimination of the mass analyzer and the ion detection system. In addition, the internal ener distribution of the incident ion will play an important rye in the colliiion dynamics and ultimately in the dissociation reactions. The internal energy dependence for CID will be especially important for ionization processes that produce ions having a broad distribution of internal energies, e.g., fast-atom bombardment. Ross and co-workers have shown that the [M + H]+ ions formed by fast-atom bombardment ionization contain appreciable internal energies and that the abundance5 of some of the fragment ions can be altered by collisional relaxation in a high ressure ion source (022),and the same is probably true for of the desorption ionization methods. For exam le, Smith et al. have examined the CID spectra of multiply cgarged peptide ions formed by electrospray (023). “Cold” ions are resistant to collision-induced dissociation, whereas “hot” ions readily undergo CID. Although issues related to the temperature of the ions undergoing collisional activation maybe of little (or no) significance for many applications, e.g., comparison of CID spectra of known and unknown compounds for structural verification, such issues are of crucial importance for studies on the fundamentals of collisional activation, photodissociation, or surface-induced dissociation ( 0 2 4 ) . Future fundamental studies must a close attention to the internal ener ies of the ions simp y ecause the “hot” ions have larger C I b cross sections and are more easily sampled than are the “cold” ions. The internal eneTgy deposited into the incident ion upon collisional activation has been a controversial issue for sometime. In recent years Derrick and co-workers have reported measurements of the translational energy loss accompanying collisional activation; the principal observation being unex ectedly large values for the translational energy loss (0%!Derrick argued that the large energy-loss values imply that the ener uptake by the ion u on collisional activation is also high. R e unplications of suc1 large energy-lms values are (i) collisional activation of large polyatomic ions occurs by rovibronic excitation and (ii) the yield for CID product ions is low because of the large number of degrees of freedom and the ability of large systems to accommodate excess energy. Derrick assumed that the energy loss directly correlated w t h the energy uptake by the incident ion, i.e., that target gas excitation could be ignored. Bricker and Russell presented a contrary view (0261, noting specifically that energy low and energy uptake can be related in a predictable fashion only if target as excitation does not occur. In addition, Bricker and Rusself proposed that electronic excitation (charge exchange ionization) of the target gas is competitive with collisional activation of the incident polyatomic ion; target gas ionization was confiied by additional (Fourier transform ion cyclotron resonance instrument) ex eriments (027). Boyd and coworkers have also noted t i a t energy uptake (estimated by energy loss measurements) upon colliiional activation (by ions that undergo collision but are stable to dissociation on the microsecond time scale) cannot be quantitatively correlated with the excitation of the incident ion (028). Boyd and coworkers have criticall examined energy-loss measurements and discussed the induence of instrumental factors on such . Boyd‘s results are consistent measurements (029)Although with those reported by both Derrick and Russell, he also demonstratesthat the measured ene loss is dependent upon the experimental method used and%at more accurate measurements are obtained by using a “floated collision cell”. Alexander and Boyd have reported on factors controlling (low-energy) CID of a model eptide, specifically leu-enkephalin (030). In this study t i e influence of several experimental parameters is examined, e.g., collision ener nature of the target gas, and operating characteristics oft% instrument, but particular attention is drawn to the reported crucial role of the internal ener of the ion prior to collisional activation. Cotter examinef’endothermic ion-molecule reactions of peptide ions with various neutrals to form association reaction product The resulting fragment ions of the association complex provide structural information not available by direct collision-induced dissociation ( 0 3 1 ) . The end result of this
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work is the s gestion of erforming neutralization/chemical structural characterization. The reionization peptides advantage of such an experiment is that the energetics of the chemical reionization can be controlled to form the association complex (e.g., near-thermal ionization) or to fragment the collision complex (e.g., exothermic ion-molecule reaction). These types of processes open new routes to structural characterization, specificall formation of ion-neutral complexes that dissociate to yiedhighly specific information. The ion-neutral complex dissociates to yield unique fragment ions because of the structural features of the complex, i.e., this is the same argument used to explain the unique dissociation reactions of [M + Li]+ and [M Na]+ type ions. Note also that this experiment is directly analogous to chemical ionization of laser desorbed neutrals. Questions addressing the referred mode (low-energy (tens of electronvolts laboratory tame) versus high-energy (kiloelectronvolts) collisions) for performin collision-induced dissociation are still unresolved. Gaskeb and Reilly examined the low- and high-energy CID s ectra of isovalerylcarnitine, 5-cholestanol sulfate, and anirosterone glucuronide and discussed the analytical quality of the data ( 0 3 2 ) . Poulter and Taylor examined the low- and high-energy CID of peptides and noted that low-energy CID shows a greater dependence on the amino acid composition than does kiloelectronvolt energy CID (033). For example, low-energy CID of eptide ions containing terminal arginine residues gives very Httle structural information; the spectra are dominated by the arg ions. These authors also estimate that a ractical upper mass limit for low-ener CID is approximate y 1500 daltons, but it should be noted gt this limit too is strongly dependent upon the amino acid composition of the peptide ion subjected to colliiional activation. The energy deposited in the incident ion by low-energy CID has been examined by Schey et al. (034) Although . the average ener deposited in the incident nature of the target gas ion is strongly dependent upon and the kinetic energy of the ion, heavier target gases result in smaller energy transfer (probabl because the scatterin angle is larger and products of “harJcollisions” are scatterej outside the acceptance angle of the instrument) and target gas excitation occurs for polyatomic targets. Surface-induced dissociation (SID), where the internal energy required for dissociating stable ions is derived from an ion-surface collision, yields spectra that are similar to gas-phase ion-neutral collision-induced dissociation. The early work in this area demonstrated the method for 20-60-eV ion-surface collision energies (035).Aberth introduced the use of microchannel plates for SID on magnetic sector instruments and reported SID spectra (250-eV collision energy) for leu-enkephalin [M + H]+ and [M - HI-ions that contained complete sequence information (036). The SID spectra of the [M + H]+ ion is dominated by B series fragment ions, whereas the SID spectra of the [M - HI- ion contains both B and C series fragment ions. SID has also been used for differentiation of structural isomers (037).In a recent paper Schey et al. described a tandem time-of-flight instrument for This . study also reported new data on the SID studies (038) SID process: (i) internal energy deposition in the incident ion is reported to increase linearly with kinetic ener (ii) the high abundance of low mass fragment ions, e.g., e C 3 , and C4 ions, suggests that large amounts of internal energy are transferred in the collision, and (iii) at high collision energies (>750 eV, but dependent upon the stability of the incident ion toward fra entation) sputtering of the surface occurs and the a b u n g c e of SID product ions is attenuated.
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E. UNIMOLECULAR DISSOCIATION PROCESSES Ion Chemistry of Organometal Ion Species. Since the early work in field desorption, the formation of cationized organic molecules, e.g., [M + Cat]+, typical1 Cat is Li, Na, K, Ag, Cu, etc., has been the subject of considrerable interest. For example, the abundance of [M + K]+ ions in the laser desorption spectra of polar molecules is usually quite high and seldom are such ions formed with sufficient internal energy to dissociate. Experimental data have been published that sug est that [M + Na]+ ions are formed by dissociation of M + f$a(NaX) +I cluster ions,.and thus the ions are cooled y loss of the EfaX neutral species ( E l ) . In fact, ions of the type [M + Cat]+ that have sufficient internal energies fragment
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b loss of M to form Cat+; however, if the binding energy of to M is large, low-ener frapentation reactions of M may be competitive with &Jociation to Cat' (E2). The fragmentation reactions of [M + Cat]+ ions are controlled by the relative binding energy of Cat+ to M and the energetics for bond breaking of the organic molecule. Russell and Mallis were the first to demonstrate that the dissociation reactions of [M + Cat]+ ions are different from the reactions for the [M + H]+ ions (E3),and this difference in fragmentation reactions was used to probe enzyme catalyzed positional isotope exchange reactions (E4)and competitive interactions of Na+ ions with peptides containing basic amino acid residues (E5). On the basis of these studies it was proposed that the alkali-metal ion is attached to the most basic functional group of the organic molecule. For example, in a simple tripeptide such as gly-his-gly the alkali-metal ion would preferentially (but not exclusive1 ) bind to the his residue, and the dissociation of [ ly-his-gry + Na]+ occurs by bond cleav e reactions withine!t gly residues. The data for a series of m%el peptide ions, e.g., gly-gly-his, gly-his-gly, and hisgly-gly are consistent with the proposed model. In addition, the dissociation reactions for gly-his-ar -pro [M + Na]+ ions are also consistent with the same motel. Gross and co-workers have presented a contrary view of the dissociation reactions of organo-alkali-metal ion complexes for model pe tides (E6). Grese et al. examined the dissociation reactions of rM + Li]+ model peptides and proposed that the Li+ is attached to the C-terminal residue. This assignment is based on the preferential loss of the C-terminal amino acid by a mechanism whereby the Li+ catalyzes loss of CO and NH=CHR (products confirmed by isotopic labeling), which comprise the C-terminal amino acid ( E n . In many cases the C-terminus loss reaction channel dominates the CID spectrum of the [M + Li]+ ion, and in nearly all cases this is the most important metastable ion dissociation channel. The latter results suggest that loss of the C-terminal residue is the lowest energy reaction channel for the [M + Li]+ ion. Grese and Gross examined both metastable ion and CID reactions of peptide [M + Cat]+ions. On the basis of these data it appears that loss of the C-terminal residue by rearrangement reactions in the low-energy dissociation, and the reactions of [M + Cat]' containing his, e.g., those described by Mallis and Russell, are high-energy dissociation reactions (E3). This view is consistent with the energetics for diesociation of [M + Cat]+ ion peptides reported earlier by Mallis and Russell. Standing's group also examined the dissociation reactions of organo-alkali-metal ions of peptides (E8).These studies also employed isotopic labeling to determine the composition of the neutral fragments lost on dissociation of M + Na]+ ions; '%-labeling studies were performed on gly-g y-phe that showed that the carboxyl oxygen of the C-terminal residue is retained by the daughter ion upon loss of the C-terminal residue. A mechanism originally proposed by Renner and Spiteller (E7) is consistent with both Standing and Grese's data, but this mechanism does not explain all the dissociation reaction channels observed for peptide [M + Li or [M + Na]+ t e ions, especially those reactions for pepti e ions that contaizighly basic amino acid residues, e.g., his, arg, lys, etc. The [M + Cat]' ions can be formed by a variety of ionization methods. These ions are observed in the field dea of nearly all polar organic sorption and laser desorption s molecules, and by addition o small amounts of alkali-metal salts to the liquid matrix these ions can be formed in high abundance in liquid SIMS. Recently Hand described a method to enhance Ag cationization of specific structural types (viz. a concave area at the eripher of the molecule) of polycyclic aromatic hydrocartons ( E d . Deprun examined the effect of metal ions on the acquisition and quality of 262Cf PDMS spectra (El0). In general, the presence of small amounts of Ca2+attenuates the yield of negative ions for phosphorylated glycolipids, but the effects are much less pronounced in the positive ion spectra. In those cases where the presence of alkali-metal metals is undesirable there are relatively simple procedures for removing the salts ( E l l ) . Allison has developed a rather novel method (the sample is placed directly onto a thermionic emitter) for producing abundant [M + K]+ ions (E12).A similar method has also been discussed by Simonsick (E13).The method has been demonstrated on molecules such as digoxin, digoxigenen, digitoxin, and ouabain. The ionization method yields intact
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[M + K]+ ions and fragment ions that are formed by lowenergy rearrangement reactions. (As noted above, ions such as [M + K]+ are less desirable for structural characterization (tandem mass spectrometry) because the binding energy of K+ to M is low relative to Na+ and Li+, and thus fewer fragment ions will be observed with K+ than Li+.) The method has also been used for rapid screenin of urine for organic acidemias (E14).Fujii et al. have Ascribed a as-phase ion-molecule reaction method for producin abunfant [M Li]+ ions of organic molecules (E15).T h e t i + ions are generated by heating lithium oxide in an aluminosilicate matrix in a nitrogen stream; the organic vapor is introduced into the carrier gas, which is connected to the mass spectrometer ion source. The field of transition-metal ion chemistry has rown significantly in the past several years, and this area wipi continue to impact both ion chemist and mass spectrometry. In fact, many of the instrumental yevelopments that are important to studies of metal ion chemistry directly impact analytical mass spectrometry, but the bulk of the ion chemistry studies over the past five years have not been directly aimed toward analytical mass spectrometry. Consequently, this area of ion chemistry will not be covered in this review. Past reviews have covered the highlights of the field, but it seems more appropriate to point out those areas of metal ion chemistry that directly impact analytical mass spectrometry. For example, Bjarnason and Taylor have examined the gas-phase ionmolecule reactions of Fe+ with benzene derivatives, the objectives being to differentiate structural isomers (e.g., ortho, meta, and para) by bond formation between two substituents of the molecule (E16).On the basis of this type of work, it is now quite clear that metal ions can be used as selective reagent ions for ion-molecule reactions. Clearly, the selectivity of such reactions is due to the different interactions between the organic molecule and the metal ion (El7)and the eneretics of the bond forming/bond breaking rocesses (E18). bore detailed studies, such as thwe on [M + Ei]', [M + Na]+, etc., may provide unique approaches to structural characterization of complex molecules. This is especially true in those cases where structural features ( olar functional groups) of the intact neutral molecule are enfanced by formation of an ionic complex that has available low energy dissociation channels. To date there have been very few studies reported where these parameters have been evaluated. Fragmentation Reactions. The bulk of the novel studies entation reactions of organic ions is now centered E o f i g m p l e x molecules and especially biomolecules. Early in the development of desorption ionization methods it became quite a parent that the most rapid development in this area would {e with peptides, and the work over the past two years has reenforced this view. Without question the dissociation reactions of peptides are the most studied and, therefore, the best understood. Recent apers by Kulik (El91 and Biemann and co-workers (E201anfthe papers discussed in the section on ORGANOMETAL IONS clearly demonstrate the range of current understanding of this class of ions. [See also the section on AMINO ACIDS, PEPTIDES, AND PROTEINS.] Many of these studies deal with the major dissociation channels, but the study cited above by Biemann and coworkers deals s cifically with dissociation reactions involving the amino a c i E i d e chains. In particular, two novel fragmentation processes are described: one channel leads to formation of N-terminal ions that permit differentiation of leucine and isoleucine, whereas the other leads to a set of C-terminal ions. It is suggested that these fr mentation reactions are formed by charge-remote reactions. each case the dissociation reactions are in some way related to the site of protonation, and there seems to be a stron dependence upon the degree to which the charge is fixef at that site. Tecklenburg et al. also reported highly specific reaction channels for hotoexcited (absorption of 488-nm photons) peptide ions (!$21). Distinct reactions are observed for leucine and isoleucine ions, and some of the photofragment ions appear to be formed by dissociation reactions involving distonic ions. The latter reaction channels are also probably examples of remote-charge site fragmentation. Gross has continued to investigate the phenomena of remote-charge site dissociation reactions of fatty acids. In a recent paper he em hasizes the role of functional groups having high proton aknities on the occurrence of remote-site
+
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fragmentation (E22). Gross has also discussed the analogy between remote-site frapnentation reactions of even-electron ions and the thermolytic degradation of the molecule (E23). Adams and Gross note that remote-site fragmentation reactions are not limited to high-energy (kiloelectronvolt) CID and the link between dissociation of ionic systems and gas-phase thermolysis. Cody has also examined the energy requirement for remote-site fra mentation and noted that such reactions are favored for !ow-energy CID and long observation times-consistent with rearrangement reactions. Specific details on the dissociation reactions of biomolecules is contained in the latter sections of this review.
F. AMINO ACIDS, PEPTIDES, AND PROTEINS As discussed in the OVERVIEW, the past two years have seen a growth and broadening of the structural roblems that have been solved by the most recent methods an: instruments. Ma'or books and chapters have already been cited in section B (&COPE) Studies ofthe ne ative ion chemical ionization mass spectra of 4-nitro henyliso&ocyanate derivatives of amino acids have continue1 in the context of increasing the speed, sensitivity, and selectivity of the Edman de adation for peptide sequencing (FI).Thermospray L C / b S has been employed to c o d m the identification of phenylthicarbamyl (PTC)amino acids (F2). From studies of the collisional activation of metal ion pe tide adducts in the gas phase, evidence is presented that alhi-metal ions generally interact with the carboxylate function of the C-terminus of a peptide and that this interaction induces the metastable loss of the C-terminal amino acid residue (F3, F4). An acid-resistant fluorescent molecule has been identified from human extracellularmatrix as imidau,[4,5b]pyridinium, consisting of a lysine and an arginine residue cross-linked by a pentose (F5).Various li amino acids represent 60% of the extractable cellular l i p i k in the opportunistic patho ens Flavobacterium meningosepticum and F. indologenes ($6). The autoinducer for the luminescence system of Vibrio harve i was identified as N-@-hydroxybutyry1)homoserinelactone A variety of neuropeptide hormones have been isolated and identified, including those of the co ora cardiaca of horse flies (F8), of the corn earthworm moth x l i o t h i s zea (F9), and of the corpora allata of the virgin female cockroach Di Zoptera punctata (FIO). An antimicrobial cationic pe tide [as been identified from horseshoe crab TachypZeus trigntatus (F11). High-energy CID s ectra were used to establish the identity of antifungal peptixes of the Iturin family active against the mycelial growth of the spoilage organism causing peach brown rot (F12). Methods of tandem mass s ectrometry have been applied to elucidation of cyclic pepti&s isolated from bluegreen algae (F13, F14). Similarly,tandem and hi h-resolution methods have been used extensively by Rinegart and coworkers in the characterization of a variety of biolo ically active peptides obtained from insecta and marine inverte%rates (F15). S-(2-Chloroacetyl)glutathionehas been identified after microsomal activation of 1,l-dichloroethylene (F16).Liquid SIMS has been employed effectively in characterizationof the products of trans lutaminase incubation using a variety of substrates (F17, Baillie and co-workers have continued their studies of reactive intermediates in drug metabolism, with further identifications of covalent adducts with mercapturic acid, glutathione, and related S-linked con'ugates. During this period they have reported on further d i e s of acetaminophen (F19), -benzoquinone (F20), N-methylformamide (F21, F22), and ambuterol (F23). In some of these examples effective use was made of LC/thermospray/MS and MS/MS, as well as various derivatization strategies, and liquid SIMS. 2azCfplasma desorption time-of-flight mass spectrometry has enjoyed more extensive use during this period. It is of clear utility for molecular weight mapping of components obtained from proteolytic digestion of proteins and a variety of other substan-, as noted m earlier reviews. This relatively inexpensive instrument is notable for its ease of o eration, and molecular weight data are readily obtainable. Aowever, a caveat of caution still exists concerning its ability to measure nominal mass unambiguously. There is always the possibility that the nominal mass may be in error by 1mass unit, and at higher masses possibly 2 or more mass units. Exploitation
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of the advanta es of this method has been discussed in considerable detaif b Roepstorff and his colleagues (F24, F25) Cotter (F26), andrMacfarlane (F27). There continue to b i widespread claims that this methodolo will become an integral part of the protein biochemical lagratory, particularly for characterization of synthetic peptides and peptides from recombinant proteins (F24, FB). However, it seems caution should be exercised due to the rapid commercialization of electrospray and other instruments, which may turn out to have competitive analytical capabilities. Matrices for PDMS are still being evaluated, including glutathione and nitrocellulose (F29, F30), and various washing protocols may be of advantage in improving the quality of the spectra and suppressing components in mixtures (F31).Work on the practical aspects of mass calibration and the effects of non-protein com ounds on spectrum quality have been discussed (F32, F33f including proteins isolated after separation on polyacrylamide geIs (F33). Since the presence of detergent in the case of liquid SIMS may present problems, the use of polyacrylamide gel electrophoresis containin urea and acetic acid without SDS has been suggested 7F34) Glucagon-like peptide-1 has been identified from human and pig small intestine (F35), and proglucagon processing has been characterized in extracts from catfiih pancreas by us* methods (F36). Similar mass mapping has identifie recomPDMS binant interleukin-2, human growth hormone (F37), recombinant human growth hormone (F31), and interleukin-3 (Fa). Rat urine collected in the presence of a serine protease inhibitor contained both a low and high molecular weight form of epidermal growth factor, while urine collected without such additives contains only low molecular weight form of EGF (F39).The two forms presently identified are a 51 amino acid form and a 45-kdalton form. In a recent stud , Roepstorff and colleagues have demonstrated that activateicomplement component Cls cleaves B2-microglobulin at the position identical with that at which B2-microglobulinis cleaved in the serum of patients sufferingfrom lung cancer. This is the first demonstration of a noncomplement protein substrate for the proteolytic activity of Cls (F40). Further concerns about rimary beam induced reactions in the liquid matrix have Eeen explored by measuring the precise isotope peak ratios for peptides under xenon liquid SIMS conditions (F41). Indications of functional groups most prone to reduction in addition to the disulfide bridges are presented. Investigation of formylation of peptides during chemical digestion with cyanogen bromide has been carried out. It was noted that use of 70% trichloroacetic acid as solvent eliminated formylation (F42). Further studies of artifacts in the production of biosynthetic human growth hormone have been concerned with the oxidation of methionine and the alteration of cysteine by reaction with acrylonitrile impurities present in acetonitrile solvents (F43). Using basic ancreatic trypsin inhibitor, a 58 amino acid protein as a mo8e1, use of enzyme-thermospray LC MS for protein sequencing has been investigated (F44). urther application of thermos ray LC MS has been illustrated by the identip Lin variants (F45). Additional results on fication of h emoglo hemoglobin variants using other methods are reported below. Hunt and co-workers have continued their use of the tandem triple quadrupole instrument in sequencing of peptides and proteins. Thii group has reported studies on photosystem I1 proteins of spinach chloroplasts that contain N-acetyl-0phosphothreonine N termini (F46), identification of a single affinity labeled histidine residue of a benzyladenine binding site peptide isolated from wheat cytokinin-binding protein (F47), sequence analysis of two isoforms of mouse calbinthe sequence of sex steroid bindin protein of din-Dgk(F48), rabbit serum (F49), and proteolytic fragments of t i e nicotinic acetylcholine receptor (F50). It is interestingto note that these results for the receptor on native and reconstituted membranes are qualitatively the same as results obtained in other laboratories on denatured intact affinity isolated acetylcholine receptor (F51). This issue of topographic relevancy was not discussed by Hunt and co-workers in their publication. Isotope labeling has been used to probe the assumed specificity of external proton donation versus internal proton transfer to the charge site for the Y + 2 ions under low-energy collision induced dissociation with a triple quadrupole mass spectrometer (F52). '80 labeling and N-acetylation were used to aid in the interpretation of the fragmentation of collisionally
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activated modified peptides using a tandem hybrid instrument (F53). The complete amino acid sequence of porcine gastrotropin has been reported and found to be similar to rat liver fatty acid binding rotein (F54). Linked scanning with collisional activation on [quid SIMS desorbed rotonatexi molecular ions was used to sequence the blocked $-terminal peptide of this protein. Further work using linked scans for studies of the peptides obtained in a proteolytic digest of pyruvate decarboxylase has been reported (F55). This methodology suffers from the same tential spectral admixture roblems as the triple quad, in t at the mass acceptance winlow from the ion source is several mass units wide. Hi h-ener CID on a tri le-sector instrument has been carridfor e n g p h i n and A8TH peptides up to molecular weight 2000 (F56). Bradykinin and kininogens have been identified in bovine milk (F57). Mass spectrometry continues to be the method of choice in the identification of N-terminal blocking groups on proteins such as oat phytochrome (F58),sarcoplasmic calcium-binding protein from crayfish (F59), oxytocin (F60), calcium-bindingprotein from the optic lobe of the squid Loligo pealei (F61), recombinant he atitie B surface an en protein (F52),and recombinant fibrohast growth factor ( 3). In the case of recombinant he atitis B surface antigen protein, only 70% was found to be Jacetylated, and a similar finding was reported for human basic FGF. Recently, prenylation of the carboxy terminal cysteine of the Ras protein has been re orted as the thioether linkage (~64).s u n i h l lam in^'18 s own to contain a geranylgeranyl modified thioezer-linked cysteine (lX5).cy-Tubulin was found to be posttranslationally modified by successive addition of lutamyl units on the y-carboxyl group of a glutamic residue, &luM6(F66). With mass spectrometric molecular weight ma ping of a t tic di est of 5 nmol of sterol carrier protein SC,! from r a t E e r , it \as been shown that the C-terminus was extended beyond the previous1 published sequence (F67). Studies of a catalytic fragment o phosphorylase kinase obtained by chymotryptic di estion showed it to be isolated from the N-terminal region of t i e y-subunit. It contained multiple carboxy termini generated at residues 290,296,298, and 386 of the y-subunit. This work illustrates the difficulties of determining C-terminal protein heterogeneity by using conventional protein microsequencing and the importance of the use of mass spectrometry in C-terminal characterization(Fss). Molecular weight ma ping of a recombinant hirudin roduced by yeast was uselto verif the cDNA sequence (&9). Similar mass mapping of proteo&tic digests of recombinant human erythropoietin has led to the identification of a desarginine 166 erythropoietin (F70). Following biosynthetic incorporation of tritiated ethanolamine into protein synthesis mass spectrometry was em loyed elongation factor la (F71), in various ways to characterize a novel amide-linke ethanolamine phosphoglycerol moiety (F72) and several other posttranslational modifications including dimethyllysine and trimethyllysine (F73). Similar molecular weight mapping of a tryptic digest of tyrosine aminotransferase has been carried out, including reduction of the holoenzyme with sodium borohydride, leadin to the identification of phosphopyridoxamine bound to kysine 280 (F74). With a combination of Edman microsequencing and mass spectromet ,complete sequence of bovine factor VI1 has been reportedF75). Once again,li uid SIMS was important in establishing the carboxy termina? sequence of the heavy chain. In studies of east external invertase, a combination of liquid SIMS andJ lucosamine analysis established that 8 of the 14 potential 3-linked glycosylation sites are either completely or almost completely glymylated, while 5 others are glycosylated to the extent of 50% or less (F76). Clarification of the cDNA sequence was carried out from the same molecular weight map of the proteolytic digest and detected only one discrepancy at osition 390. Proline was found in place of alanine and coulfhave resulted from a single base change in the triplet specifying the latter amino acid (F76). It has been shown that single amino acid substitutions in the reactive site of antithrombin lead to thrombosis. Two of these have been recently identified as congenital substitution of arginine 393 to cysteine in antithrombin Northwick Park and to histidine in antithrombin Glasgow (F77).Similar mass mapping of proteolytic digests has been used to establish that yArg-275 is substituted by cysteine in an abnormal fibrinogen Osaka
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I1 (F78).Mass analysis of specific proteolytic digests have been used to characterize the sites of proteolytic activation of Newcastle disease viru$ membrane glycoprotein precursors (F79), and a combination of Edman micnwequencing and mass spectrometry has established the sequence of human erythrocyte phosphoglycerate mutase (F80). In a landmark paper, molecular weight mapping of specific proteolytic d' ests of rabbit skeletal muscle glycogen synthase was employJto delineate multiple phosphorylation sites and the stoichiometry of phosphorylation at those sites as a function of the physiological state of the animal. Experiments were carried out injecting the animals with adrenalin or pro ranolol, and it was then possible to determine the quakative and quantitative differences among the activities of different protein kinases responsible for regulation of specific sites (F81). While it is easy to dist' ish between phosphorylated and nonphosphorylated peptxanalogues due to the absence of loss of H2P0 at M-80 in phospho analogues, this is not as easy in the suko case. Investigation of matrix conditions have been reported aimed at an attempt to increase the molecular ion intensity relative to the M-80 sulfate elimination in the sulfo analogues (F82).The status of the identification and sequence analysis of both phosphorylated and sulfated eptides by liquid SIMS and tandem mass spectrometry has L e n discussed in detail (F83). It should be clear at this point that molecular weight ma ping has been used very effectively, particularly in concert wit{ the micro-Edman sequencing method, for solving a variety of problems in protein chemistry, including N-terminally blocked proteins, verification of cDNA sequences, discovery of inborn errors in hemoglobin, and other physiologically important proteins. We have noted that the accuracy of mass measurement in measurement of molecular weights of proteolytic digests are important and are in general more reliable from liquid SIMS magnetic sector instruments than from plasma desorption time-of-flight instruments. We have also noted that mass acceptance window in tandem experiments using triple quadrupoles and B/E linked scans on magnetic double-focusing instruments can give rise to other components in the spectra. To this point, we have said little about the im ortance of finding all of the components in any given proteobic digest. Of equal importance are methods for ensuring that the purity or homogeneity of a protein is defined properly or that the complete protein obtained from assembly of the overlapping roteolytic digests establishes the correct natural or recominant protein (or mixture of proteins). The remainder of this section is based primarily on CID spectra obtained from four-sector tandem mass spectrometers. These sequencing experiments are carried out on the 12Cmolecular ion isobar with accuracy of mass measurement of the resulting CID daughter spectrum *0.2% of a mass unit, such that the mass selected and the mases detected from that activated ion are unambiguously established. This capability not only permits identification of leucine and isoleucine isobars from their different side-chain fragmentation losses but also minimizes the need to do a variety of other chemical derivatization or modification experiments on each peptide being se uenced, as appears to be necessary using the strategy reported\ Hunt and co-workers with the wide mass acceptance and nonnomind mass resolution in the daughter ion s ectrum in the triple quadrupole procedure (F46-F50). Witt multichannel array detection systems on four-sectorinstruments, it appears that the operational sensitivities are comparable, while the fidelity of the CID spectra from four-sector systems appears, at this point, to be superior. Finally, have we identified correctly the entire protein or mixture of proteins from the assembly of appropriate proteolytic digests? In this we mean in the most global sense, including glycosylation of the N- and O-linked types, as well as other posttranslational modifications, including the phosphoinositol glycan anchors. The stage is set for the first time, with the availability of both electros ray and UV absorbing matrix laser time-of-flight capabitt , for rigorous structural characterization of even complex ggcoproteins to proceed with increasing momentum. Many of these points were made in a recent news update (F84). A recent overview of multisector magnetic tandem mass spectrometry discusses the design and performance criteria
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and capabilities of these instrument t es (F85).Further considerations of four-sector tandem stu ies of peptides have recently been reviewed, including collision energy mechanisms of excitation and fragmentation, and a compilation of ptide sequences and fragment ions observed from the puEished literature (F86).Several book chapters have been prepared on the use of high-ener CID s ectra in the se uencing of peptides (B31, B33,F83,%7497f Rather detaile% discussion of side-chain-specificsequence ions occurring in high-energy collision-induced fragmentation of pe tides has been presented (F98).In addition, the particular 8ID fragmentation characteristics of peptides w t h N-terminal glutamine have been described (F99). Derivatization of pe tides with groups containing a quaternary charge center has &n shown to direct the fragmentation preferentially (F98,F100). Interpretation of high-energy collision-induced dissociation tra of peptides is often time consuming and complex. The Eelopment of computer algorithms to speed this process is of considerable importance in eventually realizing the inherent high speed of tandem mass spectrometry for protein sequencing (F101). Of course, it should be em hasized, once again, that the methods for rotein isolation, &estion, separation, and overall sample imn& are of extreme importance in exploiting these mass spectrometric methods in protein biochemistry. These have been discussed in considerable detail by Shively and and Gibson et al. (F104). Paxton (F102),Shively (F103), High-energy CID spectra were used to demonstrate clearly that as aragme 36 versus glutamine 38 or aspara ine 47 was deami& in a cyanogen bromide trypain digest of an isoform of 11-la used for X-ray crystallographic studies (F105). Hi h-energy CID spectra were used in the correction of a C D ~ derived A protein sequence of prostatic spermine binding protein, and the determination of the amineterminal blocking group as yroglutamic acid (F106,F107). Information regarding t e glycosylation state of this protein was also obtained from this study. The primar structure of a protein carboxyl methyltransferase that serectively methylates Lisoaspartyl sites has been carried out on protein isolated from bovine brain (F108)and from human erythrocytes (F109). High-energy CID spectra were used to identify the N-terminal acetyl-blocking moiety and se uence of the f i i t nine residues for cellular retinaldehyde-binling protein from bovine retina (F110).A similar result was obtained on the N-acetyl Nterminal block of rat heart fatty acid binding protein, including correction of two erroneous peptide se uences (F111).More recently, the complete sequence of L a r k liver fatty acid binding protein has been deduced primarily from high-energy CJD spectra, except in the case of a peptide sequence that gave redundant isobaric N- and C-terminal ions. This sequence was established b the Edman sequencing method and later digestion, providing a labeled peptide confirmed after analogue that showed the C-terminal sequence ions shifted 2 mass units (F92,F112). Biemann has continued studies on sequencing a series of thioredoxins using tandem four-sector mass spectrometry. The most recent examples are thioredoxin from rabbit bone marrow (F113),and a small heat-stable rotein, disulfide dithiol hydrogen donor for E. coli ribonucgotide reductase, called glutaredoxin (F114). In addition to linear sequencing issues, there are many problems related to the location of disulfide link ea in proteins. While these have not been given the detail3 attention that sequencing has,as illustrated above, nevertheless several reports addressed these issues. One report concerns the use of performic acid oxidation with mass spectrometry for identification of disulfide pe tides by liquid SIMS (F115). Since it is known that disulgde linkages are reduced as a function of time in the matrix during irradiation with the primary beam, attention has been paid to matrix additives that might be able to suppress primar beam-induced reduction. It appears that the addition oltri fluoroacet ic acid prevents reduction of disulfide linkages in the usual liquid matrices (F116).It has been noted that appropriate electrochemical detectors may be used to locate which peaks in a digest contain componenb bearing disulfide linkages (Fl17). Subsequent analysis of those fractions after treatment with performic acid (F115) confirms which peptides are disulfide-linked. A combination of Edman sequencin and PDMS was used to assign disulfide bonds in neurop ysin
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(F118).Mass spectrometric methods have been applied to distinguishing two forms of recombinant IGF differing by a simple interchange of two cysteines, C4' and C4 (F119).Other examples are work reported on recombinant human insulinlike growth factor I1 (8'120)and recombinant hirudin variant 2-Lys 47 (F121).Two reports of the use of collisional activation for sequencing through a disulfide-linked dipeptide have appeared. One makes use of a MIKES spectrum on a and the other a four-sector VG homodimeric peptide (F122), instrument on a homodimer N-terminally blocked with pyroglutamic acid (F123). The phosphorylation site of the chemotaxis response regulator protein Che Y has been determined by se uencing of the putative phosphoaspartyl borohydride-redud~ornoserine lactone using high-energy CID spectra obtained at the 30-pmol level with multichannel array detection (F91,F124). Highenergy CID spectra were used to establish identification of S-farnesyl cysteine in the appropriate peptide obtained from digestion of a2 mating factor from Saccharomyces cereuisiae (F125).The im ortance of use of unambiguous methodol in establishing g e structural nature of such p t t r m l a t i o ~ y modified proteins has been discussed independent1 (F126). It should be noted that even more recent results iscussed above suggest the prenylated moiety could be attached to the carboxy-terminal cysteine as a thioether linkage, in the case of ras protein (F64),although no tandem mass spectrometry of the posttranslationally modified peptide was presented in support of the nature of the linkage. Recent studies of recombinant soluble CD4 receptor will serve as an introduction to the next section on glycoconjugates, as well as a further example of four-sector tandem mass spectrometry investigation of a protein (F127).This paper illustrates the power of present mass spectrometric methods for the sequence analysis and structural characterization of the posttranslational modifications of recombinant glycoproteins. Use of this tandem methodology is further exemlified by studies of backbone-modified peptides that may Ee important in design of therapeutic peptides to alter ligand-binding characteristics, increase resistance to enzymic degradation,or alter physicochemical properties of the peptide (F128). Tandem mass spectrometry is of particular importance in its ability to scrutinize xenobiotically covalently modified pe tides. This topic represents a whole range of issues invohng active site photoaffinitylabeling, xenobiotic activation, and electrophilic covalent binding, carcin en binding, identification of occupational chemicals, lev% of exposure in human dosimetry, etc. One important example is the inactivation of dopamine &hydroxylase by p-cresol (F129).It was possible to characterize two modified tryptic peptides from the inactivated enzyme, including the site of attachment and the chemical nature of the adduct formed, even though the modified peptides had the same primary sequence. This study demonstrates the power of the combined use of Edman sequencing, tandem mass spectrometry, and isotopic exchange experiments applied to this kind of problem. Identification of adducts formed in vivo from covalent binding of acetaminophen to mouse hemoglobin have been reported (F130).Study of CID spectra of products of incubation of S-(N-methylcarbamoy1)cysteinewith reduced oxytocin revealed the presence of two monoadducted peptides that arose from carbamoylation of either Cys-6 or Cys-1 in the ratio of 5:l. These were distinguished from isotopically induced shifts in the CID ions upon treatment of appropriate deuterated starting material (F131). Xenobiotic alkylation of N-terminal valine of hemoglobin has been used for studies in human dosimetry by virtue of the ease of isolation of the modified N-terminal amino acid, and its subse uent quantitation by GC/MS (F132).Recently, high-energy 8 I D studies on all of the tryptic components of hemoglobin have been carried out to identify all putative sites of covalent modification by occupational chemicals, which form electrophilic species upon metabolic activation. The f i t example utilized styrene oxide and led to the surprising result that major sites of modification were histidine residues on the periphery of the hemoglobin preferentially to the more nucleophilic 893 cysteine residue. This method would appear to have general utility in identifying exposure in humans to unknown electrophilic agents which may represent a genotoxic health threat (F133). ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
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Finally, in addition to mappin the molecular weights of proteolytic and chemical digests o?proteins and glycoproteins, measurement of intact molecular weights of roteins and components of protein d’ ests that are not r e a d y measured by either liquid SIMS orfDMS is of the utmost importance. Electros ray ionization has been elegantly articulated by the work of Fenn and collaborators and recently reviewed by them (B2).Their work has led to the production of commercial instrumentation that will enjoy wides read use in a variety of a plications in the near future. An ogous versions of this metkd, which may be called nebulization aqisted eleospray (ionspray) may have their own advantages in prowding ease of correlation of retention times of proteolytic digests under LC MS conditions more readily than continuous flow FAB M (F134),although this point remains to be firmly established. There are several interestin examples of the early use of electrospray, such as the rapid etection and characterization of hemoglobin variant species. This recent work uses the accuracy of molecular weight characterization of the intact hemoglobin to 1-2 daltons for detection of variant species together with subsequent digestion and hi h-ener CID analysis for exact delineation of the epti e modi ication (A10).A further example concerns t1e measure of intact molecular masses of thioesterase 11species, where it is possible to observe mixtures of C-terminally truncated species without their separation (F135).In addition, electros ray was used to verify the completeness of the sequence o rat heart, rat liver, and shark fatty acid bind proteins (F92)and in the and characterization of native and recombinant proteins (A9), the presence of minor com onents at the 1%level. During the same time period, bothbanaka (F136)and subsequently Hillenkamp and Karas (B3)have shown the im ortance of laser-energy matrix modulation in the ejection an$ ionization of high-mass proteins by time-of-fli ht mass spectrometry. In the initial exam le absor tion a n i ionization e’ection was modulated by fine& hividefmetal powder (F136!,but more recent dramatic results have been obtained by using chromophores with the correct acid-based properties, such as nicotinic acid (F137-Fl40)and cinammic acid derivatives (F141). At this writin catalase, at around 230000 daltons, is the largest protein t at has been measured by laser desorption time-of-flight, approximately a factor of 2 higher than signals for the dimer of albumin by electros ray. As mentioned in the OVERVIEW and other sections,e!t main concern is over the reliable measurement of accurate chemical mass with laser time-of-flight systems. This, however, does not detract from the importance of obtainin molecular size significantly more accurately than that whic is possible with 2-D gel electrophoresis or any other method available. Another factor that may be of importance is the relatively low mass resolution on time-of-flight systems, even reflectin time-of-flight systems, compared with that possible when e ectrospray methods are adapted to ma netic sector double-focusin instruments (F142).This wdpermit the detailed scrutiny ofclosely related components of proteins throughout a very important molecular size range. It remains to be seen whether either of these methods have fundamental limitations on molecular size. It would seem that, judging from the Millikan oil drop experiment, had oil droplets been eas proteins molecular weight limitations would have vanis ed ecades ago, once the charge on the electron had been determined.
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G. OLIGOSACCHARIDES AND GLYCOCONJUGATES Structural investigations of most classes of oligosaccharides and glycoconjugates present their own set of formidable challenges and hence cannot be thought of as classes of structures that will yield to one overall strate as is in many ways the case with proteins. Almost all proKems in carbohydrate structure require dealing with a heterogeneous set of individual discreet molecular structures. There is a lack of reliable methods for degradation of oligosaccharides and glycocon&gates in a structurally specific manner, unlike the availability of specific endoproteases in protein chemistry. There are additional structural issues such as the linkages, branching, and stereochemistry. On the one hand, there are fewer monomeric units comprising many oligosaccharide structures compared with the number of different amino acid 280R
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components in a rotein, but the manner in which they may be structurally ekborated far exceeds that encountered in protein problems. Mass spectrometry has played an important historical role in defining carbohydrate linkages from analysis of the partially methylated alditol acetates by GC/MS, which continues to the present time. These points have been reviewed in some Laine (G2), detail recently by Hellerqvist and Sweetman (GI), and probably in the most extreme form for the animal kin dom by Radamacher et al. ((33). Ruoslahti has recentfy commented that “it’s important to discover what carbohydrates do, because this is, in some ways, the last frontier in macromolecular chemistry and function” (G4).The rest of this Ciba Foundation sponsored volume (G4)is devoted to discussion of the state of our knowled e about carbohydrate structure and function, but others adiressed these issues in some detail as well, including Paulson (G5)and Sharon and to mention only a couple. Lis (G6), Another aspect of mass spectrometry of historical as well as immediate importance is the sustained pioneering development of mass spectrometric methodology for the characterization of glycosphi olipids by Karlsson and co-workers (0. In recent years,%s effort has been joined by several other major groups in Japan, Germany, and the United States particularly. Most recent1 , reinvestigation of the spectral characteristics of these sugstances was carried out usin a four-sedor tandem mass spectrometer by Domon and Coste%o (GB). Many of the sphingolipids occur as mixtures of related structures, which have been investigated in the past by partial fractionation on the direct inlet probe of an electron impact ion source (G9),or using thin-layer chromatograph for separation of the individual components (G9,GlO). course, tandem mass spectrometry will permit the recording of CID spectra on individual components from such mixtures with the simultaneous exclusion of much of the interfering signals from matrix ions which are of particular annoyance in interpretin the low mass portion of these spectra. The highenergy C b spectra obtained in the positive ion mode prowde information on the ceramide portion, whereas the negative ion s ctra provide more information regarding the sequence and Kanching of the oligosaccharide portion (GB). More recent studies of the CID spectral quality in glycosphingolipid characterization have reported use of a modification of the procedure for reduction of the permethylated derivatives employing a microscale procedure with diborane ((711). In addition, use of trideuterioborane permits localization of the olefinic bonds in these glycoli ids. As reviously established by Karlsson and co-workers 612),reiuction of the permethylated derivative improves the quality of the fragmentation observed. In connection with their studies of the hi h-energy CID s ectra of glycosphingolipids, Domon and costefio have pro& a systematic nomenclature for putative carbohydrate fragmentation processes, which is an adaptation of the widely used Fohlman and Roepstorff nomenclature for (714).It should be noted that use CID spectra peptides ((213, of the thin-layer chromatography plates for separation of small uantities of glycosphingolipids is becoming widespread with i! fferent overlay techniques for localization of biolo ically active components on the thin-layer chromatogram, or example, the binding of bacteria (G15)or labeled antibodies to specific com onents of the glycosphingolipid mixture of the TLC plate (%16). Virtually all of the mass spectrometric ionization methods have been investigated in connection with various types of glycolipids, and as one might expect, differential capabilities seem to exist, depending on the ionization method, as well as the ion optics and possibilities for collision-induced dissociation at low or high energy. All of these methods are illustrated in various apphcations throughout the remaining portion of this section. Laser desorption mass spectrometry was used to detect and characterize three different forms of monophosphoryl lipid A obtained from the li polysaccharide of Rhodopseudomonas sphaeroides (Gl7).&e forms differed only by the presence of absence of unsaturation and a keto group in the fatty acids. Identification of a lipid A precursor at the site of incorporation of 3-deoxy-~-manno-&ulosonatein Pseudomonas aeruginosa was identified by negative ion liquid SIMS ((218).The identification of N-glycolylneuraminic acid containing gangliosides isolated from cat and sheep erythrocytes was carried out by plasma desorption time-of-flight mass spectrometry
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on the methylated derivatives (G19). PDMS was also ap lied to the characterization of species and type-specific glycoyipid antigens from mycobacteria, includin the trehalose-containin ,acylated lipooligosaccharides, t e C-mycoside glycopepticfolipids, and the henolic glycolipids (G20). Laser desor tion mass spectra laberosides isolated from the giant cocLoach show fra mentation that is useful in the characterization of the lipif portion of these novel glycolipids (G21). The presence of a diglycosylated henolic lycolipid has been characterized in Mycobacterium ovis BC usin desorption chemical ionization methods (G22). a-Gatactose (afucose)-asialo-GMlglycolipid has been identified in subsets of rat dorsal root ganglion neurons by using methylation analysis and mass spectrometry of the native anti ens and permethylated anti ens by liquid SIMS (G23). It fks been shown that inducecftumor tissue from subcutaneous transplantation of PC 12h pheochromocytoma cells accumulates four minor neutral gl colipids in the globo series (G24). The antibody ACFH-18 {as been shown to recognize a fucosyl residue lus an internal repeating N-acetyllachamine moiety proximi to ceramide novel tumor-associated developmentally lycoli id antigen (G25). Four major neutral glycosphingobpids Eave been identified in the liver of the English sole Parophrys vetulus (G26). Subsequently, the acidic fraction was shown to contain a novel an lioside with a Forssman antigen determinant (G27). In &der work on identification of monoclonal antibody antigens, the mouse IgM monoclonal antibody NUH2 was shown to bind to a-series of disialogangliosideswith bin 2 3 sialola e stru-es that are oncodevelopmenzy regulated an=GB). In studies of human placenta gangliosides, it was observed that considerable improvement in the molecular ion yield ether with reduction in chemical noise was observed by use o the cyclic polyether 15-crown-5, together with a gly$erol/thiogl cero1 matrix (G29). It was also suggested that nitrobenzyl dcohol and 15-crown-5may be an alternate useful matrix for studies of permethylated glycosphingolipids at higher mass. These studies permitted Characterization of novel tri- and tetrasialo poly-N-acetyllactosamyl gangliosides (G30). Negative ion secondary ion mass spectrometry has been used to characterize monosulfated globopentaosylceramide from human kidney (G31). In further studies of monoclonal antibody recognition, the mouse monoclonal antibody AA4 has been shown to bind to a novel a-qalactosyl derivative of ganglioside GD~,,.This antibody also inhibits binding of IgE to hi h affinity receptors on the rat basophilic leukemia cells (G327. This work represents an interesting example of combined use of TLC separation, overla techniques, and mass spectrometry in characterizin the g ycolipid antigen. Very recently, Brennan and co-wor ers provided an elegant overview of the applications of mass spectrometric methods to the characterization of lycolipid antigens of m cobacteria (G33). A variety of methds including liquid SIN& mass spectrometry have been used to postulate a tentative overall structure for the group B antigen of Streptococcus (G34). The structure and heterogeneity of oligosaccharides from the lipooligosaccharides of a pyocin-resistant Neisseria gonorrhoeae have recently been re rted (G35),where advantage has been taken information to assess the structural nature of h h-energy of t f e components in mixtures of these closely related structures. It has been shown that signal-to-noise may be considerably improved in studies of oligosaccharides b the use of multichannel array detection system on a doub e-focueing mass spectrometer (G35). The structures of the re-
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The use of tandem mass spectrometry has been explored for direct stereochemical assignment of mono- and disaccharide subunits of larger lycosides usin low-energy collision-induced dissociation ( 8 4 1 ) . This has t e e n used to identify the dimannosylthreonine on recombinant IGF al. It has also been applied to studies of the antibiotic papulacandin B (G42) and a novel oviposition-deterring pheromone pro-
duced by the female European cherry fruit fly Rhagoletis cerasi L (G43). Further details on the identification of interglycosidiclink es in disaccharide subunits by this method have been r e p o d (G44). It has been shown that collisionally activated decomposition of molecular anions of isomeric glycocon'ugates can be distinguished in the negative ion chemic2 ionization tandem mass spectrometry experiment (G45). Collision-induceddissociation studies have been carried out on three synthetic lin e isomeric trisaccharidesto assess whether ion patterns cou d distinguish among the three possible different linkage positions of the terminal fucose in the otherwise identical structures. Molecular modeling was also carried out to support the suggestion that steric factors and the position of fucose linkage relative to GlcNAc contribute to bond stability during collisional activation (G46). Protein Glycosylation. Protein glycosylation comprises a challenging, complex array of structures in three protein linkage classes: (1)to asparagine; (2) serine and threonine; (3) protein C-terminal amino acid by ethanolamine. The wded N-linked, 0-linked, and glycophosphoinositolanchors represent sufficiently different analytical challenges that presently, three classes of strategies are being employed to investigate the nature and extent of structural variations, with the aim of delineating which exact structural isomers occur at which specific sites. This information is necessary to proceed with more biological questions of structure-function relationships since, due to the heterogeneity, the chemical notion of rigorous structure is in many cases still unknown. This study of many biolo ical effects have been attempted with what might be consifered weighted average structures, and this is clearly an undesirable state of affairs. This area of research is replete with opportunity and challenge in the most global sense. Finally, the field has been plagued by the lack of highresolution chromatographic methods for unambiguous se aration of structural, hkage, and stereochemical isomers. &is has begun to change with the introduction of polyanionic chromatographic methods over the past couple of years, but it will remain to be seen as to how far that is able to advance the understandin of mixtures of closely related oligosaccharides (G47-849). There are a number of book cha ten of direct relevance to this topic. They include metho s for determinin monosaccharide composition ( G I , G50) and linkage anafysis (GI, G51-G52), derivatization methods for se uence and branching studies of intact oli osaccharides, iniuding permethylation and peracetylation fA6, B21, B27, G53, G54), reductive amination with an appropriate hydrophobic chromophore (G55),and high-energy tandem mass spectrometry for structural studies (G56, G57). Characterization of a library of neo 1 colipids from N-glycosylated proteins has been reported ?&8). Structural analysis of free sialooligosaccharidesfound in the unfertilized eggs of freshwater trout Plecoglossus altiuelis represent typical types of bi-, tri-, and tetraantennary structures with the free chitobiose reducin terminus intact. This would imply that an enzyme with PkjGase F-like activity is responsible for liberatin these structures from glycoproteins during oogenesis (G597 Carr and co-workers have pioneered the use of PNGase F for liberation of glycosylation at asparagine linkage sites for the identification of carbohydrate moieties attached to a specific site (B31, G60). Carr has used this method on a variety of examples, most recently recombinant tissue plasminogen activator (G61). Further studies on the oligosaccharides expressed on recombinant soluble CD4 reveal the presence of 14 structures liberated by hydrazinolysis and sialidase treatment of the glycopeptide containin two Nlycosylation sites, Asp 271 and Asp 300 (G62). following 8arrk procedure, it has been shown that Asp, 143 of the asubunit of the nicotinic acetylcholine receptor is glycosylated with a high mannose oligosaccharide. The N-linked glycopeptide in this case was too hydrophilic to be observed in a mixture of liquid SIMS molecular ions dominated by other peptides, and upon treatment of that fraction with PNGase F, the aspartyl analogue of the aspar ine-linked peptide was observed. The ne ative ion liquid SI& spectrum of this high mannose oligosacc?mide was recorded as the ABEE derivative using a 1024-channel electrooptical multichannel array detection system (F90).A significant increase in sensitivity over postacceleration detection was demonstrated, which makes possible high-sensitivity studies for components such as these
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derivatives of N-linked oli osaccharides that fragment well in the negative ion mode (663). Further studies have been carried out on the effect of enhancing the hydrophobicity of the aromatic amine used in reductive cou ling to the reducing terminus of N-linked oli osaccharides d e r a t e d by PNGase F. This work has estabfished that significant increases in sensitivity will be obtained in going from the methyl to the octa- or dec leater (G55, G64). A pTeliminary report on these results for t i e extended alkyl p-armnobenzoic acid esters has been presented (C65), as well as analo ous use of tetradecylanaline, showing that high-energy C b spectra may be recorded at the 10-pmol level with oligosaccharides by using this derivative (G66). In a further a lication of the use of these high-quality s ectra obtained)Prom negative ion li uid SIMS analysis of tge alkyl p-aminobenzoates couple8 to N-linked oligosaccharide-reducingtermini, Gillece-Castro and Burlingame have reinvestigated the structure and branching of the Man GlcNAcz core oligosaccharide of the mnn mutant of Saccharomyces cerevisiae, in collaboration with Ballou (G67). This led to the surprising result that addition of the outer chain previously linked a1 6 to the 1 6 arm is actually attached to the a1 3 linked mannose on the a1 3 arm. The new structure has recently been reported ((267). Liquid SIMS has been applied to sequence and structural anal sis of high1 sulfated heparin-derived polyoligosacciarides (G68-670), as well as to sulfated N-linked carbohydrate chains on porcine thyroglobulin (G71). A series of reports on the characterization of immunogenic e itopes carried on mucin glycoproteins has been reporte (C54, G71-G75). These studies have defined that the second generation monoclonal antibod (CC49) definea the mucin-Carried carbohydrate epitope GalS6-3) [NeuAca(2-6)]GalNAc (G72) and that a novel type of linearly extended poly-N-acetyllactosamine backbone with G a ( 1 - 4 ) GlcNAcP(1-6) re ting units occurs on human skim milk mucins (G73, G74). r k l a r studies on monoclonal antibody FDC-6 r ition define the oncofetal structure of human f i b r o n e c t 3 7 6 ) . In further studies of the posttranslational modification of factor VI1 (F78), it was recently reported that a disaccharjde and a trisaccharide are 0-gl cosidically linked to a serine residue in the first epiderma growth factor-like domain of human factors VI1 and IX, as well as protein Z and bovine protein Z (G77). These are rather curious di- and trisaccharides with
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h- rformance liquid chromatography/massspectrometry $fPI%/MS) , supercritical fluid chromatography/mass spectrometry (SFC/MS), and tandem mass spectrometry (MS/MS). Since the last review period a number of useful books, proceedings of symposia, and other reviews have appeared. Two comprehensive reviews by Harve (HI, B39) on a plications of mass spectrometry in the stu y of drug metaboLrn, pharmacokinetics, and toxicology have been published that are part of a continuing series from the Royal Society of Chemistry (Specialist Periodical Reports). The first of these covers the period of July 1984 to June 1986 and the second July 1986 to June 1988. Applications of fast atom bombardment (FAB)and thermospray (TSP) mass spectrometry to drug metabolism over the period 1981 to mid-1987 have been reviewed by Straub (H2). A short review is available by Sullivan and Franklin (H3) on the use of stable isotopes in combination with liquid chromatography and mass spectrometry in drug and xenobiotic metabolism studies. Other applications of stable isotopes are presented in the volume edited by Baillie and Jones (B37) taken from the Thjrd International Symposium on the Synthesis and Applications of IsotoDicallv Labelled ComDounds 1988 held in Innsbruck, Austiia. Reviews on the ab lication of s ecific techni ues such as HPLC/MS (H4, H57, SFC, and Sf!FC/MS (H6,%7) have appeared along with compendia of papers taken from the Seventh International Symposium on Mass Spectrometry in Life Sciences (B40),and from the 5th Symposium on LCMS, SFC-MS and MS-MS (B41).Reviews on the mass and the spectrometry of methylthio drug metabolites (H8) study of trace level residues in food and drugs by mass HIO)are available. spectrometry (H9, Specific pa rs on the application of mass spectrometry in drug metabogm and pharmacology can be broadly divided into two categories. The first of these involves the use of mass spectrometry in combination with other techniques (NMR, for example) for structure elucidation of individually isolated and purified major metabolites. Introduction of samples to the mass spectrometer in these studies is usually accomplished by a batch inlet system, althou h combined chromatography systems are occasionally emp oyed for convenience. The second category involves the prim use of combined techniques such as as chromato r a 3 mass spectrometry (GC/MS), HPLC$MS, SFC/M\, anikIS/MS for identification and/or measurement of metabolite rmxtures (major and minor) in extracts of biological matrices. Electron ionization mass spectrometry (EIMS) with direct insertion robe (DIP) sample introduction has been em loyed in a numger of studies including, for example, the i entification of in vitro metabolites of lovastatin isolated from rat the identiand human liver microsome preparations (HII), fication of stable metabolites of mianserin also derived from human microsome preparations (HI2), studies on the metabolites of the antiarrhythmic drug encainide (HI3),and the identification of the major metabolites of ethacizin (a new N-acylphenothiazine drug with antiarrhythmic and an properties) isolated from human urine b HPLC ( 14) Because EIMS often fails to produce moLcular ions and therefore molecular weight information, it is often complemented by the use of positive chemical ionization (PCIMS) with DIP sample introduction in, for example, the identification of the major metabolites of alfentanil from hepatocyte cultures (HI5), studies of the in vitro metabolism of the cardiotonic steroids gomphogenin and calactin (HI6),a study of the brotransformationof 5-(2-~hloroethyl)-2’-deoxyuridine in male NMRI mice (HI7), and investigations of alfentanil WUand pharmacokinetics and metabolism in humans (H18). co-workers (2319) found the desor tion chemical ionization (DCI) probe to be useful for EIMZand CIMS sample introduction in their study of the metabolism of bepridil in laboratory animals and humans. Increased structural information on the in vitro metabolites of 3-ethyl-2,6-dimethyl-4Hyrido[l,2-a]pyrimidinn-40ne was obtained by EI and MS/MS y! Jemnitz et al. (H20).Lee and Ypst (H21) have re orted a comprehensive study on the utility of EIMS/& and CIMS/MS with DIP sample introduction and various scan modes for the rapid identification of drug metabolites. An example is given of the identification of isomeric metabolites of the new antie ileptic drug, zonisamide, in which complementary neutrayloss and parent scans and differences in
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reported. Analysis of the phosphatidylinositol-specific phospholipase C resistant inositol phospholipid revealed further substitution of an inositol hydroxyl group with a palmitoyl moiety (C79). Some studies have been reported on the analysis of N-linked glycopeptides by mass spectrometry, including those from ovomucoid and asialofetuin (G80),natural murine interferon-@ (G81),and novel pol cos lated N-linked glycopeptides with blood group A, H, and $ determinants from human small intestinal epithelial cells (G82). Plasma desorption mass spectrometry has been used to reveal the lycosylation site heterogeneity in glycopeptides from each of &e three N-linked glycosylation sites of bovine fetuin (G83) and the glycopeptide 432 of the variant surface glycoprotein of trypanosomes
H. DRUG METABOLISM AND PHARMACOLOGY Over the past two years, mass spectrometry has played an increasin role in the study of drug metabolism and in the field of ptarmacology. This is a arent not only from the number of citations in the availaEg scientific literature but also from the increasing investment that the pharmaceutical industry has made in mass spectrometry facilities and instrumentation. The latter observation probably means that the actual scope of mass spectrometric application in these fields is underestimated by a simple review of scientific literature. This is especially true re arding the application of the newer and more innovative tectniques such as combined 2821
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vaporization time off the probe are employed. EIMS and CIMS, being gas-phase techniques, have the disadvantage that sample volatilization must be accomplished before ionization can occur. Fast atom bombardment (FAB) and liquid secondary ion mass spectrometry (LSIMS) are surface ionization processes and do not require sample heating and volatilization prior to ionization. Direct robe FAB and LSIMS are therefore useful in the structur8elucidation of thermally unstable, olar, and higher molecular weight drug metabolites. Metagolic conju ates such as lucuronides, into these sulfates, phosphates, and glutat%one adducts categories. Since FAB and LSIMS are relatively "soft" ionization processes, protonated molecular ions (MH+)and/or molecular adduct ions often result providing molecular we' ht information. Exam les of the a plication of FAB and LS&S in combination wit! DIP EIM8 include the identification of metabolites of the antiinflammatory selenoorganic drug ebstudies on novel selen from isolated perfused rat liver (H22), oxidation products of tenoxicam (H23),and a mass spectrometric investigation of bisbenzylisoquinoline alkaloids (2324) that also employed MS/MS. FAB and LSIMS have also been utilized to study lucuronides of hydroxylated metabolites of amitriptyline ani nortriptyline isolated from rat bile (H25), the formation of conjugates durin the metabolism of 5fluorouracil and 5-fluoro-2-deoxyuri ine in isolated perfused rat liver (H26),and in studies of 5-aminosalic lic acid (H27), the investi ational drug sertraline (H28), a n i t h e antibiotic complex o f retamycins (H29). Several particularly interesting studies employing FAB for the study of isolated glutathione conju ates have appeared. Haroldsen et al. (H30) utilized FAB/hS/MS on a hybrid sector/quadrupole instrument to study the collisionally activated decom osition of MH+ ions from some glutathione conjugates. *hey observed abundant structurally useful entation processes from various speciH studied and were ab e to employ their techniques to identify an acetaminophen-glutathione con'u ate in rat bile. Pearson et al. (H31) employed F A B / M S / a to study derivatized glutathione conjugates of the antineoplastic agent N-methylformamide. They were able to obtain structurally useful daughter ion s ectra from the MH+ ions of S-(N-methylcarbamoy1)gluta&one and a series of its alkoxycarbonyl methyl esters. Other reports in this area include the isolation and characterization of glutathione adducts of elliptinium (H32) and morphine (H33, H34). Field desorption mass spectrometry (FDMS) continues to maintain a niche in drug metabolite identification studies because of ita capability to produce molecular weight information from difficult to volatdh structures. Koch et al. (H3.5) employed FDMS in conjunction with DIP EIMS to identify metabolites of the muscle relaxant drug sirdalud (Bchloro4-(2-imidazolin-2-ylamino)-2,1,3-benzothiadiazoleh dro chloride). They were able to determine that a novel oxiiativi biotransformation pathway of the benzothiadiazole ring system hone anal e of that in the parent compound produced a SUI structure. Cocchiara et al. (H36) utiked FDMFagain in combination with EIMS) to study the metabolites of rifabutin, a new spiropiperidylrifamycin antimycobacterial agent. Metabolitee were isolated from the urine of human volunteers. Other applications of FDMS include a metabolic study of isobutylnaphthyl acetic acid (a new antiinflammatory agent) in rats (H37),a comprehensive study of the metabolism of a new positive inotropic agent (OPC-8212) in rat, mouse, dog, and a study of glutathione adducts monkey, and human (H38), of acetaminophen (H39). Thermospray (TSP)is a powerful, yet clear1 underutilized, ionization proceas in dnq metabolism studies. b e d as a direct liquid inlet or in combination with HPLC, TSP offers the potential for direct determination of olar, involatile, and thermally labile molecules (H40).$any drugs and drug metabolites fall into one or more of these categories. Like CI, FAB, and LSIMS, TSP is a relatively "softn ionization process that forms mainly MH+ or [M - HI- ions that in many cases exhibit little fragmentation. To circumvent this "problem", tandem mass spectrometry can be employed with TSP ionization to induce fragmentation by collision processes and increase the information content of the mass spectrum. Draper et al. (H40)used TSP/MS/MS in both positive and negative ion modes to study polar urinary metabolites and metabolic conjugates, such as glucuronides, sulfuric acid
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conjugates, and alkyl phos hates. The concluded that MS/MS greatly enhances tRe power of &P by roviding fragmentation information that is useful both in &e identification of unknowns and for improved s ecificity. Straub used TSP in conjunction wit\ FAB to provide et al. (H41) evidence for the formation of novel amine conjugates of studied isolated glucuronide benzaze ine. Lay et al. (H42) metaboEtes of methapyrilene with TSP/MS and FAB. employed a battery of mass Bernstein and Franklin (H43) spectrometr techniques includink EIMS, FDMS, FAB, and T S P - L C / d t o study the metabobm of the cardiotonic agent 14C-indolidan. In an extremely interesting series of experiinvestigated the formation of the ments Getek et al. (H44) acetaminophen-glutathione and -c steine conjugates in an electrochemical cell with on-line T i p ionization. They determined that formation of con'ugates occurred only after acetaminophen was oxidized e/ectrochemically by a twoand reacted electron transfer to N-acetyl-p-benzoquinonimine in a mixing tee with glutathione or cysteine. Gas chromatography combined with either EIMS, PCIMS, or negative chemical ionization (NCIMS) continues to be the most widely utilized combination of high-efficiencyseparation and mass spectrometric detection for drugs and drug metabolites. Any chemical com ound that can pass through a gas chromatograph without &composition or can be chemically derivatized to do so is amenable to GC/MS analysis. This and the requirement that analytes be extracted from their biological matrix into a volatile solvent limit the range of chemical structures to which GC/MS can be applied. GC MS allows the separation of complex mixtures of drugs and rug metabolites and their individual detection. Numerous studies have been reported over the past two years in which GC/MS techniques were employed in the identification of new metabolites. These include metabolites of bucannabinoids (H46-H52), phenelzine (H53), sulfan (H45), (-)-deprenyl (H54), methaphenilene and pyribenzamine (H55), plaunotol (a new antiulcer isoprenoid) (H56), trimi ramine (H57), sodium valproate (H58),doxylamine (H59),Jltiazem (H60), hexamethylene bisacetamide (H61, H62),fenetylline (H63), verapamil (H64), the investigational anticancer a ent chlo romazine and romazine (#66), spirogermanium (H65), N,N'-diallylpentobarbitol(%7), the thromEoxane A2 antagonist (f)-5-(Z)-7-(3-endo-phenylsulfonylaminobicyclo[2.2.1]hept-2-exo-yl)heptenoicacid (Ha), and isoxicam (H69). The necessity for chemical derivatization of many dru metabolites for GC/MS analysis has already been mentionel In a study of the metabolism of the tricyclic antidepressant trimi ramine in man Maurer (H70) was able to identif 15 meta&litee in patient urine by GC/EIMS. A number of dese metabolites contained hydroxy groups and were con'u ated in the urine (as glucuronides, for example). To allow &&MS analysis of these structures, it was necessary to cleave the conju ates, extract into a volatile solvent, and derivatize by acet ktion. Trimethylsilylation and methylation were used to i&ntify two metabolites of the uricosuric drug benzbromarone in human plasma (H71). Deconjugation of glucuronides and aryl sulfates with trimethylsilylation was also required in an investigation of the metabolism of the antianxiety drug buspirone in the rat (H72). The in vivo biliary and urinar metabolites of (A)-N-methyl-N-(l-methyl-3,3-diphenylpro ) formamide from male Wistar rats have been characterize8iy Slatter et al. (H73). They found that most of the metabolite carbinolamide liberated by @-glucuronidasetreatment of urine decomposed in the gas chromatograph to N-(1-methyl-3,3diphenylpropyl)formamide,unless stabilized as a trimethylsilyl derivative. Increased selectivity and sensitivity can be obtained if GC NCIMS or GC/MS/MS are employed. Kassahun et al. (H4 ) identified 15 metabolites of valproic acid in human serum, urine, and saliva usin GC/NCIMS of their pentafluorobenzyl derivatives. Wi% this method they were able to identify two previously unknown metabolites. EIMS/MS was employed by Dumasia and Houghton (H7 ) to characterize a dihydrodiol metabolite of propranolol excreted in horse urine after administration of a single dose to thoroughbred racehorses. A triple-stage quadrupole mass spectrometer and trimethylsilyl derivatives were utilized. GC(MS techniques are also capable of providing accurate and highly sensitive quantitative information for drugs and their metabolites. This is particularly useful in pharmaco-
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kinetic studies in whiFh lar I! databases of such quantitative information are requlred. 8rtori et al. (H76) have reported an investigation of the kinetics of unchanged aminoglutethimide and its major metabolite, N-acetylaminoglutethimide, in human sub’& with a new multiple selected ion monitoring (SIM) GC/E!MS technique. The report that this method gave kinetic findin s similar to d o s e provided by a more time-consuming H P f C method and allowed the detection of the N-acetylamino metabolite in plasma a t longer time intervals. Studies on the oxidation of halothane b he atic microsomes have been reported that employed GC/l$IMffand SIM for measurement of trifluoroaceticacid production (H77, H78). The trifluoroacetic acid was analyzed as its methyl ester after headspace s a m ling. A new sensitive and specific GC/EIMS assay has &o been re rted for the quantification of monoh droxy metabolites of pKncyclidine from biological samples yH79). GC/NCIMS has also been utilized for development of new quantitative methods. NCIMS is extremely sensitive, especially when enhanced by the formation of appropriate chemical derivatives from specific analytes. Kawabata has reported a method which combines GC/NCIMS with a rapid purification on sephadex G-10 for the measurement of trace amounts of phenylacetic acid as a metabolite of phenylethylamine in rat brain (H80). The tnethod utilizes the pentafluorobenzyl ester together with heptadeuterated phenylacetic acid as an internal standard. The GC/NCIMS method of Murray et al. (2381) for the histamine metabolite N-methylimidazoleacetic acid uses the isopropyl ester 3,5bistrifluoromethylbenzoyl derivative. This derivative has very ood chromatographic properties,. and the electron-capture kC1 spectrum contained only molecular ion. A deuterated internal standard was also employed, and a 1-pg detection limit was obtained from urine and plasma. Funke et al. (H82) have reported a GC NCIMS method for pravastatin and its major metabolites t at uses derivatization with both entafluorobenzyl bromide and N,O-bis(trimethylsily1)trihoroacetamide. As described, earlier, thermospray ionization (TSP) is a powerful technique for the identification of thermally unstable, olar, and hi her molecular weight drugs and their metaboespecialf when combined with the separating capability of HPLC. Aythou h not employed to the same extent as GC/MS, TSP L C / b S is playing an increasing role in dru metabolism studies. Lindberg et al. (H83)used TSP LC/M8 to study the oxidative metabolism of (R,S)-bambuterol in rat liver microsomes. With use of deuterium-labeled and unlabeled bambuterol, six metabolites formed via hydroxylation, demeth lation, and hydrolytic reactions were identified. A TSP L ~ M method S for the quantificationof the antimalarial arteether and six of its metabolites was reported by Baker (H84). TSP spectra of these analytes exhibited intense [M + NH4]+ ions, which are commonly observed when ammonium acetate buffer is employed in the thermospray process. Rashed and Nelson (H85) investigated the reactive metabolites of 3‘-hydroxyacetanilide,which is a nonhepatotoxic regioisomer of acetaminophen. TSP LC/MS was employed to study reaction products of these metabolites with cysteine or N-acetylcysteine. It was re orted that the TSP spectra of the mono- and bisthioether afducts showed protonated molecular ions and structurall useful fragmentation (which is not always the case with TJP ionization). The combination of chromatogra hic separation and TSP ionization allowed the unequivoca identification of reaction products in urine of mice. Metabolites and reaction products of antibiotics and anticancer dru s have also been investigated by TSP LC/MS. Suwanrumpf a and Freas (H86) identified two previously unknown components in human urine which were demonstrated to be metabolites of ampicillin. Using TSP LC/MS techni ues, the confirmed that these components were the (3S,5Rcf and (345s) epimers of ampicillin penilloic acid. It was found that only one epimer was produced in vivo, the other being the result of interconversion in the urine sample. The interaction of the anticancer drugs mitomycin C, porfiromycin, and thiote with calf thymus DNA was investigated by Musser et al. E87). It was demonstrated that TSP LC/MS was of utility in the identification of nucleoside adducts and depurinated base adducts of these anticancer drugs.
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Mitom cin C metabolites were also investigated by Heeremans et al. (3;88) using a number of ionization processes including TSP. TSP LC/MS has also been utilized to develop a screening method for clenbuterol in bovine urine (H89), to study the pharmacokinetics and metabolism of a 4’-methylthio derivato quantify tracer levels of tive of propranolol in dog (HM), isotopically labeled carbamazepine epoxide in the presence of a steady-state level of the anticonvulsant carbamazepine and to determine moricizine and its epoxide metabolite (H91), The overall utility of TSP LC MS in human plasma (H92). for the structural identification of drug metabolites has been summarized by Beattie and Blake (H93). They conclude that the introduction of TSP ionization has made the acquisition of LC/MS data relatively easy and routine, eliminating the need for rigorous isolation and purification of individual metabolites and allowing the determination of minor metabolites in the presence of major ones. As mentioned in the previous discussion TSP spectra often exhibit little, if any, fragmentation of the MH+ or other molecular adduct ions initially formed. Tandem mass s ctrometry is, therefore, often employed with TSP LC/MGor identification of unknowns. Vajta et al. (H94) utilized TSP LC/MS/MS for the identification of zolpidem metabolites in rat urine. They were able to obtain successful results by direct injection of concentrated urine with the observation, however, of a suspected thermolytic cleavage from a hyused droxymethyl amide metabolite. Blake and Beattie (H95) TSP LC/MS/MS in identifying the sites of metabolic transformation of SKF 95448 by rat hepatocytes in vitro and by rats in vivo. These same workers also reported studies on the analysis and characterization of isomeric metabolites of and for the identemelastine (a novel HIantagonist) (H96) tification of cynomolgus monkey and human metabolites of SKF 101468 which is a dopamine D receptor a onist (H97). Walhagen et al. (H98)demonstrate4 that TSP &C/MS/MS utility could be further increased by independent optimization of the chromatographic system. Peak compression allowed the detection of 61 ng/mL of metoprolol enantiomers in used TSP LC/MS/MS and plasma samples. Lan et al. (H99) other mass spectrometric techniques to identify numerous metabolites of tripredane and elucidate three major brotransformation pathways for this substance in liver homogenates. A number of other HPLC/MS interfaces/ionization processes are available, and some have seen recent application in drug metabolism studies. The direct liquid introduction (DLI) interface has been utilized to study highly thermall It was reported that wit{ labile rifamycin antibiotics (H100). use of n ative ionization, abundant molecular ion species were observe? and that the spectra also contained structurally significant fragment ions. DLI LC/MS was also employed to analyze the glucuronide of N-(I-hydroxyeth 1)flurazepam (H102). With this method the glucuronide couldfbe quantified in the urine of human subjects after a single intravenous administration of 50 mg of parent drug. The continuous-flow (CF) FAB technique is one of the newest HPLC/MS combinations. As yet, only a few applications to drug analysis have been reported, such as the study of dextromethorphan in plasma by Kokkonen et al. (H102). Preliminary data have been re orted by Caprioli and coworkers (H103) utilizing CF-FA[ LC/MS which may represent a new horizon in the application of mass spectrometry to pharmacology, the direct in vivo measurement of drugs and their metabolites. In this study the ability of microdialysis and on-line CF-FAB LC/MS to rovide in vivo pharmacokinetic data over prolonged p e r i d of time for the evaluation of multiple-dose antibiotic thera y is assessed. The microdialysis device (Carnegi Medicin7 was inserted inside a 22gauge needle cannula that was surgically placed in the carotid vein of a male rabbit. The device was then coupled directly to the CF-FAB probe of a triple-stage quadrupole mass spectrometer and was used to follow the multiple-dose pharmacokinetics of the antibiotic piperacillin in live rabbits. This innovative analytical technique was demonstrated to be highly effective for this application. It should also be possible given the flow requirements of the microdialysis device to interface it with other LC MS techniques such as DLI and electrospray ionization. odifications could also be made to the system for interface to thermospray ionization sources.
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The mass spectrometer is an isotope se arating device, and many workers in the drug metabolism feld have taken advantage of this by utilizing stable isotope labeled analogues of drugs in their experimental design. Appropriately placed labels will appear in metabolites, making the identification of these structures much easier. The use of stable labels and mass spectrometry also precludes the need for radiolabeled com ounds, which are often inappropriate for in vivo human stuges. As noted earlier, a recent volume has a peared that presents a number of such applications (B37). Tiese include, for example, a study of the use of deuterium labeling in mechanistic studies of the metabolic chiral inversion of ibuH105),studies on the profen in adult male volunteers (H104, and studies on the metabolism of carbamazepine (H106), pharmacokinetics of stable isotope labeled phenytoin (H107). Other studies have also appeared that utilize stable isotopes investigated and mass spectrometry. Kang and Hong (H108) the in vitro metabolism of tranylcypromine in rat liver microsomes using the unlabeled drug and two deuterium labeled analogues. GC MS techniques were employed to identify labeled metaboiites. Baty et al. (H109) administered acetanilide with the three hydrogens of the acetyl group replaced by deuterium to human subjects and male Sprague-Dawley rats. Capillary GC/MS was used to determine if the major metabolite ( aracetamol) was deuterated or a mixture of deuterated ppus undeuterated structures produced by deacetylation followed b reacetylation. The deuterated metabolite was producedlat hi her levels in both man and rat. This same group further setermined that nonenzymatic acetyl-group exchange of deuterated compounds did not occur; however, exchange did occur in vitro when incubated with human or rat whole blood (HIIO). The remainder of reported studies with stable isotopes also utilize GC MS as the rincipal analytical technique. These include a IM GC(E1hS method for deuterated ritanserin for the study of this drug's pharmacokinetics (H111), an investigation of the metabolism of trimebutine by the simultaneous administration of unlabeled parent drug and deuterium-labeled metabolite (H112), a GC/MS method for the determination of [13C, 16N2]-labeledand unlabeled theoinvestigations of the metabolism of the anphylline (H113), tianxiety drug bus irone in humans using l4C/lSN-1abeled an! the development of a novel stable isotope material (H114), technique for investigating the metabolism of ketamine (H115).Other reports of note include investigations of the metabolism of 2-propylpentanal acetals to valproic acid (H116), the deimidazolation of antimycotic croconazole (H117), and the biotransformation of valproic acid to the hepatotoxic olefin 2-n- ro lpentenoic acid usin the enantiomers (R)and (S)-[&-@A] as metabolic proies (H118). Chace and Abramson (HI191 have employed a ca illary GC chemical reaction interface mass spectrometq (CWIMS) tecl! nique to selectively detect 13C-, l6N-, and H-labeled phenytoin and its labeled metabolites in urine. The CRIMS 18 microwave powered and converts eluting analytes into small molecules (C02, NO, H2) that can be detected with high sensitivity by mass spectrometry (EIMS). Since several instruments of this same eneral philosophy have recently become available, it is l i d , that more studies of this type will appear. It is antici ated that the next two ears will see a significant increase ine!t use of on-line HPLZ/MS and MS/MS techniques in drug metabolism studies in order to avoid the need for rigorous isolation and purification of each major metabolite and to aid in the identification of minor metabolites. Thermospray ionization should continue to be the most widely employed technique, but significant gains will likely be made by CF-FAB and electrospray ionization. HPLC/MS/MS techniques should see increasin application in structural elucidation studies. Recent devefopments in the application of LC/MS techniques to in vivo drug studies are exciting and deserve close attention by all workers in the field. The use of stable isotopes in drug metabolism studies should continue to increase.
I. TOXICOLOGY Mass s ctrometry has been extensively utilized in the field of toxico&y over the past two years. Toxicologists have employed mass spectrometric techniques to elucidate the
molecular structures of toxic substances, to study the metabolism of xenobiotics and natural product toxins, and to develop sensitive and accurate methods for the analysis of drugs of abuse and other toxic substances in biological matrices. The number of citations to be found suggests that the role of mass spectrometry in toxicology is a vital and expanding one. Some of the most interesting structure elucidation reports that appeared involve marine neurotoxins and related natural products. With use of positive and negative ion FAB and MS/MS in combination with bioassay and other analytical techniques, domoic acid was found to be responsible for a serious outbreak of shellfish-relatedfood poisoning in Canada in late 1987. The chemical detective work in this episode is summarized by Quilliam and Wright (11). Domoic acid is a potent neurotoxin, resulting from its effect as a glutamate agonist. In this case, the domoic acid was determined to be a secondary metabolite of the diatom Nitzschia pungen forma multiseries, which was bioconcentrated by the shellfish. Thibault et al. (12) describe the FAB mass spectrometry of domoic acid and investigated an approach for the trace-level quantitative analysis of this substance by formation of the volatile tert-butyldimethylsilyl derivative. Quilliam and coworkers (13) applied the new technique of ion-spray mass spectrometry to domoic acid and two other marine neurotoxins,saxitoxin and tetrodotoxin."With this technique, which can be interfaced with HPLC, all three compounds gave positive ion spectra with si nificant MH+ ions and little gave a negative ion spectrum fragmentation. Domoic acid with a significant [M - HI- signal. Sensitivity was such that domoic acid could be determined in toxic shellfish tissue extracts. Additional studies on marine and other natural product toxins include the identification of tetrodotoxin in the frog Atelopus oxyrhynchus by GC/MS analysis of the basehydrolyzed trimethylsilyl derivative (141, the a plication of secondary ion mass spectrometry (SIMS),GC Mg, d esorption chemical ionization mass spectrometry (D I) thermospray mass spectrometry (TSP), and MS/MS to the analysis of anatoxin-a (E),the identification of the toxic tetramine from the marine astropod Neptunea antiqua utilizing FAB (16), a GC/CIM#study of the chemical composition of toxic skin secretions from trunkfish (17),characterization of neurotoxic cyanobacteria blooms with GC/MS techniques (18),and the use of FAB in the characterization of peptide toxins derived from cyanobacteria (19-111). Mycotoxins are another important class of natural product toxic substances amenable to characterization by mass spectrometry. Krishnamurthy and co-workers (112-115) have published extensively on macroc clic trichothecenes and related compounds, employin De1 with MS MS (112), and TSP HPLC/MS and TSP H!PLC/MS/MS 413-115). They report TSP to be effective at ionizing roridins and biologicall active baccharinoids, producing ammonia-molecular ion a 2 ducts in great abundance. Roach et al. (116) applied supercritical fluid chromatography/mass spectrometry (SFC MS) to the trichothecene m cotoxins, T-2 toxin, deoxyniva enol, and roridin A. NCIMB under electron capture, proton abstraction, and chloride attachment conditions was employed. These authors also report an evaluation of various SFC arameters such as capillary restrictor temperature. OtRer applications to mycotoxins include FAB/MS/MS analysis of crude aflatoxins and sterigmatocystin related compounds (117), a comparison of GC/MS and various MS/MS protocols for the analysis of trichothecene mycotoxins (I18),and the use of reactive collisions with ammonia for the specific characterization of trichothecenes (119). Mycotoxins have the potential to be utilized in chemical/biological warfare, and much of the work on these substances has been performed by researchers in defense-related establishments. D'Agostino et al. (120,121) reported on the analysis of another class of chemical warfare agents, mustards and related compounds. These workers developed a capillary column isobutane PCI GC/MS method to identify mustard and 17 mustard-related substances in soil samples from an area that was subjected to chemical attack during the Iran Iraq war. They were further able to characterize a n u n r of ether/thioether macrocycles and vinyl alcohols that had not been previously reported, as decomposition products of munitions-grade mustard (121). Rohrbaugh et al. (122)
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employed a GC/EIMS and PCIMS method to detect impurities and degradation products of 2-chloroethyl ethyl sulfide, a mustard simulant. A short review on the US. overnment's in-house program and other applications of ourier-transform ion cyclotron resonance mass spectrometry (FT-ICRMS) to chemical/ biological agents was written by Chou (123). A number of other mass spectral characterization studies on toxic substances have appeared and include a study of hilanthotoxins by LSIMS and linked scannin at constant g / E to generate structurd information (124),TS! HPLC MS investigations of diquaternary pyridinium oxime salts 125) and organophosphorus acids by Wills and Hulst (1261, and the structural elucidation of the A and B subunits of Shiga toxin and of Shiga-like toxin I employing FAB (127). The application of mass spectrometry to the identification and measurement of xenobiotic metabolites follows the trend of similar studies in drug metabolism in that GC MS with EI, PCI, and NCI processes is the most widely uti ized techni ue. A good example of this is found in studies involving popycyclic aromatic hydrocarbons (PAH), which are some of the most widely distributed xenobiotics in the environment. They are es ecially widespread in areas of concentrated industrial anzother human activity. Iwata and Maesato (128) studied the effect of oral administration of naphthalene on cataract formation in rabbit lens. The ap earance of naphthalene metabolites in the lens correspon&d with decreases in GSH level and GSH S-transferase activity and subsequent lens opacification. GC/MS was used to identify naphthalene metabolites of the mercapturic acid pathway. The biotransformation of the carcinogen 7,12-dimethylbenz[a]anthracene was studied by Myers and Flesher (129). With capillary GC/EIMS they were able to identify three hydroxyalkyl metabolites of this substance in the dorsal subcutaneous tissue of the rat. Other studies in which GC/MS techniques were employed include the identification of excreted metabolites of chrysene from rats (130),identification of 6,7-dimeth lquinoline and 6,8-dimethylquinoline metabolites from rainLw trout (131)and the characterizationof three metabolites of 3-methylcholanthrene in rat liver cytosol (132). Also available are a comprehensive mass spectral study of derivatized metabolites of benzo[a]pyrene (1331,a report on the characterization of nitro-PAH metabolites by direct-exposure probe NCIMS (1341, and a review of the mass spectrometry of methylthio metabolites of PAHs and other substances (135). Halo enated compounds, including certain pesticides and her bicicfes, are another important class of xenobiotics whose metabolism is amenable to study by mass spectrometry. GC/EIMS was utilized in two re orts on the metabolic fate of 3,3',4,4'-tetrachlorobiphenyl in g e mouse (1%)and rat (137). GC MS methods were also employed to study the de adation of I4DT by a model cytochrome P-450 system (138y Three degradation products were identified as conjugates of cysteine with a net loss of two or three of the five ori DDT chlorine atoms. The metabolism of the hexach or0 1,3-butadiene degradation product S(pentachlorobutadieny1)-L-cysteinewas investigated b using GCIEIMS to identify roducts formed (139). GC N&MS is useful in the study of ialo compounds because o its relatively high sensitivity for such structures. Using this method, Artigas and co-workers (140,141)were able to identify several metabolites of lindane in rat brain homogenates without extensive sample cleanup. A number of other studies have appeared on the metabolic fate of xenobiotic compounds (142-155). Several among these are of articular interest. Stable isotope infusion followed by GC/E€'MS analysis was used to follow the transformation of ethylene carbonate to ethylene glycol in male Fischer 344 rats (152). The observed rapid conversion tends to ex lain the similarity of toxicity profiles between these two sdstances. Webster and Anders (153) were able to understand the metabolism, and therefore the cytotoxic and nephrotoxic effects, of ~-thiomorpholine-3-carboxylic acid (TMC) by incubating this compound with rat kidney cytosol and adding sodium borodeuteride at the end of the incubation period. The GC/EIMS identification of deuterated TMC indicated the formation of an imine intermediate. TSP HPLC/MS was employed by Woodburn et al. (154) to study the photolysis of picloram in dilute aqueous solution and b Holme et al. (155)to identify a direct acting genotoxic metagolite from the
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food mutagen 2-amino-l-methyl-6-phenylimidazo[4,5-b]pyridine. The metabolic fate of toxins derived from natural product sources has also been investigated by mais spectrometry. For example, it is known that the major portion of mutagenic activity in fried meat is attributable to heterocyclic aminoimidazo compounds that form during the cookin process. Overvik et al. (156) investigated the importance o f creatine, amino acids, and water as precursors for these mutagens in cooked pork, employing HPLC/MS with a moving belt interface to identify muta enic compounds. The use of the moving belt allowed EIME to be utilized. HPLCICIMS MS, also with the moving belt interface, along with LS MS, GC/MS, and high-resolution EIMS were employed by Shigenaga et al. (157) to determine the metabolic fate of 8nicotyrine in rabbit lung and liver microsomal preparations. The HPLC/MS/MS experiments played a particularly important role in the identification of unstable metabolites. Positive ion FAB was used by Heur et al. (158) to study microsomal denitrosation of N-nitrosodimethylamine. As with xenobiotics, however, the bulk of studies on natural product toxin metabolism have employed GC/MS techniques. Examples include studies on the intoxicating beverage kava (159,ZM),nitroso compounds (161,1621,plant-derived natural products such as pulegone (163)and terpenoids (164),and T-2 toxin (165). The metabolic processes that operate on lipophilic xenobiotics in humans and other higher organisms are designed to solubilize these substances to assist in their elimination. These metabolic processes can be divided into two categories (166): phase I reactions which include oxidation, reduction, and hydrolysis; phase I1 processes which include conjugation and biosynthesis. Some of the most interesting and innovative work reported over the past two years in the field of toxicology has involved the use of mass spectrometry for the characterization and study of xenobiotic and other metabolic conjugates. Conjugates of xenobiotics and members of the glutathione (GSH) family including glutathione, cysteine, and N-acetylcysteine have been studied by several oups (also see previous references). Tomer and co-workers rave a lied FAB with tandem mass spectrometry (167,168)and !%P (169) to the study of these species. They determined that while TSP spectra could be obtained for all Compounds studied, molecular ions were often produced in relatively low abundance with extensive fragmentation being observed. A significant art of this fragmentation was hypothesized to be the resuE of thermal decomposition rather than unimolecular cleavage. FAB ionization resulted in the generation of molecular ion species under both positive and negative ion conditions. Collisional activation induced characteristic fragmentations in the GSH species with MS/MS analysis providing useful structural information. Conchillo et al. (170) reported that whereas glutathione adducts of potential metabolites of 3,4e xyprecocene I1 ave poor TSP sensitivity, the correspongg cysteine an% Nacetylcysteine derivatives gave much better sensitivity and interpretable fragmentation patterns. The methyl esters of these derivatives afforded particularly good sensitivity, It is likely that observed TSP sensitivity and degree of f r a r e n tation, more so than those from FAB, are affected y instrumental conditions, source design, and other factors unique to a particular analytical system. This fact should be considered when evaluating the conclusions of various authors regarding the utility of TSP for structures such as GSH conju ates. Other studies on GSH conjugates have been reportej (171-181). Notable among these are studies employin FAB to characterize GSH conjugates of propachlor formec! in soybeans (171), bisglutathionyl adducts of polyaromatic uinones (172),and GSH conjugates of polycyclic aromatic Iihydrodiols and insecticides (173). FAB and TSP ionization have also roven to be very useful for the analysis of glucoronides andp sulfates formed from xenobiotic species. Bank et al. (182) characterized the alltrans-retinoyl lucuronide formed in rat liver and kidney by They determined that exposure of rats to negative ion F ~. B tetrachlorodibenzo-p-dioxin(TCDD) caused increased retinoic acid glucuronidation. This result may partially explain the hepatic depletion of vitamin A and increased excretion of vitamin A metabolites that follow TCDD exposure. Takeuchi
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et al. (183)utilized LSIMS to identify quaternary ammonium glucuronide of the antifungal ent croconazole excreted by rabbits. Studies of sulfate andgucuronide metabolites of the food mutagen 2-amino-3-methylimidazo[4,5-flquinoline analyzed by various mass spectrometric techniques including TSP HPLC/MS have ap eared (184, 185). Also reported are studies on the identigcation of heme-derived products from bromotrichloromethane by FAB (186)and hemeglobin adducts from animals exposed to gasoline and diesel exhaust (187). The latter study employed a modified Edman degradation that specifically cleaves off alkylated N-terminal amino acids, followed by GC/MS analysis. Draper et al. (166)have reported a comprehensive study metry of a range of xenobiotic on the thermospray mass s glucuronides, sulfates, an phosphates. Using an instrument capable of filament-assisted TSP ionization, they demonstrated that positive ion spectra with molecular wei ht information could be obtained for glucuronides a n d some phosphates but not for sulfates. Negative ion spectra were roduced from all compounds studied. The combination of FSP with MS/MS was shown to provide structurally useful spectra for the identification of unknowns and for improved specificity. These workers correctly point out that certain xenobiotic conjugates, includin glucuronides, sulfates, and cysteine adducts (138,188) can {e analyzed by GC/MS after appropriate derivatization. Over 75 reports describing GC/MS methods for the identification and determination of tar et drugs (particularly abused drugs) and toxic substances in%iologicalmatrices have appeared over the past two years. These studies often report im roved sensitivity and specificity compared to previously puhshed methods owing to a novel extraction procedure, a different chemical derivative, or an alternative ionization process (NCIMS, for example). GC/MS methods have historically been favored as confirmation tests by drug/toxin analysis laboratories because of the robustness of the instrumentation and the ease with which simultaneous qualitative and quantitative information can be obtained. Two reports that give an overview of drug testing and underscore the important role that GC/MS has played are available (189, 190). A articular1 interesting series of reports that may portend the uture in irug testing have appeared in which hair is emplo ed as the matrix for extraction and screening by GC/&. Baumgartner et al. (191) state in a comprehensive overview article that hair is in many ways su erior to urine and blood as a sampling matrix. Hair can l e more easily collected under close supervision (avoiding embarassment) and is not subject to popular evasive maneuvers such as temporary abstention, excessive fluid intake, and substitution or adulteration of the sample. It is also not subject to false ositive results caused, for example, by poppy seed ingestion 1921, spikin of food or drink, and mixup or contamination of samples. Sepending on the length of the hair sample, a wide window of detection ranging from months to years is possible. It is also possible to assess exposure of an individual to environmental pollutants and other toxins by GC/MS analysis of hair extracts (193). Balabanova et al. (194) re orted the detection of cocaine by GC/MS analysis of shee air extracts up to 58 da after its administration at 2.5 The cocaine could be c&xted in hair samples 12 days3ter administration. These workers also reported the successful detection of cocaine in the hair of human cadavers following known cocaine abuse. In an additional report this group developed a method for the determination of benzoylecgonine in the hair of cocaine abusers (195). GC/MS analysis of the pentafluorobenzyl derivative allowed the detection of ap roximately 50 of the compound. Significant retrospective letection (1y e 3 of dru use from the analysis of hair segments was reported. Metho& for other abused dru s such as heroin and morphine (196)have appeared, a n d i t is likely that over the next review period a significant number of references on the analysis of hair extracts by GC/MS and other mass spectrometric techniques will be available. Althou h GC/MS remains the most widely employed analyticaftechnlque for target drug confirmation, it is not without certain disadvantages. First, the analyte must be in a form capable of passin through the gas chromatograph without decomposition. %his requires extraction from the
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biological matrix (urine, blood, hair) into a volatile organic solvent and, in many cases, chemical derivatization of the analyte. Second, the target analyte(s) must be reasonably well separated from interfering substances to allow for positive identification and accurate quantitation. Time-consuming capillary GC runs are often required to effect such se arations. To increase the speed of GC/MS confirmation of frugs and toxins, compromises are often made in either the extraction or chromatographic processes (197). Recently, a number of workers have investigated tandem mass spectrometry as an alternative to GC/MS to provide enhanced sample throughput while retaining accuracy of identification and determination. Phillips et al. (197)have reported an MS/MS method for the identification and quantitation of morphine in whole blood. The method employs a single-step liquid-li uid extraction procedure followed by pentafluoropropionicAydride derivatization. A short (5 m) capillary GC column was used as an inlet to the E1 source of a triple-stage quadrupole instrument. Two collision-induced molecular transitions were then followed in the multiple reaction monitoring (MRM) mode. The authors report linear calibrations for morphine concentrations ranging from 1.0 to 500 ng mL and detection limits of at least 1ng mL. The use of S/MS allows both the extraction and c romatographic processes to be comromised, resulting in a si f i a t reduction in analysis time. !‘he short GC column a l E s greater control of sample introduction and reduced matrix interference compared to direct insertion probe techniques (see the direct exposure probe for example). Many DIP systems method of Papa et al. (198), can be slow to heat and cool, and the constant insertion and withdrawal of the probe is hard on the seals and high vacuum system of the instrument. MS MS in combination with GC has also been employed in the ollowing reports: the analysis of T-2 and HT-2 toxins in whole blood employing MRM in a hybrid sector/quadru ole instrument (199,1100); the detection of residues of cdorinated phenols and phenoxy acid herbicides in the urine of school children using GC/ PCIMS MS (1101,1102). HPL /MS techniques are also being investigated as alternatives to GC/MS for target drug and toxin analysis. A comprehensive overview of this application has been written by Bowers (1103),who discusses the potential of all the major HPLC/MS interfaces, including the “particle beam separator” which can interface to an E1 or CI source without the mechanical problems associated with the moving belt interface. HPLC MS techniques have the advantage that labile and unstab e species can usually be analyzed without chemical derivatization, thus allowing compromises to be made in the extraction rocedure. Potential disadvantages for drug screening laioratories include high initial e ui ment costs, low sensitivity in many cases relative to GC/%dtechniques, and the lack of robustness of HPLC/MS systems. Recent applications include detection of the herbicide diuron and its metabolites in human plasma and urine employing the moving belt and EI/CI rocesses (11041, and the determination of various steroid &ugs and their metabolites in equine urine and plasma with the ion-spray technique and MS/MS (1105, 1106). It is reasonable to assume that the role of mass s ctrometry in the field of toxicology will continue to e x p a n r The next two years should see an increase in the use of “soft”ionization processes such as FAB and LSIMS for structural elucidation and the use of on-line HPLC/MS techniques. The introduction of new HPLC/MS interfaces, notably particle beam, electrospray, and continuous-flow FAB, to com lement the well-established thermospray technique shoula make this powerful combination more attractive to the anal ical toxicologist. The role of tandem mass spectrometry wi continue to expand, although whether MS/MS and HPLC/MS will replace GC/MS as the standard technique for target drug analysis is open to serious debate.
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J. EICOSANOIDS Mass spectrometry has been particularly successful in this field, being a requirement for identification of arachidonic acid metabolites since the pioneering work of Nobel laureate B. Samuelsson. During the past two years, a prolific literature on metabolic studies involving mass spectrometry has appeared. Owing to the limitations of space and the focus on new trends in mass spectrometry, much of this application ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
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literature has been intentional1 (although reluctantly) excluded from the review. Capiflary column GC MS with electron-capture negative chemml ionization W -I4CI) retains its standing as the method of choice for quantitative analysis requiring high sensitivity, but there has been a significant increase in the application of tandem mass spectrometry (MS/MS) and a corresponding drop in the use of LC(MS. The ap lications of mass spectrometry in prostaglandin research Rave been reviewed recently by Kelly ( J l ) . A comndium of 750 mass spectra of eicasanoids has been published y Pace-Asciak (B25). According to Garland, who reviewed the book (J2),this publication will be more useful to experienced researchers who use mass s ectrometry to study biotransformation of eicosanoids than &ow interested in their quantitative analysis, since no consideration was given to negative ion CI spectra. It is generally accepted that pentafluorobenz 1 estermethoxyamine-trimethylsilyl ether (PFB-MO-Td) derivatives of eicosanoids give the best results with GC/EC-NCI (methox amine is employed to derivatize eicosanoids containing Jdehydic or keto groups) since most of the ion current resides in the carboxylate anion [M - 1811- generated by dissociative electron ca ture. A ood example is the analysis of prostaglandins El, 82, F l a , #2a, and 6-oxo-Fla in cerebrospinal fluid (CSF) of humans and rhesus monkeys (53). The concentrations in healthy humans were below 15 pg mL. Murphy et al. reported a method for quantification o arachidonic acid itself as the PFB ester a t subpicomole concentrations ( J 4 ) . Comparative evaluation of GC/EC-NCI with radioimmunoassay (RIA) or enzyme immunoassay (EIA) is usually done to validate a new RIA or EIA method. Although RIA is simpler to perform and usually more sensitive, there are always concerns about cross reactivity. In one study, excellent correlation was re orted in blood between RIA and GC MS for PGE2, PGD2, bGF2a,and TXBB but not for 6-oxo- la (J5) because RIA overestimated ita concentration. For leukotriene B4 (LTB4) in human neutrophils, correlation between the methods was ood in the range 0.1-100 ng/mL (J6). The use of immunodiity columns for recovery of specific metabolites at ver low concentrations is a valuable asset to GC/ECN C I d analysis. Knapp et al. showed that analysis of 6oxo-Fla by this m e t h d was interference-free (Jn,and Gaskell et al. reported an isotope-dilution assay for LTB4 in human serum with low pg/mL sensitivity (J8).It was necessary to add a lipoxygenase inhibitor immediately on drawing a blood sample to suppress ex vivo formation of LTB4 that would otherwise result in overestimation of its concentration. For broad-spectrum analysis, the standard method of extraction is by solid-phase cartri e, usually C18. This method was used with isotope-dilution C/EC-NCIMS to obtain eicosanoid profiles in human broncheoalveolar lavage (J9).Prostaglandins E2, D2, F2a, 9a-ll&F2,6-oxo-Fla and TXBS were detedable in 5 mL of the fluid at 0.1-0.2 p /mL levels, the actual concentrations of E2, D2, TXBS, and 82a being less that 2.6 g/mL. Prostanoid profiles of respiratory tract lavage could dia ostically useful. Such a profile from human cultured lung fieoblaats has been described, in which the analysis time was reduced by using a short (6 m) capillary column and using hydrogen as carrier gas (JIO).Eleven prostanoids were resolved in less than 5.5 min with detection limits below 0.1 pg for most. Arachidonate metabolism by human tracheal epithelia revealed F2a and E2 as the major cyclooxy enase products (J11),and Malle et al. studied eicosanoid metafmlism in human platelets (J12). Analysis of prostaglandins in urine of rats supplemented with fish oil indicated that in vivo PG13 formation increased without PGI2 suppression, as observed in humans (J13). Other applications of the GC/EC-NCI method of quantification include determination of prostaglandins D2, E2, F2a, 6-oxo-Fla, and TXBB in lyophilized rat brain (J141,transformation of PGD2 to PGF2 isomers by human eosinophl (J15),and 11-dehydro-TXB2in urine using the unconventional but effective phenylboronate cartridge for solid-phase extraction (J16). Many studies are still conducted by standard GC EIMS, usually with the methyl ester-methoxyamine-T S (or TBDMS) derivative. An unusual derivative of 2,3-dinor-6oxo-PGFla, the l-propylarnide-6-methoxyamine-9,11,15tris(dimethylisopropylsily1) ether, enabled a detection limit of better than 2 pg a t 10000 resolution despite its long re-
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tention time on the column (J17). Another exam le of a specific derivatization is the use of the $ethylhydrogensil 1 cyclic diethylsilane derivative to analyze 11dehydro-Td3; (Jl8). The following examples of GC/MS in metabolic studies were selected from a large and diverse group. Evidence for a novel 12-lipoxygenase pathway of leukotriene formation in human platelets was described (JI9). Eling et al. compared the metabolism of arachidonic acid in human nasal and bronchial epithelia, indicating the formation of 15-lipoxygenase products (J201, and reported novel 12S,19and 12S,20-dihydroxyeicosanoidsformed by murine lymphocytes (J21). Gelpi et al. showed that l B " E is the major eicosanoid in normal human nasal secretions (522). Minor amounts of 18-hydroxy-PGEl and -PGE2 discovered in human seminal fluid (J23) are thought to be derived from cytochrome P-450 oxidation. Pulmonary cytochrome P-450 mediated eicosanoid metabolism in the pregnant rabbit has been reported (J24, J25), also the existence of a P-450-dependent monoxygenase pathway in bovine adrenal cells (J26). Murphy e t al. have characterized (12R)hydroxyeicosatetraenoic acid, a novel P-450-dependent metabolite of arachidonate in corneal microsomes having otent vasodilatory properties (J27). Isotope labeling a n t mass spectrometry played a crucial role in the structural elucidation. The discovery of LTB4-coenzyme A thioester in rat liver microsomes by techniques including LSIMS (J28) sug ests that further metabolism by enzymes of the mitochondriafand peroxysomal &oxidation pathwa is likely. Several new metabolites of LTE4 in isolated rat xepat..yt.. were identified by Stene and Murphy (J29) as @-oxidationproducts, one of which required the action of 2,4-dienoyl-coA reductase. LSIMS, GC/EIMS, and GC/EC-CIMS were instrumental in the characterization of these structures. A re ort of prostalandin synthesis from arachidonate catalyzed fiy peroxidases [as appeared (J30). Nicotinic acid, a potent hypolipidemic agent, was shown to markedly induce the release of PGD2, the likely mediator of the undesirable vasodilative side effects of the drug (J31). The facile rearrangement of (5S,12S)-dihydroxy-6,8,10,14(E,Z,E,Z)eicosatetraenoicacid during GC analysis to a stable derivative formed by ring closure between C6 and C11 was reported (J32) and is apparently selective for 5,12-dihydroxy derivatives with the trans-cis-trans triene unit. Evidence that the 1-ether-linked phospholipids are a major source of arachidonate for leukotriene synthesis in human neutrophils has been presented (J33). The ap lication of on-line LC/MS in eicosanoid research has dwindped significantlysince the last review, probably owing to inadequate sensitivity. Gelpi et al. reported that the methyl esters of &oxo-PGFl-a, PGFZa, PGE2, and PGD2 give better results with thermospray LC/MS than their underivatized counterparts (J34),but derivatization ne ates the only significant advantage of LC/MS in this fie& Off-line DCI in combination with HPLC was employed by Jamieson et al. to study conversion of LTA4 to LTB4 by human liver microsomes (J35,536). Informative mass spectra were obtained from 10 ng-1 fig material by using ammonia-CI, and quantification in the 10 pg-10 ng range was achieved by SIM. This technique was also employed by identify reaction products from the pyridinium dichromate oxidation of prostaglandins E l , E2, F l a , and F2a (J37). Tandem mass spectrometry is being ap lied more extensively in this field and can now be regardezas an established method in eicosanoid research. A brief review emphasizin the sensitivity and selectivity of MSf MS has been publishe! (J38).Several studies of basic fragmentation behavior under MS/MS have appeared (J3%JM), with the general objective of establishing parent-daughter ion relationships that might be useful for selected-reaction monitoring (SRM). Strife et al. studied the high-energy CA (MIKES) spectra of four PGs and eight (El, D2, E2, ll-deoxy-13,14-dehydro-15-oxo-El) deuterium-labeled analogues as their ME-MO-TMS derivatives (J39)and later compared the fra entation of a PGE2 derivative usin ammonia-CI by CA-&ES and by MS/MS in an ion trap &40). This information is, however, of limited utility to most researchers, who utilize most1 triple-quadrupole or hybrid instruments with low-energy 8A. In a series of papers, Meese et al. studied the fragmentation of PGs, E2, D2, 6:oxo-Fla, and 2,3-dinor-6-oxo-Fla (J41)and TXB2 and 2,3-dinor-TXB2 (J42)as ME-MO-TMS derivatives using E1 with low-energy CA in a tandem quadrupole. They also
MASS SPECTROMETRY
studied the decom osition of M - PFBI- ions enerated by EC-NCI of the PA-MO-TM derivatives of #Gs F2a, E2, D2,6-oxo-Fla, 2,3-dinor-6-oxo-Fla and of TXB2, 2,3-dinorTXB2, and 11-dehydro-TXBP (J43). The results were somewhat difficult to inter ret, but in general, EI-CA fragmentation was more usefu for structural elucidation than SRM,whereas EC-NCI-CA,which gave rise to fragmentation on the peri hery with essentially no carbon skeleton fragmenta, was Eetter suited to SRM. Although some derivatives were resistant to fragmentation or underwent extensive
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a tandem uadrupole and concluded that the detection limits for the E8-NCI derivatives were in the 1-10-pg range oncolumn and about 2 orders of ma itude better than the E1 derivatives (J44). Interestin ly, g k e l l et al. found the [M - PFB]- ions from PGs, E l , and F2-a resistant to highenergy CA fragmentation in a magnetic trisector instrument and reported modest sensitivity in GC-MS MS. However, [M LTB4 exhibited a useful fra mentation, [Id - PFBI- PFB - TMSOHI- that e n h e d isotope-dilution quantification at low pic am levels in human serum (J45).Dawson et al. used an anxfious method to quantify LTB4 in synovial fluid in concentrations down to 10 pg mL. In their case, the PFB-TBDMS derivative was used an the reaction monitored was the M - PFBI- [M - PFB - TBDMSOH - fragmentation ( 46). The true advantages of GC-MS/M are exemplified in a report by Lorenz et al. (J47),who utilized a rapid extraction procedure on phenylboronate cartridges to recover 11-dehydro-TXB2 from urine and analyzed the PFB-TMS derivative by GC-MS/MS at low picogram concentrations with a precision equivalent to that of a much more tedious reference method. Perhaps even better results could be achieved by combining these techniques with short capillary columns and hydrogen carrier as described earlier (JIO),since it is the combination of speed, sensitivity, and precision that makes MS/MS attractive.
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K. BIOGENIC AMINES This diverse grou of compounds with high biological activity presents a cdenging analytical problem owing to their very low endogenous concentration in biol 'cal matrices. The method of choice for their anal sis is s# isotope-dilution GC/MS and this is unlikely to g e replaced soon, although some interesting new techniques have emerged that merit discussion. Two com rehensive reviews have appeared, one the other a major on GC/MS and GC &/MS methods (KI), re rt (over 970 re erences) of biogenic amines in body fluids ( E a ) . Boulton et al. have ublished a short review of procedures for the isolation an! analysis of several monoamine neurotransmitters and their deaminated metabolites ( K 2 ) . The analysis of catecholamines by methods including radioimmunoassay, HPLC, and GC/MS has also been reviewed ( K 3 ) . Boulton et al. have shown that lon -term storage (up to 9 months) of urine specimens at -18 O8results in a fall in the concentrations of most indoleamines and catecholamines as measured by GC/MS, whereas plasma samples appear to be stable (K4).General improvements in detection limits to the 1-10-p range for catecholamines were achieved b in&en cluding a Jephadex G-10 column cleanup step ($3). lower absolute detection limits (e1pg) were achieved by GC with electron-ca ture negative chemical ionization (EC-NCI). Midgley et al. grmed 3,5-ditrifluorobenzoyl derivatives in aqueous solution, which were then extracted and further derivatized as TMS ethers (K6).The molecular anion carries most of the ion current, resulting in high sensitivity and selectivity. This method was used ta quantify bio enic amines in the brain of the American cockroach (K7).&e principal amines detected were tyramine, do amine, p-octopamine, 5-hydroxytryptamine,and noradrenafne. Similar derivatives of indoleamines, in which pentafluorobenzyl and trifluoroacet 1 roups were introduced, proved to be suitable for G C ~ E ~ N C I Manalysis S and enabled the identification of 1-meth 1-1,2,3,4-tetrahydro-fi-carboline in rat brain and lung tissue &8). GC NCI was utilized to confirm the presence of dopamine in je yfiih tissue (K9). In most cases,GC/EIMS methods have adequate sensitivity for biogenic amine analysis,
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as exemplified by quantitative analysis of do amine, noradrenaline, and their metabolites in portal b l d o f the shee and rat (KIO)and of monoamine metabolites in human CS! and serum ( K I I ) . GC MS and GC-MS/MS were employed to identify and quanti y tryptoline, 5-hydroxytryptoline,and 5-hydroxymetht toline as their heptafluorobutyryl derivatives in rat bra1?&12). High-resolution mass spectromet was used to study the interaction of biogenic amines w i g certain components of cigarette smoke (KI3),which produced cyanomethylenederivatives, am0 others. These compounds were also readily formed by inc%ation of the amines with human saliva obtained after cigarette smokin . Liquid chromatography/mass spectrometry ?LC/MS) has not been widely applied in this field. However, Gelpi et al. have analyzed catecholamines by thermospray LC/MS with subnanogram detection limits ( K I 4 ,K15).Improved sensitivity should be achieved by using capillary zone electrophoresis CZE/MS, as re rted by Smith et al. for analysis of epinephrine and relateramines (K16). This technique is based on electros ray ionization, as is ca illary isotachophoresis, a relate8 technique that has higi sensitivity for amines in dilute solutions (KI7).At present, LC-API/MS appears to be a more radical method, for which low pic detection limits have k e n reported for biogenic amines A series of papers by Lubman et al. ( K I S K 2 1 ) has demonstrated the analysis of indol- and catecholamines by resonant two-photon ionization spectrometry in a time-of-flight mass spectrometer. Compounds were introduced in solution by liquid in'ection (K19) or volatilized by laser desorption and entrained in a supersonic CO, *etexpansion for analysis ( ~ 2 0 ) . The laser desorption method was used to analyze spots of material on silica gel TLC plates (K21). While this esoteric technique appears to be at the laboratory curiosity stage at present, its sensitivity and versatility are impressive. The "imaging" of epinephrine and acetylcholine dispersed on silver substrates using SIMS has been reported (K22), and LSIMS/MS was employed to characterize ethylenediaminetype antihistamines and their N-oxides (K23) and to investigate polyamines related to philanthotoxins (K24). The occurrence of putrescine and N-methylputrescine, bios thetic precursors of nicotine, in tobacco roots was confumed y using DLI/MS (K2.5). LC/MS was also employed to study the oxidation of polyamines by pyrroloquinone (K26). Numerous reports of choline (Ch) and acetylcholine (ACh) analysis in various biological systems have ap eared (K27K36). A brief review of quantitative analyticar methods for Ch and ACh has been published (K27). Although techniques such as FD, LC/MS, and thermos ray LC/MS have previously been shown to provide exceient sensitivity for these quaternary ammonium compounds, the predominant method used in these studies was GC/MS. This is somewhat surprising, since extensive sample workup, followed by demethylation of the substrates with sodium benzenethiolate (Jenden's reagent) are required. Turnover of ACh, measured in specific brain areas of the rat, was used as an index of A similar neuronal activity after percussive head injury (K28). study has been carried out on the effects of morphine and traumatic head injury (K29). Quantitative analysis of Ch, ACh, and various derivatives including phosphocholine, phosphatidylcholine, and lysophosphatidylcholine was accomplished by isotope-dilution in 100-mg samples of tissue from rat liver, muscle, heart, kidney, brain, plasma, and red blood cells (K30). A complex analytical scheme was required to separate the metabolites, which then were hydrolyzed and assayed as demethylated choline by GC/MS. The effects of middle cerebral artery occlusion or cerebral cortex Ch and ACh concentration in the rat were studied by Jenden and Scremin (K31). The release of ACh from the medial septum/diagonal band of the rat brain (K32) and the effect of cocaine on hippocampal cholinergic metabolism (K33) have been investigated. The effect of propionate on free and bound ACh concentration in frog muscle was studied (K34, K35). It is difficult to escape the conclusion that in the field of biogenic amine analysis in general and Ch and ACh anal sis in particular, techniques such as LSIMS/MS and L C / h S are significantly underutilized.
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L. STEROIDS, STEROLS, AND BILE ACIDS Mass spectrometric techniques, especially capillary column GC/MS, continue to play a dominant role in the identification ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
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and analysis of these compounds. Most of the current techniques and their a plications have been reviewed in three recent books (L1, E2, B24). In the publication edited by Lawson (LI),there are chapters on steroids (by Gaskell) and bile acids (by Setchell and Lawson). A chapter on reference methods (by Siekmann) includes several references to steroids. Chapters on steroid and bile analysis (by Shackleton et al.) and gas chromatography/mass spectrometry of sterols (b Fischer and Trzaskos) appear in McEwen and Larsen's boo$ (L2).A third book reviews the analysis of sterols and steroids derived primarily from lants (B24)and contains chapters on oxysteroids (b Paris!), brassinosteroids (by McMorris), steryl glycosides &y Grunwald and Lishar), ste 1 esters (by Lorenz et al.), and hytosterols (by Rahier andxenveniste). A short review of in the analysis of steroids and terpenoids from natural products has appeared (L3). Most ap lications of GC(MS in hormonal steroid analysis rely on preiminary extraction of the tar et compounds from biologicdl matrim using HPLC or s o l i d - p L extraction, most often with CIScartridges. Derivatives suitable for either E1 or EC-NCI are then prepared. A wide variety of derivatives have been utilized, especially for NCI, but it is not clear in most instances whether the choice of one derivative has any particular advan e over another. The use of octafluorotoluene, for exam e, developed in 1984 by Jarman and reviewed recently has apparently been restricted to one laboratory. The challenging analysis of estrogens in human breast tumor tissue has been rformed by GC/NCIMS usin the unusual (trifluoromethylrenzoyl derivatizing group, an 5a-androst-16-en-3-0newas assayed in saliva as the pentafluorobenzyl (PFB) oxime derivative (L5). PFB-oxime derivatives were also used to quantify C19 and C21 steroids in insect larvae (L6)and for isotope-dilution assay of the synthetic steroid norgestomet in bovine plasma at the ppt level (L7). Corticosteroids have always presented problems for GC/MS analysis. Watson et al. have developed a novel procedure that involves prior oxidation with reagents such as yridinium chlorochromate to re are for formation of a statle derivative for NCI (L8). 1 gley et al. reported a GC/NCI method for prednisone, dexamethasone, and betamethasone in aqueous humor using methoxime-TMS derivatives (L9).Prednisolone acetate was also determined in aqueous humor by this method (LIO). Midgley et el. claim that although PFB-oximeTMS derivatives of corticosteroids are formed with only modest yield, their sensitivity to EC-NCI gives improved detection limits compared with other derivatives (L11).However, it is still not clear, despite their increasing popularity, whether NCI methods for steroid analysis offer significant advantages over the more well established E1 methods. Wolthers et al. used E1 with heptafluorobutyrate and MO-TMS derivatives in the development of ?eference" methods for progesterone, cortisol, testosterone, and estradiol and Ishibashi and co-workers assayed cortisol in serum (L12), and 6@-hydroxycortisolwith low picogram sensitivity in urine using the unusual metho aminediethylhydrogensilyl cyclic diethylsilene derivatives &3). PFBO-TMS derivatives were used to investigate and quantify metabolites of 16androstadien-&one, the odorous bacterial products in human hair (L14,,515). An interesting application of EC-NCI with C/MS and GC/MS/MS was reported by Houghton et al. in their investigation of the recently proposed sesterterpene pathway for steroidogenesis. Putative intermediates on this pathwa were discovered as end enous components of human a d r e n d tumor tissue (L16).This group has also utilized GC/MS to investigata steroid metabolism in equine placental tissue (L17)and e uine testicular tissue (L18).Human metabolism of the syn%etic corticosteroids fluorometholone (~19) and betamethasone (LZO)has been studied. Metabolic studies with stable isotopes have included an investigation of cholesterol and bile acid biosynthesis in human fibroblasts and he atocytes using D20 (L21),the in vivo conversion of [138]testosteroneto its metabolites in healthy men following iv infusion (L22)and the use of [9,12,12-2H]cortisolto study in vivo metabolism and production rates in adrenalectomized piglets (L23).Yergey et al. reported an isotope-dilution assay of underivatized cortisol in human plasma using LC/MS with thermospray ionization (L24)and suggested that the method could be applied to determine produchon rates in vivo. A new GC/MS assa for cortisol in plasma was also reported (L25). Analyses of Xbrmonal steroids in other organisms have in-
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cluded the determination of ovarian steroid levels in African catfish plasma before, during and after ovulation (L26), analysis of steroids in the ovary, eg s, and hemolymph of a metabolism of hyfroxysteroids in cultured crustacean (L27), guinea pig skin tissue ( L B ) ,determination of the boar taint in adipose tissue (L29), and steroid (5cu-androst-16-en-3-one) analysis of hormonal steroids in plasma from the female possum (1530). Steroid conjugates are usually analyzed by the established methods of grou separation, hydrolysis, and analysis of the liberated steroifs by classical GC/MS methods. A direct isotope-dilution analysis of cholesterol sulfate in plasma by LC/MS with thermos ray ionization has been developed by Shackleton (L31).Tge [M - HI-ion is monitored after a simple solid-phase extraction and short HPLC separation. An alternative method developed by Gaskell utilized a selective purification by immunoadsorption chromatogra hy coupled with LSIMS-MS MS to uantify intact dehydkepiandrosterone sulfate in uman b ood serum (L32).Low-energy CA of the [M - HI-ion produced HSOc (m/z 97) as the only significant product. Scanning the parents of m/z 97 in a tandem mass spectrometer could simplify the analysis of steroid sulfates as a group. Tomer et al. have investigated the high-energy CA fragmentation [M - HI- ions of 35 steroid conjugates, including sulfates and glucuronides, by LSIMSMS MS (L33). Coupled with the novel continuous-flow LS S interface for LC/MS developed at NIEHS (L34), this technique looks very powerful. Reviews of LC/MS (L35)and SFC/MS (L36) in the analysis of steroids, inter alia, have recently been published. Das et al. compared thermospray ionization (fiient-on) using 100% methanol plus ammonium acetate as the mobile phase with ammonia-DCI for a series of steroids and not surprisingly found similarities in the mass spectral behavior (L37). Mass spectrometry continues to make significant contributions in anabolic steroid research. The use, metabolism, and analytical identification of anabolic steroids, with emphasis on broad-spectrum screening by GCIMS, has been reviewed (L38). Another review focuses on the isolation, enzymic hydrolysis, and GC MS analysis of anabolic steroid metabolites in urine (L39). he authors present an integrated analytical method usin capillary GC/MS with sequential selected-ion monitoring ~ I Mof) characteristic ions in groups that change appropriately with retention time. The development of quality criteria for the detection of anabolic agents in test samples has been discussed (L40).Two studies of oxandrolone analysis and metabolism in man have been published (L41,L42). A method has been reported for overcoming the potential interference of oral contraceptives in and Fennessey et al. monitoring anabolic steroid usage (L43), have studied the effect of extended anabolic steroid use on the metabolism and excretion of endogenous steroids (L44). Immunoaffinity column chromatography was employed to concentrate ppb levels of nortestosterone in bovine muscle for GC/MS assay (L45)and in calf liver, kidney, urine, and bile for HPLC assay with GC/MS confirmation (L46).This technique should improve the detection of illegal anabolic steroids in tissue from the injection sites of animals, usually performed by GC/MS after standard LC purification (L47). Henion has utilized HPLC/API-MS/MS with ion-s ray to identify methandrostenolone metabolites in human anzequine urine (L48).Boldenone sulfate and related steroid sulfates were analyzed in equine urine by the same technique (L49). With use of SIM, 10 pg of boldenone sulfate was detectable on-column by using negative "ion-spray", and the compound was detectable by full-scan MS/MS up to 17 days after ingestion. Testosterone esters in oil injectable5 were characterized in the absence of analytical standards by LC/MS/MS with thermospray ionization (L50).Greater utilization of MS/MS in this field is warranted. The aforementioned reviews (B24,L1-L3)ex lore most of the current methods for analysis of sterols. I n improved candidate definitive method for serum cholesterol has been reported from the National Institute of Standards and Technology. W,-labeled cholesterol is employed as the internal standard in a GC/MS assay of the TMS ether that has a coefficient of variation of 0.22% (L51).A simultaneous isotope-dilution GC/MS assay for cholesterol and lathosterol in serum has been described (W2).Mass spectrometry helped to elucidate the structures of four new steroidal cyclopropanes
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( W 3 ) . GC MS was employed in the analysis T g g x i n pollen ( 4 ) and in a species of companul a m e (WS). Setchell et al. used thermospray LC with tandem mass spectrometry to identify phytoestrogens in soy protein preparations (L56). The technique was useful for the class identification and structural assignments of phytoestrogens in dietary reparations. Games et al. conveniently anal zed polar steroE (ecdysteroids)by packed-column SFC/MS (&7) An alternative method for this compound class utilizes TLC
this review period, there has been an increase in the use of S for sterane and triterpane “biomarker” research in organic eochemistry. A useful discussion of the occurrence, utility, anif detection of biomarkers has been published (L61). Applications of GC/MS have included analysis of oils, sediments, and mature rocks from around the world (L62-L69). Several reports of the use of MS/MS have appeared (L70,573) claiming increased s eed and accuracy compared with GC/MS. Both quadrupope and sector tandem instruments have been employed, but systematic studies of the fr tation behavior of these compounds with both low an%ii collision ener ies are lacking. Some basic fragmentation studies of setcted steroids by MS MS have included a MIKES study of selected steroids (L 4 ) and a LSIMS-MS/ MS study of steroid conjugates and bile salts (L33). Since fundamental studies have been few and far between, it is also worth mentioning the laser desorption FTMS study of underivatized steroids by Wilkins and Fung (L75). Ap lications of MS in vitamin D research have been dimostly toward the uantitative analysis of hydroxylated vitamin D metabolites. %he best examples are to be found in papers by Coldwell et al. describing the analysis of vitamins D2 and D3, 25-hydroxy-D2, 25-hydrox -D3, 24,25-dihydroxy-D2, 25,26-dihydroxy-D2, and 1,25-1&hydroxy-D3by isotope-dilution GC/MS in approximately 3 mL of plasma (L76, L77). Mixtures of reversed-phase and normal-phase cartrid es and HPLC were used for sample purification. N - B U & O ronatm were formed across vicinal hydroxyl groups, where appro riate, followed by TMS ether formation. GC/MS with {IM was compared to a radioreceptor assay for 1,25-dihydroxy-D3in serum, with good correlation (L78).An isotope-dilution assay for 1,25-dihydroxy-D3was employed to investigate the pro erties of the la-hydroxylase enzyme in guinea pig kidney (E79). In vivo and in vitro metabolism of dihydrotachysterol 3, a vitamin D analogue, was studied in the rat by using GC/MS (L80).Mass spectrometry was also used to demonstrate conversion of 1,25-dihydroxy-D3to calcitroic acid, the excretory end-product, in two target cells of vitamin D3 (L81).Recently, Setchell et al. have described the application of thermospray LC/MS to study 25h droxy-D3 metabolism in renal mitochondrial preparations (iL.2). ~n isotope-dilution m y was utilized for quantification in the ran e 0-40 ng, the detection limit being 0.25 ng oncolumn. Ais method has the advantages of minimal sample preparation and preservation of structural integrity, the lack of which have plagued GC/MS methods for this compound class. Owing to the metabolic impprtance of bile acids and their conjugates, interest in their analysis is currently very active. Applications of maas spectrometry have been comprehensively reviewed in the books previously referred to ( L l ,L2) and in another major report authored by Setchell and Lawson containing 311 references (L83). An excellent review of chromatogra hic methods for bile acid analysis has been published (L84). , {he results of a collaborative interlaboratory study, organized by Nakamaya, of methods for bile acid analysis, including enzymic analysis, RIA, TLC, HPLC, GC, and GC/MS with SIM,have been reported (12.5). The advantages of sensitivity and specificity of GC/MS were considered to be offset by the greater need for preliminary cleanup and fractionation compared with re uirements for enzymic assay and RIA. Recent publications$y Goto et al. (L86,L87)exemplif the state of the art in sample preparation for isotope-dution GC/MS analysis of unconjugated bile acids. The high sensitivity achieved with the PFB-ester-dimethyl-
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ethylsilyl ether derivatives in the EC-NCI mode, which enerated intense carboxylate anions, [M - 181]-, ena led quantification in human urine and in small (2-10 mg) samples of human liver tissue. Jakobs et al. employed the same technique to analyze bile acids in the plasma (0.2 mL) from healthy fasting adults, with low picogram detection limits (L88).Setchell and Vestal have investigated the analysis of intact bile salts by ne ative-ion filament-assisted thermospray LC MS (L89). The tile acid con’ugates gave [M - HI-ions wit fragments corresponding to !os, of up to 4 molecules of water. The glyco and tauro conju ates of cholic, chenodeoxycholic,deoxycholic,and lithochofic acids were detectable under LC/MS conditions by SIM in the range 10-20 pmol. Bile acid profiles were reported from the bile of a gallstone patient and from the plasma of two patients with cholestatic liver disease. Eckers et al. (L90)used plasma discharge-assisted negative-ion thermospray LC MS to analyze bile acids. Post-column addition of polyethy ene glycol enabled highresolution (1OOOO) mass measurement with up to 3 mmu accuracy. The direct analysis of bile salts by LSIMS in small samples (5-10 mg)of rat liver, following detergent and ion-pair extraction and group separation,has been demonstrated (L91). Sjovall et al. have also employed LSIMS for analysis of bile acid glucosides and N-acetylglucosinamides,a new t e of bile acid conju ate in man (1592,,593). Extensive use o CIScartrid es and HPLC was required before analysis of negative ion Applications of metabolic profiling of bile acids and alcohols for recognition of disease states are increasing. Unusual bile acids were detected by GC/MS in amniotic fluid of term neonates and in sera and urine of adults with liver diseases (L94, L95). A detailed study of the qualitative and quantitative composition of bile acids in human fetal allbladder bile has been carried out, using GC/MS and L&MS (L96) and a new metabolite, 3a, 4@,7a-trihydroxy-5&cholanoic acid was characterized therein (~5971,indicating that C4hydroxylation is a unique and important metabolic athway in early fetal development. The profile of bile alcobls, liberated by enzymic hydrolysis, in normal human serum has been published (L98).The occurrence of three unconju ated C27 bile acids as normal constituents of human blootf was reported by Sjovall et al. (L99).Bile alcohol glucuronides were analyzed in urine after enzymic hydrolysis by GC/EIMS of the TMS ethers (L100). Although their excretion was higher in patients with primary biliary cirrhosis (PBC) than in controls, there was no PBC-specificprofile. The synthesis of bile acids by human liver cells in culture has been investi ated by GC/MS (LlOl). With D20 in the medium, it was sfown that 3/3-hydroxy-5-cholenoicacid is derived from cholesterol and should be considered a primary bile acid. A method has been reported (L102) for studying the kinetics of cholic acid and chenodeoxycholic acid turnover in vivo, using bolus oral administration of deuterium-labeled analogues and following isotopic enrichment by GC/MS. Innovative application of LSIMS by Setchell et al. has enabled the discovery of a new inborn error of bile acid synthesis in identical twins (L103). The elevated excretion of taurine conjugates, later shown b acid: GC/MS to be derived from 7a-hydroxy-3-oxo-4-cholenoic was the key observation made by LSIMS. To close this section, some general comments on future direction in the analysis of steroid and bile acid conjugates are in order. Despite the popularity of LSIMS and the commercial availability of the continuous-flow interface for LC MS since 1985, there has not been a single report of on-iine LC/LSIMS analysis in this field. The application of this technique, with its high sensitivity and intuitively obvious potential for combining sample preparation with analysis, would seem to be overdue. Also, the use of electrospray ionization in this field should increase.
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M. APPLICATIONS IN CLINICAL MEDICINE For the purposes of this review a plications of mass spectrometry in clinical studies can be Broadly divided into two categories: diagnosis of specific disease conditions and use of stable isotopes to investigate human metabolic processes in vivo. Although the number of citations in this area is relatively small compared to ap lications in drug metabolism and toxicology, it is anticipateathat the next decade will see significant growth. One reason for the slow growth in clinical applications of mass spectrometry to this point is the conANALYTICAL CHEMISTRY, VOL. 62,
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servative nature of c l i n i c h , who are often slow to accept new diagnostic techniques or modify traditional study designs to incorporate new technologies. Diagnosis of inborn errors in fat and amino acid catabolism has received considerable attention during this review period. Such conditions are usually diagnosed by identifying and/or uantifying specific abnormal metabolites (organicacids and %eir con’ugates, for example) excreted in urine or accumulating in h o d and tissue. Mass spectrometry, particularly combined gas chroma aphy/mass spectrometry (GC/MS), has been well establish in this area for two decades. In their report of a new GC MS method for or anic acid “profiling”, Hoffmann et al. ( I ) state that the ‘a5iditional information, specificity, and sensitivity provided by mass spectrometry is nearly indispensable for the quantitative multicomponent analysis of complex biological samples needed for the diagnosis and study of patients with new or ill-defined disorders, for research in organic acidopathies, and for resolving an ambiguity of identificationin severely ill atienta who are receiving numerous exogenous medications”. h e method of Hoffmann et al. (MI)involves extraction of organic acids, aldeh des, and ketones on silicic acid columns followed by GC/EIMiianalysis of the correspondin trimethylsilylderivatives. The extraction procedure is applicgle to various biological matrices including urine, plasma, and amniotic fluid. No deproteinization of samples is required. Oxoacids, aldehydes, and ketones are stabilized as the corresponding 0-(2,3,4,5,6-pentafluorobenzy1)oximes prior to extraction. Identification and quantitative determination of diagnostic metabolites is performed under computer control with a target compound analysis system. A number of other applications of GC/MS analysis to the rofiling of metabolic acidosis and similar disease conditions ave appeared (M2-MIO). Notable among these are the studies on valproic acid metabolitea by Rettenmeier et al. (M2) and Kassahun et al. (M3), a study of the excretion of Nacetylated branched-chain amino acids in patients with maple syrup urine disease by Lehnert and Werle (M4),the identification of a novel case of toluene-induced metabolic acidosis and the discovery that a generalized by Jone and Wu (M5), dicarboxylicaciduria is a common finding in neonates (MIO). Stable isotope dilution GC/MS techniques for the accurate quantification of specific abnormal metabolites have been applied to the diagnosis of inherited metabolic disorders. A ver interesting series of reports has ap eared in which such t e c h q u e s were utilized for the p r e n a d diagnosis of various disorders, including methylmalonic acidaemia, propionic acidaemia, glutaric aciduria type 11, tyrosinaemia type I, isovaleric acidaemia, and galactosaemia (MII-MI3). Amniotic fluid is employed as the matrix for these analyses. Kretschmer and Bachmann (MI2) have shown that methylcitric acid, a metabolite of abnormal propionyl-coenzyme A metabolism, is elevated in the amniotic fluid when the fetus is affected with pro ionic acidaemia or methylmalonic aciduria. These worters employed a deuterated analogue of methylcitric acid as an internal standard. Holm et al. (MI31 confirmed these findings and further demonstrated that methylmalonic acid was elevated significantly (23-255 times normal) in cases of methylmalonic acidaemia. The highest level was found in a case where the fetus was at risk for a vitamin B12 unresponsive mutase defect, and the lowest was found with a fetus at risk for the cbl-C genetic complementation group. Medium-chain acyl-coenzyme A dehydrogenase (MCAD) deficiency is one of the most common inborn errors of fat catabolism, makin it one of the most widely encountered organic acidurias. k n a l d o et al. (M14-MI6) have reported a new diagnostic method for this condition based on the isotope dilution GC/MS analysis of n-hexanoylglycine, (3henylpropion llglycine, and suberylglycine in patient urine. Fhese particurar glycine conjugates are formed from the medium-chain organic acids (and acyl-coenzyme A compounds) that accumulate behind the metabolic block. The method employs a series of 13C- and ISN-labeled internal standards and ammonia positive chemical ionization to produce intense MH+ ions for quantification by selected ion monitoring (SIM). In a comparison of their diagnostic test with a previously reported method, acylcarnitine profiling, Rinaldo et al. (MI5) claimed greater reliability for the acylglycine isotope dilution quantitative measurement. This claim was disputed by Millington and Roe (MI 7).
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Acrylcarnitines are quaternary ammonium salts and thus are not amenable to analysis by as-phase ionizaton processes such as EIMS and PCIMS. L8IMS has been employed to identify and quantify disease-spec& acylcarnitinesin human urine and other matrices. Millington et al. (MI81 re orted the use of LSIMS with tandem m u spectrometry (M8/MS) for acylcarnitine analysis from human urine, blood, and tissue. Low-energy (15 eV) argon collisions were utilized in a triple-quadrupole mass spectrometer to produce a common daughter ion ( n / z 99) from all acylcarnitine methyl esters. An acylcarnitine profile could then be acquired with a precursor ion scan function. Significant improvements in selectivit and sensitivity were reported for the LSIMS/MS methodrcompared with LSIMS. These workers further reported the application of continuous-flow (CF)-LSIMS/MS and HPLC/MS to urinary acylcarnitine rofiling and suggested that this technique could be emppoyed in neonatal screening for a number of metahlic diseases, including MCAD deficiency (MI8, MI9). Kodo et al. (M20) employed the LSIMS/MS technique as the basis for an isotope dilution assay for free and total carnitine in biological matrices. Simultaneously re rted was a method for acylcarnitine profilin based on LSIdonization followed by high-energy collisionelf activation and linked-scan analysis in a magnetic sector mass spectrometer (M21). Of the various linked-scan modes evaluated, the constant neutral loss scan function was demonstrated to be the most useful. Shackleton and Reid (M22)have reported a new method for the diagnosis of recessive X-linked icthyosis, a condition caused by a deficiency of steroid sulfatase, which is an enzyme responsible for the release of steroids from sulfate con’ ation. The method utilizes thermospray (TSP) HPLC/@ with isotopically labeled internal standards for the accurate quantification of cholesterol sulfate in patient plasma. The mass spectrometer was operated in the negative-ion mode and SIM was employed. This method represents a clear advance over those that involve measurement of free cholesterol after hydrolysis of cholesterol sulfate. These latter methods are prone to error due to interferences from free cholesterol present in patient plasma. Other applications of mass spectrometry to the diagnosis and stud of metabolic disease include the use of isotope dilution JC/MS to quantify metabolites that are diagnostic of deficiencies of biotin (M23),cobalamin, and folate (M24), a method for the determination of methylmalonic acid in serum and urine by solid-phase extraction and isotope dilution GC/MS (AI%), the determination of myoinositol in diabetic sera using GC/EIMS with SIM (M26),and the identification of a new inborn error in bile acid synthesis by Setchell et al. (M27). Mass spectrometric techniques have also been employed for the diagnosis and study of other disease conditions. Lam et al. (M28) used LSIMS to confirm the presence of LTE , a leukotriene, in the bronchial fluid of 15 of 17 patients with mild to severe asthma. They concluded that analysis of these mediators in brochial fluid may be useful for determining their role in the disease process and for determinin the effect of pharmacologic intervention. Duncan et al. (&9) measured levels of norepinephrine and 3,4-dihydroxyphenylglycolin the urine and plasma of patients with pheochromocytoma by GC EIMS with SIM and determined that such measurements ha diagnostic utility. GC/EIMS was also employed by Bolt et al. (M30) to measure 2-h postprandial unconjugated bile acids in human serum. It was suggested that such measurements were useful in the dia nosis of small bowel bacterial overgrowth. Vandeputte et .a! (M31) used laser microprobe mass spectrometry to determine that the spheroliths in the Bowman’s membrane of patients suffering from primary atypical bandkeratopathy consisted mainly of calcium hosphate. A computerized classification technique for G 4 M S screening of breath biomarkers in lun cancer was ro osed by O’Neill et al. (M32). Shoda et al. ( b 3 3 )utilize$G(?/MS techniques to identify a series of bile acids in the urine of patients with various liver diseases. Weydert-Huijghebaert also using GC/MS techniques, determined that et al. (M34), total urinary bile alcohol glucuronide excretion is significantly increased in patients with primary biliary cirrhosis. The use of stable isotopes in clincial studies has increased significantly over the past decade. In a recent review on this subject Thompson et al. (B38)state that this increase is the
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result of both advances in mass spectrometry and the availability of competitively priced labeled compounds. The importance of the latter should not be underestimated. Stable isotopes can be used to probe metabolic pathways (both normal and abnormal), to quantify energy expenditure and body composition, and to measure substrate flux and oxidation rates (B38).All of these studies can be accomplished in vivo, that is in human subjects, without the obvious problems associated with the use of radiolabeled com ounds. In their review Thom son et al. (B38) discuss the cEoice of label and tracer molec e, mode of tracer administration and samplin site, analytical instrumentation, interpretation of data, and ethical constraints. Amino acid flux and the d amics of protein metabolism can be evaluated in humans using infusions of stable isotopically labeled amino acids. Jahoor et al. (M35) used infusions of labeled leucine, valine, lysine, and urea to assess the protein metabolic response to burn in’ury in children. The enrichments of these essential amino acids and urea in plasma were determined by GC/EIMS with SIM of the appropriate trimethylsilyl derivatives. The absolute rate of protein breakdown in patients was assessed by measuring the mean plasma flux of infused amino acids, and the net protein catabolism was estimated from urea production rates. It was concluded that protein breakdown is elevated in all phases of response to burn injury, a significant net N loss occurs only in the “flow phase” of patient treatment, and a switch from net protein catabolism to anabolism occurs during convalescence des ite the elevated protein breakdown rate. Shaw and Wolfe ($36) used infusion of [15N]urea and [15N]lysine to determine rates of whole-body protein synthesis and catabolism in atients with early and advanced astrointestinal cancer anfwith normal controls. GC/PCIM\ with methane was employed to measure plasma lysine enrichments. They determined that the rate of protein catabolism depended on the type and extent of disease, and in certain cases glucose infusion or use of total parenteral nutrition (TPN) could decrease the rate of protein loss. A number of other reports have appeared that employ stable isotopes to study protein metabolism (M37-M44). Of particular interest among these are a study of the ada tive res onses of body protein metabolism to lactation by Phlotil et (M37),an investigation of alanine flux in obese and healthy humans by Hoffer et al. (M38), and a method for determinin deuterium-labeled tryptophan in proteins by HPLC/TSPM8 and field desorption mass spectrometry (FDMS) (M43). Another area in which stable isoto es and mass spectrometry have been employed is the stu y of metabolic disease. Thompson et al. (M45, M46) employed ( l-13C)propionate infusion to compare the total production and oxidation of propionate with the urinary excretion of ropionate metabolites in children with methylmalonic aci8emia. Propionate is the immediate metabolic precursor to methylmalonate. They determined that propionate oxidation is an important route of propionate disposal in methylmalonic acidemia and that measurement of urinary metabolite concentration alone may not accurately reflect clinical status or res onse to treatment. Millington et al. (M47) have describei clinical applications of labeled L-carnitine infusion in which LSIMS was utilized to measure isotopic enrichments in acylcarnitines. They observed a rapid incorporation of label into propionylcarnitine in a atient with propionic acidaemia and octanoylcarnitine an$ other medium-chain acylcarnitines in a patient with MCAD deficiency. It was stated that the labeled carnitine could be employed to measure in vivo production rates of carnitine and acylcarnitines in these disease states and therefore to determine whether impaired biosynthesis of carnitine contributes to secondary carnitine deficiencies. used infusions of [l-14C]palmitate, [3,4Bowyer et al. (M48) 13C acetoacetate, [6,6-2H2]gl~c~~e, and [5,5,5-2H3]leucineto stu y the effect of intravenous L-carnitine on the metabolism of fatty acids, ketone bodies, glucose, and branched-chain amino acids in normal subjects and patients on long-term home parenteral nutrition. GC/MS techniques were used to measure enrichments in target compounds. Other areas of application of stable isotope labeled organic and substrates include steroids (M4SM51),bile acids (M52), sugars (M53, M54). Additional citations may be found in the recent volume Synthesis and A plication of Isotopically Labeled Compounds edited by $)aillie and Jones (M55).
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Stable isotopically labeled inorganic species also have clinical application. Hillman et al. (M56) used two stable isotopes of calcium to measure true fractional calcium absorption in premature infants. W a and %a were measured in urine by thermal ionization mass spectrometry and expressed as a ratio to naturally occurring W a . McMillan et al. (M57) employed l80analysis in urine by continuous-flow isotope ratio mass spectrometry to study total body water and water turnover. Dever et al. (M58)developed a new a proach The for measuring erythrocyte life span by labelin with erythrocyte-bound Wr was determined by infuctively coupled argon plasma mass spectrometry. The effect of extrusion cooking of a bran-flour mixture on iron and zinc retention was measured in normal adults by using the stable isotopes @Fe and 67Znby Fairweather-Tait et al. (M59). Zinc enrichments were measured by fast atom bombardment mass spectrometry. Javitt and Javitt (M60)have described the use of D20 in the study of cholesterol and bile acid synthesis. Although increased significantly,the use of stable isoto es in clincial research continues to be limited by the availabilty of appropriately labeled compounds. Many clinical laboratories are not able to accomplish custom synthesis and must, therefore, rely on commerical sources. As the use of stable labels becomes more popular, it is anticipated that many new compounds will become available.
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ACKNOWLEDGMENT We are grateful to the following experts who reviewed sections of this review prior to publication for accuracy and completeness: Thomas A. Baillie (Universit of Washington, Seattle, WA), Simon J. Gaskell (Baylor Codge of Medicine, Houston, TX), David J. Harvey (Oxford University, UK), Robert C. Murphy (Nationd Jewish Center for Immunology and Respiratory Medicine, Denver, CO), and Kenneth D. R. Setchell (Children’s Hospital Medical Center, Cincinnati,OH). We wish to acknowledge discussions and information from many colleagues too numerous to cite individual1 A.L.B. is indebted to the expertise and talent of Candy itone, for library searching, typing, and editing the entire manuscript. Financial support was provided by NIH DRR Grant 01614 (to A.L.B.), NSF Grant DIR 8700766 (to A.L.B.), and Grant AM 27643 (to A.L.B.).
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LITERATURE CITED A. OVERVIEW
(AI) Thompson, J. J. Rays of Positive Electricity and Their App/icaHon to Chemical Analysis; Longmans Green and Co.: London, 1913. (A2) Thompson, J. J. Recollections and Reflections; CambrMae Universitv Press: Cambridge, 1937. (A3) Lowell, P. (1655-1916), astronomer and founder, Flagstaff Observatory, Arizona. (A4) Barber, M.; Bordoli, M.; Sedgwick, R. D. In B&bgh/Mass S p e c t r m try; Morris, H. R., Ed.; Heyden & Sons: London, 1961; pp 137-152. (A5) Burlingame, A. L., McCloskey, J. A. Eds. B/o/ugh/ Mass Spectromeby; Elsevier Science Publishers: Amsterdam, in press. (A6) McCloskey, J. A., Ed. Methods €nzym/.. In press. ( A 7 ) Nelson. R. W.; Rainbau. M. J.; Lohr. D. E.; Williams, D. Science 1989. 246. 1585-1586. (As) Wltkowska, H. E.; Green, E. N.; Smith, S. J . B M . Chem. 1990, 265, 5662-5665 - --- - --- . (A9) Pouker, L.; Green, E. N.; Burlingame, A. L. In B/o/og/calMass Spectrometry; Burlingame, A. L., McCloskey, J. A., Eds.; Elsevler Sclence Publishers: Amsterdam, 1990. (A10) Green, E. N.; Oliver, R. W. A.; Faiick, A. M.; Shackleton, C. H. L.: Roitman, E.; Wkkowska, H. E. In Biorogrca/ Mass Spectromby; Burlingame, A. L.; McCloskey, J. A., Eds.; Elsevier Science Publishers: A m sterdam, 1990. 8. SCOPE
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..., -.. --..
A74 317-397
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Atomic Mass Spectrometry David W. Koppenaal Pacific Northwest Laboratory,' P.O.Box 999, MS P8-08, Richland, Washington 99352
INTRODUCTION AND SCOPE This is the second fundamental review on the currently topical sub'ect of atomic mass s ectrometry. Following the format of tke initial survey on t k s subject (I),the intent of this review is to assess the scientific activit ,as evidenced in the published literature, in the growing fie d of atomic mass s ectrometry. Boundary limits for this review are set by title efinition,the aims of this journal, and the late 1987-late 1989 time period. Atomic mass spectrometry is defied as the mass spectrometric measurement of atomic ions, for the primary purpose of elemental and/or isotopic compositional determination. The atomic mass spectrometry term is preferred over the more widely used and all-encompassing inorganic mass spectrome or elemental mass spectrometry terms, as it more clearly in icates the elemental and isotopic emphasis of this review while excluding other legitimate inorganic mass spectrometry topics such as organometallic and chelate compound structure identification, metal cluster ion formation and reaction studies, and gaseous metal chemistry,all of which are active1 investigated by using mass s ectrometry techniques (aniwhich are reviewed elsewhere 3)). The atomic mass spectrometry label is also grammatically parallel with terminology for other analytical techniques based on atomic phenomena (i.e., atomic absorption, atomic emission, and atomic fluorescence). Consistent with the aims of thisjournal,
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this review places emphasis on developments and applications of atomic mass spectrometry for quantitative analytical purposes. This review is technique oriented and organized. Primary atomic mass spectrometric techniques reviewed include spark source, glow dischar e, inductively coupled plasma, stable isotope ratio, therm$ (surface) ionization, laser microprobe, resonance ionization, accelerator, and secondary ion mass spectrometrymethods. These sub'ecta are given the acron SSMS, GDMS, ICPMS, SIRMS, h S , LMMS, RIMS, and SIMS, respectively. Miscellaneous tecnhiques that fit within the atomic mass spectrometry definition or relate to it are also included where appropriate. Mechanistics of this review are based on a computerized Chemical Abstracts search of titles, keywords, and abstracts of literature published from late 1987 to late 1989. Government reports, unpublished conference proceedings, and obscure forei journal references are in general not cited in this review. &re foreign journal references are cited, a Chemical Abstrads accession number is also given. Over 2300 literature citations were screened for this review; as directed by the readership and editorial desires of this journal this review is somewhat critical and selective in its coverage. It is the author's hope that this approach will be of most use to the journal readers. Instrument refinements, technique developments,analytical applications, and technique status reports are all covered in this review. Surveys of the type and scope of analytical applications are judged by this reviewer to be t!.: ultimate measure of a techniques utility and e cceptance by the ana0 1990 American Chemical Society
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