Mass Spectrometry D. C . DeJongh, Department o f Chemistry, Wayne State Universify, Detroit, Mich.
I HAD AGREED to prepare the 1970 review of Mass Spectrometry, covering the two-year period from the termination of the last review ( I ) , I became aware of the scope of the field. It would be a n impossible task to prepare an all-inclusive bibliography of mass spectrometry and related topics, ranging from medical applications and space research, through organic and inorganic chemistry, to highly theoretical considerations, data handling, and instrumentation. I was urged b y the editorial staff to be selective and to have the review function as a critique. Thus, in preparing this review I followed two broad guidelines. First, I indeed was selective; but I do not claim that all parts of the review are equally valid as critiques, since m y experience is not as great in some areas of the field as in others. Second, I attempted to emphasize those areas that I felt were of most interest and relevance to the readers of AKALYTICAL CHEMISTRY. A search of the literature of mass spectrometry has been made considerably easier since the appearance of the Mass Spectrometry Bulletin in November 1966 ( 2 ) . This monthly guide to the current literature of mass spectrometry and related topics published 3600 references in 1967, 5000 references in 1968, and 5500 references in 1969. The references are obtained from 220 journals, as well as abstract journals, report lists, patents, dissertations, and books. I n addition, two new international journals devoted to mass spectrometry have begun publication. The Journal of Mass Spectrometry and Ion Physics accepts papers on a broad spectrum of topics (S), whereas Organic Mass Spectrometry is more specialized (4), as its name implies. The Mass Spectroscopy Society of Japan continues to publish the journal, M a s s Spectroscopy ( 5 ) . The mass-spectral data from the University of Goeteberg data bank are now available in the form of a threevolume atlas and on magnetic computer tape (6). The mass spectra of the 6839 compounds are organized b y molecular weight. This Atlas can be supplemented and kept up-dated with the journal Archives of Jfass Spectral Data which provides new and recently acquired spectra on a quarterly basis using the same format as the Atlas (7). Whereas the Atlas and Archives present complete mass spectra, the Zndez of hlass Spectral Data lists approximately 8000 mass spectra, including the Universityof Goeteberg collection, by molecular weight and the six most intense
A
FTER
peaks (8). The first supplement to the Compilation of Mass Spectral Data, which appeared in 1966, adds a n additional 1000 spectra t o the original 5000 (9); the ten most intense peaks are given. A review has compared these data sources (10). On a professional note, the participants at the 17th Annual Conference on Mass Spectrometry and Allied Topics adopted a constitution in their formation of The American Society for Mass Spectrometry. The Annual Conferences had previously been organized by the officers of the ASTM Committee E-14 on Mass Spectrometry. The new Society will continue to work closely with Committee E-14. Also, a n intermediate conference of the Triennial International Mass Spectrometry Conference was held in Japan, in September 1969; the Proceedings of this conference will be published ia book form in 1970. A number of local mass spectrometry societies have been organized in the United States. Mass spectrometry groups in Britain have been discussed (11). Also, a number of symposia, conferences, and meetings on aspects of mass spectrometry are held around the world each year; these are announced monthly as Forthcoming Events, in Organic Mass Spectrometry (4, including dates, locations, deadlines, etc. The ilCS Short Course on Mass Spectrometry is again being offered at local sections and in conjunction with national and regional meetings. A number of analytical laboratories now offer commercial technical services in mass spectrometry; these are usually advertized in the Directory Section of Chemical and Engineering News. Also, the National Institutes of Health have established mass spectrometry centers as resources for health-oriented research. Increasingly, journals are accepting molecular weights and formulas which have been determined by mass spectrometry. For example, the Journal of Organic Chemistry accepts molecular formulas determined by accurate mass measurements ( + 3 x mass unit) if combustion analyses cannot be obtained due to sample limitations; wherever possible, mass-spectral data presented in the journal must be supported by criteria of purity. Thus, mass spectrometry continues to grow as a professional field, increasing in impact, identity, and organizational structure. I n this review I will attempt to indicate areas in which substantial contributions to the field have been
made over the past two years via publications. Inevitably some key references will be omitted or left unemphasized, because of errors in judgment of the author, or because of late publication or late arrival of journals at the end of 1969. The approach to the subject is topical, as has been the custom ( I ) , within the broad guidelines mentioned in the first paragraph. I n the remaining paragraphs of this introduction, books and reviews of general interest are mentioned. Others dealing with specific topics will be included under subsequent headings. Reviews of ion sources, sample handling, and vacuum systems (12),of massanalyzer systems and detectors ( I S ) , and of commercial mass spectrometers (14) have been written as aids for those who use or, especially, teach the use of modern instrumentation. Book chapters on mass spectrometry as an instrumental method of chemical analysis are also useful for the nonspecialist (16, 16). The status of mass spectrometry as seen in 1968 has been reviewed (1, 17, 18). Organic mass spectrometrists are considering and using suggestions which were made for the use of symbols and abbreviations in papers dealing with topics in that area (19). The tables of uncertified mass spectra of diffusion pump oils will be an aid to those who interpret spectra suspected of containing peaks due to artifacts (IO). The limits of measurement with mass spectrometry have been reviewed ( d l ) , as have instruments and their applications in mass spectrometry (2%’). An important book presents the proceedings of the International Mass Spectrometry Conference held in Berlin in September 1967 (bS), and another presents proceedings of a symposium on mass spectrometry (24). Books that introduce instrumentation and techniques of mass spectrometry ( 2 5 ) , that discuss mass spectrometry in science and technology (I6), and that present modern aspects of mass spectrometry (27, 28) have appeared. Instrumentation has been treated in a Russian book (29). Time-of-flight mass spectrometry is covered in a recent book (SO), and a series has begun which is intended to provide a medium for publishing the latest developments and applications in the field of dynamic mass spectrometry (31). Books also cover mass spectrometry in inorganic chemistry (SI), mass spectrometry and ion-molecule reactions (SS), and mass spectra of organic com-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5 , APRIL 1970
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pounds (34, 56). Applications of high resolution mass spectrometry to organic chemistry have been reviewed (36). Mass-spectrometric investigations of carbonium ions (37) and their thermodynamic aspects (38) have been treated in a book covering general aspects and methods of investigation of carbonium ions. THEORETICAL STUDIES
Developments in the theory of mass spectra since 1961 have been discussed (39), and the quasi-equilibrium theory has been applied to large aromatic molecules (40, 41). The observed kineticenergy distribution in the double ionization of molecular hydrogen agrees closely with theoretical predictions based on the Franck-Condon principle (42), and Franck-Condon calculations for the double ionization of molecular hydrogen (43) and for the ionization of HzO and D20 (44) and NHa and H2O (46) have been made. Calculations of potential-energy hypersurfaces for predicting dissociation processes have been discussed in terms of C2HSNH2.+ and C2H4 * + (46). The effect of ionizing voltage on the mass spectra of some deuterated ethanes (47) has been used in an empirical method for calculation of the spectra from a statistical model that was standardized to the mass spectrum of CzHe (48). A misunderstanding on the possibility of a direct relationship between the probability of fragmentation and the positive-charge distribution has been aired (49-52). A comparison between the charge-density diagram of a molecular ion and the observed mass spectrum can only be made in those cases where secondary dissociations do not play a dominant role (50). Experimental results concerning predissociations of polyatomic positive ions in the gas phase have been reviewed and compared with predictions of the quasi-equilibrium theory (52). The kinetic-energy distribution observed for HCO + in the mass spectrum of isocyanic acid is consistent with formation by slow predissociation via a repulsive quartet state of the parent ion, whereas quantitative disagreement between the observed deflection curve and quasiequilibrium theory prediction for X H + may be due to the formation of some ions in an excited electronic state (53). Also, a spin-forbidden predissociation in the mass spectrum of isocyanic acid has been reported (54). Interpretations of the processes of ionization and dissociation of phenanthrene and methylphenanthrene upon electron impact (66)and predissociation in the dissociative ionization of formaldehyde have been made (66). The theory of electron-impact excitation and of the ionization of atoms and ions has been re-
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viewed (57-69). calculations have been carried out, using the quasi-equilibrium theory, to show that a decrease in penetration of the accelerating field into the ion source may significantly increase the number of primary daughter ions produced from a molecular ion in the source (60). The Woodward-Hoffmann rules for pericyclic reactions (61) have been shown to be a poor probe into the energy levels occupied by ions fragmenting in the mass spectrometer, at the present state of knowledge (62). One group claims that the photochemically excited state best describes the electronic configuration of the relevant species in the mass spectra of the aromatic systems discussed (65), although this is disputed (68). Another group took a perturbation-molecular-orbital (PMO) approach to the interpretation of organic mass spectra and divided reactions into three classes based on metastable observations and the classification of reacting ions as odd- or even-electron (64, 65). These articles (69-65) discuss and review the electronic relationships between mass-spectrometric, pyrolytic and photolytic fragmentations. In another study, vibrational modes, orbital symmetries, and unimolecular reaction paths have been related (66), but the authors foresee difficulties in eventual application to mass spectrometry, although PMO theory, which determines the basic conditions of their method, does seem useful (64). The PMO theory has also been used to interpret /%cleavage reactions in the mass spectra of heteroaromatic boron compounds (67) and hexahelicine rearrangements and other electrocyclic massspectrometric reactions (65). Ionization potentials of halogenoethylenes for which experimental data are available (68) and of mixed fluoro, chloro, and bromo derivatives of ethylene for which there are no experimental data (69) have been calculated by a simple MO method, and orbital energies and ionization potentials have been related (70). Ionization potentials, electron affinities, and screening constants have been compared with Hartree-Fock calculations (71). Formulas have been derived for calculation of ionization cross sections of atoms by electron impact (72) and applied to He, Li, Be, B, C, N, 0, F, a n d N e ( 7 3 ) ; and to Na, Mg, Al, Si, P, Si, C1, and A (74). The relationships among ionization yields, cross sections, and loss functions have been studied (76). Dissociation of molecular ions in 110 theory has been presented in relation to decomposition of C H I . + (76) and “3. + (77), as has MO theory for the mass spectra of aliphatic compounds (78). M O theory has been applied to the bond scission of mono- (79) and diamines (80). MO calculations on
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
carbonium ions have dealt with the methyl, ethyl, and vinyl cations and the series C J & + (81) and with a local guide to allowed interconversions of CdH,+ ions (82). ION STRUCTURE AND FRAGMENTATION
MECHANISMS
Methods currently being used to study structures of ions and mechanisms for the production of positive ions have recently been critically reviewed (83). The review concentrates on possible pitfalls of energetic considerations, on aspects of the kinetic approach which uses metastable-peak characteristics and substituent effects, and on the care required in the interpretation of labeling studies. Earlier reviews had covered the situation prior to 1968 (37, 84). The factors influencing [A+]/ [M+] ratios a t the collector for M+-, A + have been established (85, 86-87) and reviewed (88). These factors include (88) the rate constant kA for fragmentation and its variation with internal energy E (89, 90), the fraction of M + not having the energy to fragment to A+, secondary decompositions of A+, and competitive decompositions from 1I+. These factors have been used to make calculations and qualitative predictions of the mass spectra of monoand p-di-substituted benzenes, with good consistency in most cases with experimental data (88). A transition-state probe designed t o distinguish between rate differences due to ground-state energy differences, as against transition-state energy differences, has been proposed (91). For m- and p-substituted isomers, this approach is a kinetic evaluation of the difference in appearance potentials for the m- and p-substituted daughter ions being produced. This approach has been applied (91-93). Diastereotopic hydrogens in (S,S)-and (S,R)-2-deutero-4-chloropentane have been distinguished in the losses of HCl, DCI, and C1 from the molecular ions ( 9 4 ) ; this technique serves as a probe into the transition state for the elimination. Other factors concerned with relative ion abundances and correlations include variation with accelerating potential (96, 96), effects related t o Stevenson’s rule (97) in spectra of alkyl halides (98), and the preferential loss of small us. large radicals in the a-fission of aliphatic amines (99). All fragments in the mass spectra of a number of bifunctional aliphatic compounds can be deduced from the molecular-ion species, as long as low electron energy and low ionsource temperature are used (100). Determination of AH, of dissociation products is said to be essential to the understanding of most gaseous ionic phenomena (101). Excess-energy measurements, i.e., the excitation energy of
R1R2+ above that necessary to form
R1+ + RZin
their ground state, are necessary for studying appearance potentials of fragmentations; when excess energy is included, the method is capable of yielding good values for AH, of radicals and ions (101). Structures can be assigned to ions on the basis of heats of formation (101, 102). The theoretical basis for comparing the structure of ions b y measurement of the relative abundance of metastable ions has been examined (103). Equations have been derived, using some of the postulates of the quasi-equilibrium theory, for reactions taking place from the same state of the reacting ion and from different states. The use of ratios of metastable-peak abundances has been discussed in relation to the question of decompositions from isolated electronic states (104). The spectrum of ion energies emerging from the electrostatic sector of a double-focusing mass spectrometer gives more detailed information about fragmentation processes than the mass spectrum or the metastable-peak spectrum alone (105), although this technique is new and has not been applied. ii brief discussion concerning the effect of different internal energies of common precursor ions on the further fragmentation pattern has been presented, followed b y a comparison of the relative rates of decomposition of some common ions generated via fragmentation and via direct ionization (106, 107). Substituent Effects. Work on substituent effects in t h e mass spectra of aromatic compounds prior to 1968 has been extensively reviewed (lot?),and an attempt has been made to enumerate basic factors (87). Relationships are looked for, similar to those found in solution chemistry, based on the Hammett equation (109). The kinetic approach to mass spectra has been critically evaluated (83, 110). I n one case, a Hammett correlation in mass spectrometry has been shown t o be due t o correlation of the ionization potential with the substituent constant and not to the mechanism of bond cleavage (111). Substituent effects have been studied on the McLafferty rearrangement of methyl 4-phenylbutyrates (112), in the mass spectra of some cy- and 0-substituted methyl butyrates (lis), and in the M-42 rearrangement of n-butylbenzenes (114). Mass spectra of substituted benzyl phenyl ethers (115, 116) and substituted toluenes (116) also show characteristic substituent effects. Other classes of compounds studied are substituted ethylbenzenes (117), acylacetanilides (118), acetanilides and phenyl acetates (119), Schiff's bases (120), and anilides of benzoic acids (121). Linear free-energy relationships involving ortho substituents have been
studied (126). Wide range electronenergy kinetics and metastable-ion relative-abundance techniques have been evaluated and used (116, 117). Proximity effects in the mass spectra of aromatic carbonyl compounds containing adjacent methoxy (123) and ethoxy (124) substituents have been observed. However, peri-interactions do not exist in the mass spectra of 1alkylnaphthalenes or 4-alkylquinolines (125). The steric effect has been developed a s a mechanistic tool in massspectrometric decompositions (126, 127), and the influence of steric inhibition of resonance on ion intensities has been noted (128). Ion Structures. T h e heat of formation of C H O + has been determined (129), and AH, of CH30CO+ has been redetermined for use in determining stabilization energies of cations (130). The structures of C 2 H 5 0 +in the mass spectra of 2-alkanols (131), of C4H40. + from %pyrone (132), and of YC&O. + from alkyl aryl ethers (133) have been studied. The p-fluoro labeling technique has been developed for the study of symmetry in gaseous ions containing aryl groups (134). The technique was used to demonstrate distorted tetrahedral symmetry, rather than cyclobutadiene, or other, in the (14-CO) ions in the mass spectra of tetracyclones (135), in the (M-2CO) ions of tetraphenyl-p-benzoquinones ( I % ) , and in the (M-SO2) ions of tetraphenylthiophene-1,1-dioxide (137). However, it seems quite certain, from a theoretical comparison of unionized tetrahedrane and cyclobutadiene by ab initio techniques, that cyclobutadiene is substantially more stable than its tetrahedral isomer (138). The structure of C6H7+ ions from 1methyl-~~C-cyclohexadiene-1,3 and -1, 4 and l-methyl-2,4-cyclohexadienehas been studied (139). The rearrangements of benzylic ions to tropylium ions in the mass spectra of some methylarenes and methylheteroarenes have been characterized (140); fluorinated tropylium ions (141) and the question of the nitrotropylium ion (142) have been discussed. I n studies of ion-molecule reactions in aromatic hydrocarbons, C7H7+ from toluene and xylenes adds to the neutral parent to form a dimeric ion (143, 144). Similar dimers from C7H7+ and ion-molecule reactions occur in the gas-phase radiolysis of alkylbenzenes; perhaps a rapid variation of the structure between tropylium ion and benzyl ion occurs and the benzyl ions react with the aromatic compounds to form the dimeric product (145). A paper has appeared on the structure of of CEHQ+from C ~ H S C Z Hcompounds ~X and C ~ H Ufrom + CSHSCBHBX (146). Using ratios of metastable-peak abundances, it was shown that saturated CF,+ and CE+ ions from different sources
isomerize to a common structure prior to reaction via olefin elimination (147). On the other hand, the mode of formation of C6&*+, whether directly or by fragmentation from benzenechromium tricarbonyl, affects its fragmentation pattern (148). Charge localization and charge migration in ions formed upon electron impact have been compared (149, 160), and a comparison of charge-localization and the quasi-equilibrium theories has been made (161). I n the mass spectra of pand m-amino-n-butylbenzenes, fragmentation paths can be accounted for by assuming the molecular ions have the radical electron in their highest occupied molecular orbital (168). The role of localized charges in the contrasting behavior of singly- and doubly-ionized molecular ions has been elucidated (153). Generally, when a fragmentation occurs, the positive charge remains on the fragment with the lower ionization potential (154). Evidence for interionic reactions in the mass spectra of diamino steroids has been said to strongly support the charge-localization theory (155). Evidence for localization of the radical site and its influence in skeletal rearrangements has been presented from the mass spectra of triphenylcarbinols (156). Rearrangement Reactions. A comprehensive review of t h e literature through late 1968 applies current concepts from t h e theory of mass spectra to rearrangement processes (167). Energetics and kinetics of bond formation, as well as classification of rearrangements are covered. This subject has undergone intensive investigation because of its importance in structure determination and for the information it yields on problems of ion structure and fragmentation mechanism. This review (157) contains 187 references; these will not be repeated here. Rearrangements for ions fragmenting in the metastable drift regions are strongly favored (158). I t is possible to minimize these rearrangement paths which have low activation energies by forming the ions with higher energies; higher energies have been obtained with C7H7+and CzHsO+,by collisions of the ions with neutral molecules in the metastable drift region (159). Aryl participation in the expulsion of B r . from the molecular ions of p-phenylethyl bromide and eleven of its ringsubstituted derivatives (160) and in the secondary decomposition of the evenelectron p-phenylethylaminomethyl cation (161) has been suggested. When the functional groups in certain bifunctional compounds are at the ends of a polymethylene chain, the groups fragment in a manner independent of the chain length. I t is suggested, in regard to bifunctional compounds containing trimethylsilyl groups,
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that charge transfer involving the heteroatoms is responsible for maintaining the heteroatoms in close proximity, thus resulting in coiling of the polymethylene chain (162, 163). Migration of electronegative groups from carbon to silicon in the mass spectrum of methyl 3-trimethylsilylpropionate (164), migrations of trimethylsilyl groups in steroids (165), rearrangements of trimethylsilyl ethers of aliphatic glycols (166), and methoxy-group migrations in the mass spectra of methyl ethers of linear and branched aliphatic polyalcohols (167) have been observed. The structural requirements for concerted alkyl and hydrogen rearrangements in ortho substituted diphenic acids have been elucidated (168). Oxygen rearrangement processes in the mass spectra of aromatic and unsaturated esters have been reported (169, 170). Other compounds in which rearrangements have been reported are 9,lOanthraquinones (171), acetylenic compounds (172), N-acetyl-0-aryl carbamates (I???), a-trifluoroacetaminocarbonium ions ( 1 7 4 , diphenyl pyrazoles and isoxazoles (175), alkyl and aryl isoxazoles (176), and N,N'-dimethyl1,2,4-phosphadiazetidin-3-ones(177). Skeletal-rearrangement processes in the mass spectra of organosulfur compounds have been reviewed (178). Rearrangements have been observed in the mass spectra of P-(alky1thio)propionic acids and esters (179, 180), substituted thioglycollic acids and esters (181), phenyl vinyl sulfides (182), and methyl vinyl sulfoxides (183).
Labeling Studies and Hydrogen Rearrangements. Isotopic labeling continues t o be used routinely for studying fragmentations in the mass spectrometer. Deuterium labeling is used most frequently; however, I3C, 14C,and 1 8 0 , as well as 19F ( 1 3 4 , have also been used. Rearrangements of l-phenylheptenes (184, carbon-skeletal rearrangements of butene ions (185), of aniline (186), of l-(a-thienyl)hexane (187),of toluene (188), of five classes of trityl compounds (189), of o-terphenyl (190), of aromatic amines (191), and fragmentations of methyl esters of normal long-chain monoacids (192) have been studied with 13C-labeling. A method for studying I4C-labeled compounds has been described; a mass spectrometer with a focal plane produces two different mass spectra, a normal one showing all mass lines and a second one, obtained by autoradiography, showing only 14C-containing lines (193). Another laboratory has studied 14C-labeled salicylic acid and adenine b y isotopic-abundance determination (19.4). The mass spectra of some biphenyl and dibenzofuran derivatives have been interpreted with the help of 180-labeling (195). A study of various specifically deu-
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terated aliphatic ketones has shown that H/D scrambling is much slower than single-bond cleavage processes at 70 eV; however, ions decomposing in the first and second field-free regions of the MS-9 double-focusing mass spectrometer show extensive scrambling prior to all the various decompositions (196). I n the mass spectra of methylpyridines and quinoline, a t all beam energies, complete H/D scrambling occurs in the molecular ions prior to loss of HCN, whereas, at high beam energies, CHs. is lost prior to scrambling (197). Hydrogen randomization pre cedes the formation of species formed by loss of a hydrogen atom and of a methyl radical from the stilbene molecular ion at 15 eV (198). These and other papers dealing with the question of hydrogen randomization emphasize that care must be exercized in the interpretation of mass spectra of deuterium-labeled compounds (83, 157, 199-203). For example, because of extensive rearrangement, mass spectrometry is not a useful tool for differentiating double-bond isomers in l-phenylheptenes (184). Ring size has been found to play a crucial role in determining the ease of hydrogen migration in the mass spectra of 2-(1 '-octy1)cyclohexanone and deuterated analogs (204). Hydrogen transfer via an 8-membered transition state has been observed in mass spectra of vinyl ethers (205) and 13-, or more, membered in spectra of a-substituted tetrahydrofuran derivatives (206). Also, specific hydrogen rearrangement occurs in the mass spectra of o-alkoxyanils (207). I n the molecular ions of y-phenylpropanol and deuterated analogs, a mutual exchange occurs between hydrogen atoms from OH, from the y-methylene group, and from o-positions of the aromatic ring (208). Specific exchange also occurs between the a- and o-hydrogens in the molecular ions of y-phenylpropylbromide (209). I n porphyrin derivatives, the McLafferty rearrangement occurs preferentially in even-electron systems (210). The p-fluoro label (134) has been used to study scrambling of the heavy atoms of the heterocyclic ring in tetraphenylfuran and tetraphenylthiophene (211) and to show that the phenyl carbon skeleton remains intact, even though deuterium labeling shows scrambling of hydrogens on the skeleton, in the fragmentation of pentaphenylcyclopentadienol (212). Isotope effects have been observed during fragmentation of C2HZ.+ (213), during the dissociation of HzCO. +, HDCO . +, and D 2 C 0 + by electron impact ( Z 1 4 ) , and during fragmentation of CH4 * +, CD4 +, CDaH +, CzHs * +, CzD6' +, and C2D5H.+ formed by charge exchange (215). The statistical theory of mass spectra was applied to calcula-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
tion of the H/D isotope effect in the last case, and the results were compared with the experimental data (215). Fragmentation mechanisms and deuterium isotope effects have been studied in the mass spectra of bis-1,3-dithiolanes and bis-1,3-dithianes (216). Kinetic isotope effects for expulsion of deuterium from the 4-position and transition from quantal to purely classical isotope effect are reported in the fragmentation of N-alkyl-3-cyano-1,4-dihydropyridine 4-d (217 ) .
Structural Isomers and Stereoisomers. I n general, mass spectra of stereoisomers are qualitatively similar and differ little quantitatively. Hence, care niust be taken when one assigns stereochemistry on the basis of mass spectra. T h e theoretical understanding of ionization and unimolecular ion decompositions is not a t a state advanced enough to allow prediction of the differences. A review of stereoisomeric effects on mass spectra covers the literature through mid-1968 (218). Differences in ion intensities, even though modest, in the spectra of geometric isomers can often be correlated with differences in geometry and hence furnish a basis for assigning structures (218). Common product ions from stereoisomeric hydrocarbons seem often to arise via a common transition state. The spectra of stereoisomers can be simplified and made more distinctive by lowering the source temperature and ionizing voltage. I n the mass spectra of the four stereoisomeric l-methyldecalins and the four 2-methyldecalins, the order of relative intensity of (MCH3.) parallels the order of relative stability of the molecules (219). Work on the 1,2-diphenylcyclobutanes suggests that mass-spectrometric investigations of isomeric cyclobutanes may provide a means for establishing stereochemical assignments which would be otherwise difficult (220). A proposal has been made to account for the large stereochemical bias against cis isomers for elimination of HzO from the epimeric molecular ions of both 4- and 3-t-butylcyclohexanol (221). Ionization efficiency curves have been applied to some stereochemical problems in steroid and terpenoid systems (222). The fragmentation of the hydroxy-I-amino-3 (5a)pregnane diastereomers has been described (223). Other studies include observation of C,H,+ in the mass spectra of stereoisomeric pentacyclotetradecanes ( 2 2 4 , of the benzoic ester stereoisomers of 2-methyl- and 1,2-dirnethyl-4-hydroxydecahydroquinoline (225), and of carane-3,4-diol stereoisomers (226). Kinetic energies of fragment ions from pentane isomers have been determined (227). Energetic and metastable data were used to construct ionization and fragmentation schemes
for three C6Hs isomers (228) and for seven CsHs isomers (229). Heats of formation of ions were determined; a t least in most instances, ions of the same composition from all of the isomers must pass through a common intermediate and probably are formed with the same structure and AH, (228,229). hfono- and bicyclic C7H12isomers (230) and CIOHl6 substituted cyclobutanes and cyclohexenes have also been studied (231). Evidence has been presented showing that conformation of the olefinic side chain in trans- and cis-l-propenylcyclohexenes markedly influences the course of the retro-Diels-Alder fragmentation (232). hlass spectrometry can differentiate anti and syn isomers, as illustrated in the fragmentation of geomttrical isomers of 4,5,6,6a, 6b,7,8,12b-ocl ahydrobenzo [ j]fluoranthene (233). Neutral Product Stability. The stability of small neutral fragments expelled upon fragmentation often provides a n important driving force for rearrangement (234). This concept has been corroborated in a study of the McLafferty rearrangement in a series of compounds of the type PhCOCHzXCHz R (235). A novel elimination of COZ from molecular ions of syn and anti dimers of lJ4-naphthoquinone via a transannular oxygen rearrangement has been reported (236). Other authors have emphasized losses of stable or novel neutral fragments in mass spectrometry (237-240). Comparative Studies. Attempts have been made to discuss electronic relationships among mass-spectrometric, pyrolytic, and photolytic fragmentations in theoretical terms (616 7 ) . Mechanistic relationships between mass-spectrometric and photochemical processes have been discussed and reviewed (157). The course of the -y-hydrogen rearrangement in 2-pentanone has been followed by nonempirical hIO calculations, assuming a sixmembered cyclic transition state; a rationale is given for the parallel course of the reaction under electron-impact and photochemical conditions (241). Opposing views, based on 510 calculations, are held on the stepwise us. concerted nature of the process comprising yhydrogen migration and 0-cleavage upon electron impact (65, 241). A cyclobutanol intermediate has been proposed in the decomposition of aldehydes under electron impact (242, 243), suggesting (242) that hydrogen migration occurs as a discrete primary step in analogy to the Norrish type I1 photolysis. Other comparisons in the area of yhydrogen migration under elertron impact and photolysis have been reported (244-246). The mechanism of ketene formation from cyclohexanones upon electron im-
pact correlates with photochemistry (247). Correlations and discrepancies between the mass spectra and photochemistry of bicyclic cyclopropane derivatives have been pointed out (248). A close correlation between cation radicals and T,T* excited species, leading to a retro-Diels-Alder reaction, has been observed from styrene-anthracene and indene-anthracene adducts (249). Comparisons have also been made with the dimethyl acetal of o-nitrobenzaldehyde (250), with trans-cinnamimide (261), with azanaphthalene N-oxides (252), with isoxazole (253), and with 5-phenoxy-1-phenyl-1H-tetrazole(254). Numerous references to analogies existing between pyrolytic and electronimpact induced reactions of organic compounds are contained in a paper concerned with such analogies in S-methyl xanthates and esters (255). Pyrolytic decomposition and mass-spectrometric studies have been compared for the cyclic anhydrides of some P-sulfocarboxylic acids (256) and with 1substituted 3-phenyl-2-thioureas (267). The pyrolyses of aromatic aldazines have been reported, and their similarities to and differences from fragmentation under electron impact noted (258). Other series studied are several azobenzenes (259), some aromatic imides
ORGANIC COMPOUNDS
Perusal of a journal which publishes papers on organic topics provides a convincing reminder that mass spectrometry is now being used routinely by organic chemists. Several years ago, mass-spectrometric data did not appear in these journals. Then, such data appeared in papers devoted to studies in mass spectrometry and in papers in which mass spectrometry played a key role. Now, mass-spectrometric data also appear in experimental sections as data on new compounds even though they are not used or discussed; these data are usually difficult to locate since they are not indexed or abstracted. Journals also accept molecular weights and formulas determined by mass spectrometry, under certain conditions. The number of publications containing mass spectra of organic compounds is large. Since all of them can not be covered in this review, typical references in a variety of areas will be cited as examples. If one wishes t o check on a particular compound or class of compounds, abstract journals and, in particular, the Mass Spectrometry Bulletin ( d ) , as well as journals on mass spectrometry (3-5) and compilations of mass spectral data (6-9) can be consulted. I n several reviews and articles on mass spectra of organic compounds, (260), 1,1,4,4-tetramethyltetralin-2,3new developments in the applications dione and related compounds (261), to structural analysis have appeared and I-bromo- and 1,l-dibromo-2,3(268-276). A conference in Hamburg dimethylcyclopropanes (262). I n the (277) and the 16th and 17th Annual last case (262),both reactions appear to Conferences on hfass Spectrometry and be consistent with the stabilities of the Allied Topics (278, 279) have been intermediate allylic cations expected reviewed. for a concerted disrotatory process (61). ilpplications to organic chemistry I n an inorganic example, intermediates range from mass spectrometry in the in the pyrolysis and mass spectra of analysis of reactions, to studies of groups chlorine monoxide and chlorine heptoxof related known compounds, to deide have been studied (263). Close termination of structure of unknown parallels between mass spectrometric compounds, to mechanistic studies. and pyrolytic decompositions have been An example of each of these will be rationalized as reflecting parallelism of cited: Lhe formations of tetrachlorobond energies and vibrational modes in benzyne and four dichlorobenzynes, vibrationally excited neutral molecules highly reactive intermediates, have been and their ionized counterparts (261). detected by high-vacuum pyrolysis of The photoionization and electronthe corresponding 1,2-diiodobenzenes impact mass spectra of exo- and endoor phthalic acid anhydrides in a mass norbornyl bromide and exo- and endospectrometer (280); the mass spectra of 8-bromobicyclo [3.2.l]octane show that eighteen 2-aryl-l,3-dithianes and -1, exo-Br loss is more facile than endo-Br 3-dithiolanes have been recorded, and loss; these data are compared with fragmentation schemes outlined (281); solvolysis data from the same systems the mass spectra of substituted [2.2] (264). The pinacol rearrangements of paracyclophanes a t low voltages show quinoline analogs of benzopinacol have predominant fragmentation to the subbeen studied and evidence presented stituted p-xylylene ion radicals and, for the rearrangement under the conconsequently, they have been used for ditions of electron impact (265). I n the determining the number of substituents radiation chemistry of propanol, coron each ring of polysubstituted [2.2] relation with mass spectrometry has paracyclophanes (282); and, finally, the been found (266). mass spectrum of CH&HZCHZCOOD shows no loss of C2H3D,indicating that Mass-spectrometric fragmentation the McLafferty rearrangement is conpatterns have proved useful in guiding certed or that olefin loss is very fast in along new lines experimentation into comparison to the transfer of hydrogen reactions of aromatic compounds a t back to the methylene radical (283). high temperatures (267). ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
173 R
Possibly erroneous conclusions can be reached because of pyrolytic or catalytic transformations in the inlet system or ion source prior to ionization (684). For example, when some quaternary nitrogen compounds are heated, their mass spectra are not obtained; the mass spectra of the products of a-attack, of Hofmann elimination, and of dealkylation to a norbase are observed (285). Carbonyl Compounds. T h e p , y cleavage of carbonyl compounds has been discussed as a universal fragmentation reaction in mass spectrometers (286). Doubly-unsaturated carbonyl compounds (287) and monocyclic polycarbonyl compounds (288) have been studied. The mass spectra of straight-chain aliphatic ketones (289), of acyclic a,@-unsaturated aldehydes, ketones, and esters (290), of cyclic p-diketones (291), of a$-epoxyketones (292), and of diazoketones (293) have been reported. On the mechanistic side, the loss of H20 from molecular ions of aliphatic ketones (294) and the effect of fluorine substitution on the McLafferty rearrangement of aliphatic ketones (295) have been studied. Papers have reported the mass spectra of phenylhydrazones (296, 297), nitrophenylhydrazines (298), nitrophenylhydrazones (298, 299), 2,4-dinitrophenylhydrazones (300, 301), thiosemicarbazones (302), dimedone derivatives of aldehydes (303, SOg), and Schiff bases (305). The mass spectra of 1,2-naphthaquinones (306),ubiquinones (SOT),dunnione and hydrodunnione diacetate (308), and 1-arylanthra-9,lO-quinones (309) have been discussed. Under electron impact, a double hydrogen migration occurs in the mass spectra of adducts of p-benzoquinone with bi1-cycloalken-1-yls (310). I n one system, keto-enol tautomerism was shown to be absent in fragment ions of cyclic p-diketones (311). Fragmentations of enol derivatives of acyclic /3-diketones (312), of trimethylsilyl derivatives of enol ethers (313), of nonenolized a-diketones ( 3 1 4 , and of benzocycloalkenones and their enol acetates (315)have been described. An interesting McLafferty rearrangement occurs in the mass spectra of some a-lactams (316). The mass spectra of spiro-0-lactams have been reported in conjunction with their synthesis (317 ) . Mass spectrometry can be used to characterize substituted &lactones (318) and branched-chain aliphatic lactones (319). The source of (RI-H) has been cleared u p in the mass spectrum of N,N-dimethylbenzamide (320). Mass spectra of cyclic esters of aliphatic ahydroxy acids (321), of p-thioketoesters (322), of glycerolacto esters (323), and 174R
the electron-impact induced reactions of &hydroxyamides (324) have been of a series of methyl styryl sulfoxides, reported. extensive structural rearrangement ocThe fragmentation mechanisms of curs in the molecular ion (357). maleic and fumaric acids and related compounds have been compared (325). Mass spectra of alkyl methanesulThe mass spectrum of 3,4,4-trimethylfonates (358), of 3,5-bismethylene-1,2, 5-oxo-trans-2-hexenoic acid has unusual 4-trithiolanes (359),of saturated and features (326). The ionization and unsaturated derivatives of thiacyclodissociation of formic acid, acetic acid, hexane and 4-thiacyclohexanone (360), propanoic acid, and butanoic acid have and of P-thioketothiolesters (361) have been studied using labeling and metabeen reported. Some aspects of the stable peaks (327). On the more theohigh resolution mass spectra of tolueneretical side, scission probabilities of p-sulfonamides have been noted (362). skeletal bonds in the mass spectra of Mass spectrometry has been used to alkyl acetates have been deduced (328). study the mechanism of the ring-cleavTwo papers report the fragmentations age reaction of malonaldehyde bisof tetronic acid and related compounds thioacetals (363) and to decide the (329, 330). structures of glyoxal bisthioacetals (342, Alcohols and Ethers. Mechanistic 363). studies on the elimination of H20from Mass-spectrometric methods of anat-butylcyclohexanols have been publyzing cyclic sulfides have been reported lished (221, 331). The mass spectra of (364,365),as they have been for various cyclohexanediols (332, 333), a,p-unsulfides and disulfides (366,367). Fragsaturated secondary alcohols ( 3 3 4 , mentations of some cyclic sulfites and a- and p-decalols (335),synthetic and carbonates include “ketone-like” and natural dimethylcarbinols (336), ali“epoxide-like” (M-CO2) and (M-S02) phatic 1,2-glycols (337), and branched ions (368). The mass spectra of dialkyl long-chain aliphatic alcohols (338) have sulfites (369), conformational isomers been discussed. The stepwise fragof 2-methylpentane-2,4-diol sulfite mentations of secondary and tertiary (370), and cyclic sulfones (371) have alcohols of higher molecular weight also been studied. The mass spectra of have been carefully studied (339). non-enolizable thioketones have been Reports on the mass spectra of alkylated compared to those of the corresponding bisphenols (340) and of benzofurans ketone and diazo compounds (372). (341) have been published. The mass spectra of dibenzothiophene Mass spectrometry has been used, (373), phenylthiophenes (374), and along with other data, to attempt to deuteriothiophenes (375) have been assign a structure to the ethylene glycol studied in conjunction with pyrolysis diacetal of phenylglyoxal (342). Studies experiments (267, 376). Some other on the ionization and dissociation of sulfur-containing aromatic heterocycles cyclic ethers (343)and the scission proband their sulfoxides and sulfones have abilities of the skeletal bonds of alibeen investigated (377), as have dephatic ethers (344) are of theoretical rivatives of thiazole (378), isothiazoles interest. A new fragmentation re(379581), benzothiazole (382), derivaaction was reported from the mass tives of benzisothiazole-S-dioxide(3837, spectra of cyclic acetals of alditols that and substituted oxathiolanes (384). contain the 1,3,6,8-tetraoxabicycloNitrogen-Containing Compounds. [4.4.0]decane ring system (345). The T h e fragmentation behaviors of isomass spectra of 5,6-dihydro-a-pyran pentyl cyanide a n d deuterium analogs derivatives (346),of alkyl tetrahydro(385),of n-alkyl cyanides (386, 387), pyranyl ethers and thioethers (347), of and of aromatic isocyanides (388) have some cyclic ethers (348), and of tribeen studied. Also investigated are phenylmethyl ethers (349) have been nitroso compounds (389), nitrate esters discussed. The mass spectra of organic ($go), nitrite esters (391), aliphatic peroxides (350, 351), of alkyl hydropolynitro compounds (392), and azoperoxides (352),and of ozonides (353) methane (393). The source of the loss have been investigated. of H20 from a nitro compound has been Sulfur-Containing Compounds. elucidated (394). Fragmentation reactions of sulfur The reduction of s-tetrazines to dicompounds have engaged many workhydro-s-tetrazines can occur in the ion ers. T o illustrate this point, a review source of a mass spectrometer (396). of skeletal-rearrangement processes in A mass-spectrometric study is part of the mass spectra of organosulfur comthe reassignment of structure of the pounds contains 118 references (178). dihydro-v-tetrazines (396). The mass Chemical-bond energies and other spectrum of an N-isothiocyanatodithermodynamic quantities have been methylamine dimer helped confirm the obtained from appearance and ionizapresence of the 1,2,4-triazole ring (397). tion potential data on fragmentation of The mass spectra of some azepines obaliphatic sulfur compounds (354). An tained from dimethyl acetylenedicarunusual loss of CH2 from the molecular ion of thioxanthene was reported (355), boxylate (398) and the catalytic production and mass spectra of N-hydroxybut this was later disputed (356). I n
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
alkyl-N-N-bis-arylamines (399) have been reported. The mass spectra of benzotriazinones and the first loss of 28 maes units (400), and competitive loss of C2H4 and CO from molecular ions of 1-tetralone (401) have been subjects of investigation. Among nitrogen heterocycles studied by mass spectrometry are simple indoles (402), aromatic aldazines and ketazines (4OS), acridine N-oxide (404), substituted phenazines and quinoxalines (405), quinoline and isoquinoline N oxides (406, 407), cinnolines (408), 1,2, 4-oxadiazolines (409), alkylquinolines (410), some di- and triazaindenes (411), 1,4-dihydropyridines (412), alkyl isoxazoles (413), and many others. An S ~ 2 - t y p efragmentation has been observed in the mass spectra of substituted AT-alkylpiperidines (414). The mass spectra of lli-azepines have been reported in a paper dealing with a general synthetic entry into derivatives of 1H-azepine (415). The mechanisms of fragmentation of nitrones were explored (416). Reports on the identification of amines as trifluoracetamides by mass spectrometry (417) and the mass spectra of derivatives of phenylalkylamines (418)have been published. Halogen-Containing Compounds. Two studies are concerned with ionization and dissociation processes in hexafluoroethane and in l,l,l-trifluoroethane and fluoroform (419) and in fluoroacrylonitriles (420). N-Aryl (421) and LV-alkyl (422) trifluoracetamides have been used in making volatile derivatives for mass spectrometry. Mass-spectrometric fragmentation schemes for some fluorinated alkanes have been worked out (423,424). Mass spectra of 1,1,1,2- and 1,1,2,2-tetrafluoroethane (425), of polyfluorinated aromatic compounds containing a carbonyl group (426), and of fluorinecontaining dimethyl esters (427) have been studied. Low-voltage behavior of some aromatic fluoro compounds shows that rearrangements dominate, whereas, simple cleavages provide the major ions a t 70 eV (428). Wagner-Xeerwein rearrangements occur very readily in the mass spectra of several methylnorbornyl chlorides, and compounds related by these rearrangements give very similar fragmentation patterns (429). Dichlorocyclohexanes (430), monochloro alkylcyclohexanes (431), and 1,l-dichlorocyclopropanes (432) have also been studied. The fragmentations of polychlorinated bridgedring systems have been investigated (433)* Triphenylcyclopropenyl bromide, chloride, and, more surprisingly, fluoroborate are volatilized into a mass spectrometer without difficulty and afford information on the fragmentation of 3halogeno - 1,2,3- triphenylcyclopropenes (434). I n the mass spectra of halo-
genated phenols, elimination of CO or CHO competes with elimination of X or HX (456). The mass spectra of halogenated thiophenes have been reported (436). Hydrocarbons. Fragmentation of skeletal bonds of cyclic alkanes ( 4 3 9 , of hexane (438), and loss of C H I . and CHI in the fragmentation of n-alkanes (439), have been investigated. I n the mass spectra of dideuteriooctanes, the distribution of peaks representing the higher-molecular-weight ions can be predicted using a simple fragmentation model (440). The even-mass numbered peaks due to hydrogen transfer have been explored as a source of important information on the mass spectra of branched alkanes (441). The McLafferty rearrangement has been described as a principal fragmentation reaction of 1,l-di-, tri-, and tetraalkyl substituted ethylenes (442). Mass-spectrometric studies have been undertaken on some triarylethylenes (443) and some acetylenic compounds (444, 446). The stability of aromatic hydrocarbons under electron bombardment (446) and the structures of (M-43) ions in the mass spectra of substituted nbutylbenzenes (447) have been subjects of investigation. The (M-H) ion in the mass spectrum of triptycene arises by loss of hydrogen originally bonded to one of the aromatic rings, rather than from loss of a bridgehead hydrogen atom (448). The mass spectra of fulvenes (449), of 9,lO-dihydrophenanthrene (450), of some lignans of the l-phenyllJ2,3,4-tetrahydronaphthalene series (451), and of tricyclic hydrocarbons of high molecular weight (lubricating oil range) (452) have been reported. Other systems studied are CsHlz (453), mono- and bicyclo C6H10 ( 4 5 4 , 4- and 5-methylparaffins (465), and isomerization of some derivatives of cyclobutane (456). Hydrocarbon byproducts in the formation of carbon blacks have been studied by mass spectrometry (457). Two macrocyclic ring systems with CeSs and CsSlz rings have been investigated (458). Electron-attachment mass spectrometry has been used to investigate solid saturated hydrocarbons from slack wax (459).
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NATURAL PRODUCTS AND BIOMEDICAL RESEARCH
Applications of mass spectrometry in biomedical research, often in combination with other techniques, have been reviewed (460, 461). Also, the mass spectra of a large variety of complex biological substances, from plastoquinones to peptidolipids and mycosides, have been reviewed (462). A few interesting applications are: mass-spectrometric studies of laC-variations in
corn and other grasses (465); massspectrometric gas analysis in asthma (664); studies on sterols in vernix caseosa, amniotic fluid, and meconium (465); and the use of spark source mass spectrometry for trace element survey analyses of biological materials (466). Other applications are: a study of the compositioi L of spermaceti by mass spectrometry (467); identification and estimation of choline derivatives by mass spectrometry (468); and identification of some hallucinogenic drugs (469)* A tentative structure has been assigned to antheridiol, a sex hormone in Achlya bisexualis, on the basis of a highresolution mass spectrum of a few micrograms of the hormone and spectra of derivatives (470). Mass spectrometry has helped identify a novel glycoside of an insect-metamorphosing substance (471). These examples illustrate a few ways in which mass spectrometry has played roles in natural product and biomedical research; there are many others, some of which are cited under the following subheadings. Metabolism. I n the field of pesticides, studies of metabolism of halogenated aromatic hydrocarbons by microorganisms (472) and structures of metabolites of aldrin and dieldrin (473) have been aided by mass spectrometry. Metabolism of prostaglandins has received considerable attention with the use of mass spectrometry (474-479). For example, the structures of the major urinary metabolites of prostaglandin E z in man (474) and in guinea pig (476) and of prostaglandin Fza in man (475) and in the rat (478) have been determined. 25-Hydroxycholecalciferol has been identified as a biologically-active metabolite of vitamin Ds (480). High resolution mass spectrometry was essential for characterizing chlordiazepoxide metabolites in the rat (481). Combined gas chromatography and mass spect)rometry were used to identify chloropromazine and its metabolites (482). Elucidation of a major pathway for the mammalian oxidative degradation of phytanic acid (485)and identification of some human metabolites of a progestational agent (484) have involved mass spectrometry. The mass spectrum of a new polyisoprenyl ketone from silkworm feces provided essential structural information (486). Mass spectrometry has helped to identify: urinary hippuric acid isolated by anion-exchange chromatography (486); 0-hydroxyisovaleric acid in the urine of a patient with isovaleric acidemia (487);small amounts of methamphetamine after passage through the body (488); 160-hydroxydehydroepiandrosterone in infant urine (489); and pyroglutamic acid as a peak in the gas chromatography of human urine
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
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(490). Twelve
fluoren-9-ones (491), and @-naphthylamine (492, 493), acenaphthylenes and macrocyclic polyolefins (494), and 9-methylcarbazoles (495) have been identified as constituents of cigarette smoke by mass spectrometry. Pesticides. Xetabolism studies (478, 473) of pesticides have been mentioned above. High resolution mass spectrometry has been used to identify pesticides in mixtures (496). Other studies include mass spectra of some chlorinated pesticidal compounds (497), and of some carbamates and related ureas (498). Antibiotics. A mass-spectrometric study of natural mixtures of ennatinic antibiotics (499) and studies on antiviral and antitumor antibiotics have been reported (500-502). Mass spectrometry has been used to identify new antibiotics from a mutant of Streptomyces fradiae (503). The trimethylsilyl derivatives of neomycins B and C and related compounds are volatile enough for mass spectrometry (504). A molecular-weight revision for compounds of the oligomycin complex relied on mass spectrometry (505). Chemical-ionization mass spectrometry has been used to assign a structure to the antibiotic botryodiplodin (606). Mass spectrometry has helped identify: autoantibiotics produced by a fungus (507); a nucleoside antibiotic (508); a novel chlorine-containing antifungal agent (509); and mycobactins from various species of mycobacteria CY-
(510).
Depsipeptide antibiotics (511) and the composition of antibiotic peptides (518,51S ) have been studied. A revised structure of a peptide antibiotic was established by mass spectrometry (514). The structure of beauvericin, a new depsipeptide antibiotic toxic to Artemia salina, has been studied by mass spectrometry via the lithium aluminum deuteride reduction of beauvericin (515). Peptides. One of t h e most exciting applications of mass spectrometry to natural products has been sequencing amino-acid units in peptides (516-519). An extensive review of the mass spectra of amino-acid and peptide derivatives has covered papers published to the end of 1967 (520). The structure of the antitoxic cyclodecapeptide antamanid has been studied by a combination of gas chromatography and mass spectrometry (521). A peptide from the variable part of normal immunoglobulin lambda-chains has been sequenced by chemical and mass-spectrometric means (522). Cyclopeptides have been examined (523). A new technique of N-methylation was employed in the peptide sequence analysis of feline gastrin (524). The advantages and limitations of the use of permethylated oligopeptide derivatives 176R
have been discussed (525-627). The structure of mycoside Cbl was studied after N-methylation (528), and permethylation has been extended to peptides containing arginine and methionine
Colombian arrow poison frog (664) have been determined. Classes of alkaloids studied are lycopodium (565), lolium (566), eburnaminetype (567), vobasine-type (568), benzo(529). pyrano [3.4g]indole series (569), alkaThe mechanisms of the fragmentation loids from voacanga africana (570), of peptides containing aromatic and tetrahydroprotoberberine (571), benheterocyclic amino acids have been zophenanthridine (572, 573), crypoutlined (530). Other discussions intaustoline-type (574), gentiana (575), clude sequence analysis of peptides 2-alkyl (aryl)-4-quinolines (576), lycontaining arginine (531) and containing corine-type (577), and others (578investigacystine and cysteine (532). Mass spec583). Mass-spectrometric trometry has been used to study the tions indicated that two benzodiazepine thermal degradation of proteins (535). alkaloids might be converted into two others by thermal degradation; in fact, ,4 comparison of the utility of various they do readily rearrange pyrolytically N-protecting groups has shown that by loss of methyl isocyanate as sugthe acetyl derivative provides the best combination of volatility and abundance gested by the mass spectrum (584). of "sequence" ions (534). The comMass-spectrometric analysis of quaterbined GC/MS of amino acid derivatives nary nitrogen compounds has been rehas been reported (535). A pyrimidine ported in conjunction with a study of derivative has been used for both the pleiocarpamine derivatives (685). Carbohydrates. The successes of gas chromatography and mass spectroapplications of mass spectrometry to metry of arginine in the presence of other amino acids (536). analysis of the sequence of amino acids in peptides led t o similar studies on the Investigators have used pentadeutetrasaccharide-antibiotic paromomyterobenzoyl-peptide methyl esters (537), ethoxycarbonyl-peptide methyl esters cin (586). The mass spectra of some trimet'hylsilyl ethers of di- and trisac(538), benzyloxycarbonyl derivatives charides show that' the sequence of (539), N-thiobenzoyl polypeptides (540), phthaloylamino acids (541), monosaccharide units, the position of phenylthiohydantoin amino acids (542), glycosidic bonds, and other details of structure can be elucidated by this and trimethylsilyl derivatives (543). method (587). Similar conclusions Transannular amide-amide interactions were reached from the mass spectrum in cyclopeptides have been reported under mass-spectrometric conditions of maltose octaacetate (588). This t'echnique has been applied in the identi(544). A pyrimidine amino acid has fication of new tetrasaccharide antibeen identified from pea seedlings (545), and 2,3-cis-3,4-trans-3,4-dihydroxy-~- biot'ics (503). Minor components formed in the proline has been characterized by mass trimethylsilylation of equilibrium mixspectrometry and X-ray analysis (546). tures of some monosaccharides and of the Structure determination of peptide products of glycosidation have been alkaloids by mass spectrometry has been determined to be furanose and furanodiscussed (547). A number of applicaside st'ructures by combined GC/MS tions have appeared (548-552). From (589). Studies on ring structures of a study of the fragmentation pattern of lSN-and lac-labeled desthio gliotoxin, ketoses have been undertaken in a similar manner (590, 591). Trimethylan antibiotic, it was concluded that silyl ethers have also been used to dephenylalanine can be incorporated into termine the number and position of gliotoxin by a t least two pathways and met'hoxy groups in methylated alglycine can serve as the source of both dopentoses (592) and aldohexoses (593) nitrogen atoms (553). Alkaloids. Research into the mass and to characterize aminosugars (594). Glycosides and 0-isopropylidene spectrometry of alkaloids over the ketals of deoxysugars (595) and partially past two years has followed the lines of methylated methyl glucosides (596) studies of new classes of alkaloids and have been characterized by mass specof new members of previously studied trometry. Uronic acids have been classes, and studies involving strucstudied free and as derivatives with ture determinations of new alkaloids. free hydroxy groups (597), as their 0Structures of fourteen novel Aspiisopropylidene derivatives (598), and dosperma alkaloids (554, 555), of voaas their methylated methyl ester methyl cristine hydroxyindolenine (556), of glycosides (599, 600). Degradation the hydroxyindolenine derivative of products of uronic-acid amides in the voacangine (667), of pancracine, a 5, Hofmann reaction have been investi11b-methanomorphanthridine alkaloid gated by mass spectrometry (601). (558), of two macrocyclic alkaloids Combined gas chromat'ography-mass (559), of rindline (560), of eripine, a spectrometry has been used to deternew indole alkaloid (661), of two isomine the structure of disaccharides as pavine alkaloids (562), of three tetrahytrimethylsilyl derivatives of disacdroquinolylimidazole alkaloids (563), charide alditols (602). Peracetates of and of a steroidal alkaloid from the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
unsaturated carbohydrates, of anhydrodeoxyalditols, and of a branchedchair sugar are suitable derivatives for mass spectrometry (603). Alditols have also been studied as trifluoroacetates (604,as triniethylsilyl derivatives (605), as methyl ei.hers (606), as benzylidene acetals (COY), and as 0-isopropylidene derivatives (608). Mass spectrometry has been discussed as an aid for determining structures of natural glycosides (609), for analysis of sugars in bacterial endotoxins (610), and for the study of cellulose derivatives (611). Several natural Cglucosyl compounds (612), derivatives of 2-substituted glyco [1’,2’:4,5]-2oxazolines ( G I s ) , substituted glucosylamines containing the indole nucleus (614), and glycosides of alkaloids (615) have been investigated. Nucleic-Acid Components. X sulfur-containing nucleoside from yeast t-RKA has been assigned the struct’ure 2-thio-5-uridine acetic acid methyl ester on the basis of its high resolution mass spectrum, chemical properties, and ultraviolet spectrum (616). Another sulfur-containing nucleoside has been identified as 5’-S-methyl-5’-thioadenosine (617). The identifications of 6-(3methyl- 2- butenylamino) -9-p-D-ribofuranosylgurine (618) and its 2-methylthio analog (619), as well as minor nucleosides (620),have been reported from
t-RXA. The trimethylsilyl derivatives of nucleotides, nucleosides, and bases have been studied by mass spectrometry (621). Amino groups, enolizable carbonyls, and sugar and phosphate hydroxyls are generally silylated, whereas, silylation of the bases of thymidine, 2’-deoxyuridine, and cytidene-5’-phosphate occur only to a small extent. Fragmentations of subst’ituted uracils (626),of thymidine (623),of pyrimidine and purine bases by a combination of paper Chromatography and time-offlight mass spectrometry (624)),of purines (626, 626), and of pteridines (627) have been characterized. Mass spectra of carbon-carbon linked nucleosides (628, 629) and of photoreduction and dimerization products of 1,3-dimethyluracil (630) have been reported. Steroids. Characterizat’ion of st,eroids by gas chromatography and mass spectrometry has been briefly discussed (631). Considerable work has been done using these two techniques in combination. For example, 3ahydroxyandrost-5-en-17-onehas been identified in human plasma, urine, and bile (632); and steroids have been identified in feces from rats (633435) and in rat, liver microsomes (636). Some neutral steroids have been found in human lymph chylomicrons (637), and 5cu-pregnane-3a,20c~,21-triol has been identified in human pregnancy plasma (638). Derivatives of aldosterone have
been developed for gas-phase analysis (639). Four androstene tetrols have been characterized from urine (640). Structure determination of a plant steroid has used mass spectrometry (641). lp- and 2P-hydroxytestosterone have been identified in the human fetal liver (642). Two 26-hydroxy compounds were isolated from two fermentation products of cholesterol with ;Mycobacterium smegmatis and mass spectrometry was used to help locate the hydroxy groups (643). Differences in the fragmentation of 5a-cholestane at 50 eV and a t extremely low electron energy have been interpreted (644). The fragmentations of androstane (645)and the course of the characteristic ring D fragmentation of steroids have been elucidated (646). Other systems studied are ethylene hemithioketals and ethylene dithioketals (647), steroidal lactams (648), adrenal cortical steroids and their lactonic derivatives (649),C-19-modified cholesteryl derivatives (650), sterols with unsaturated side chains (651), hydroxyprogesterones (662), pollen sterols (663), bicyclic and monocyclic models of the 20-keto-steroids (664), the monochloroacetate derivative of progesterone (655), and some steroidal A4- and A5-3-ketals (666). Mass spectra have been used to correlate the absolute configurations of the isomeric, optically active sulfoxides 36-hydroxy-20-thia-17 ( a and p)-pregn5-ene oxides (657) and to differentiate between 19-methyl- and 19-nor-steroids (658). &\lass spectrometry can be used to determine the configuration of steroidal alcohols (659-662),to locate hydroxy groups (663),and to locate double bonds (664). The influence of functional groups on the mass spectra of 17-p-hydroxy steroids has been studied (665). Lipids and Terpenes. T h e mass spectra of lipids have been reviewed (666, 667). Gas chromatography combined with mass spectrometry has been used to detect: the hydroxy acids and fatty acids of a 5000-year old lacustrine sediment (668);isoprenoid hydrocarbons and fatty acids in shark liver oil products (669); fatty acids of an alga (670); methyl and ethyl esters of long-chain fatty acids in ox pancreas (67‘1); and hydroxy fatty acids from cerebrosides of the central nervous system (672). A considerable amount of effort has gone into using mass spectrometry in the determination of double-bond positions in olefinic compounds (673-678). Derivatives of cyclopropene fatty acids (679) and of cyclopentenyl fatty acids (680) have been investigated. Diglycerides (681), phospholipids (682), and hydroxylated octadecanols from hydroxylated stearic acids (683) have been studied. Deuterium (192, 684) and 13C-labeling (192) have been utilized.
The fragmentation of methyl esters of fatty acids into methoxycarbonylalkyl ions has been investigated with 13Clabeling (685). The molecular mass spectra of triterpenes have been described (686). GC/ A I S has been used to quantitatively analyze mixtures of diterpene hydrocarbons (687). Other studies include hydrocarbons of the carane and menthane series (688), triterpenoid dehydration products (689), aromatic diterpenes (690),triterpenes related to serratenediol (691), custunolide and its derivatives (692), and two isomers of camphor (693). The mass spectra of synthetic ceramides (694) have been used to identify ceramides from human plasma sphingomyelins (695). Sphinga-4,14-dienine has been characterized from plasma sphingomyelin (696). Sesquiterpene lactones have been studied (697, 698). Other Complex Molecules. Other types of natural products st’udied are guanidines (699),phenothiazines (700704), lichen constituents (705-.709), coumarins (710-712), gibberellins (713716), vitamin 130 (717), bile pigments (718), and bile acids (719). Medicinal barbit,urates can be identified by mass spectrometry (720). The nature of the (A1 2) peaks in the spectra of chlorophyll and haemin has been investigated (721). The fragmentation reactions of carotenoids h a r e been surveyed (722,723). A carotenoid isolated from a discomycete has been studied by mass spectrometry (724), as have carotenoid ketones ( 7 2 5 ) . Flavonoids have been investigated extensively also (726-733). Odor and Flavor. Mass spectrometry is playing a n increasingly important role in the study of constit,uents of flavors and odors (734). Combined GC/MS is a particularly effective tool for studying multicomponent mixtures. For example, volatile components of milk-fat steam distillates have been identified by GC/J\IS i(735). Forty-two compounds have been identified as the major volatile components of the juice of the American cranberry (736); also identified were thirty-eight whisky components (737) and forty-six tomato-aroma constituents (738). Catty odors in food (739), volatiles in apple essence ( 7 @ ) , nutmeg oil (741), and pecan oils (742) have been itlentified by gas chromatography and mass spectrometry. Mass spectrometry was used extensively to characterize compounds from avocado pear (743). Xass spectra of bitter compounds representing a new family of degraded triterpenoids have been studied in detail (744). Some acids have been identified from the autoxidation of methyl linoleate (745) and as volatile decomposition products of hydrogenated cottonseed
+
ANALYTICAL CHEMISTRY, VOLi 42, NO. 5, APRIL 1970
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oil (746). Volatile flavor compounds of boiled beef have been isolated and identified (747). Studies into chemical communication in the insect world can employ G C I M S effectively (748). A 2,3-dihydrofarnesol has been identified as the main component of the marking perfume of male bumble bees (749).
effects of alkyl substituents (769). The abundance of fragment ions in the mass spectra of carbonyliron complexes have been related to structure (764). Decomposition of complexes of Schiff bases of aromatic aldehydes and ketones with ennecarbonyldiiron depend on the nature of substituents (766). The decomposition of [MC~HIO]+ in the mass spectra of metal acetylacetonates occurs by means of comMASS SPECTRA OF ORGANO-INORGANIC COMPOUNDS petitive eliminations of CzH4 and CO (766). Oxygen migration has been The main approach to the study of observed in the mass spectra of porganometallic compounds has been to ketoalkylidenephosphoranes (767). correlate fragmentation patterns of The 9-methyl-9-sila-anthracene ion gas-phase ions with structures deter(768), bridged phosphofluorenyl ions mined from crystallographic data. (769), tropylium ligands in the mass The main application has been to obspectra of substituted ferrocenes (770), tain molecular weight data. Three and “triple-decker” sandwich ions from excellent reviews of the mass spectra of cyclopentadienyliron carbonyl tetramer organo-inorganic compounds were pub(771) have been detected. lished in 1968 (750-752). Mass spectrometry has been applied The mass spectra of metal chelates to the study of redistribution reactions often are dependent on the temperature on boron, with exchange of alkoxy of the ion source, due to thermal reacgroups detected by this technique (772). tions in the mass spectrometer (753The low-pressure pyrolysis of borane 755). It has been recommended that carbonyl has been investigated, giving a direct-insertion probe, a range of the bond-dissociation energy of diborane source temperatures, and caution in (773). Also, the ionic-bond energy of interpretation be used (753). Pyrolyphenylboron dichloride has been desis-mass spectrometry has been used termined (774). Potassium, rubidium, to detect phenyl- and methylphosand cesium t-butylates have been inphinidenes from thermal decomposition vestigated by mass-spectrometric, as of cyclophosphines (756). I n the obwell as roentgenographic and nuclear tainment of the mass spectra of some magnetic resonance means (775). alkali-metal derivatives of P-diketones, Group IIIA. T h e analysis of boexchange reactions occurred in the inranes and carboranes by mass specstrument; even after pumping away trometry has been described (776). Evithe first chelate, addition of a second dence for the gasems A10CN molecule chelate brought ions of the first (757). has been presented (777). The mass During the evaporation of metal chespectra of substituted cyclotetrazenolates into the ion source, extensive boranes (778), of selected heteronurearrangements may take place and clear diborane[4] ring systems (779), complex reactions may occur, leading of some 1,3,2-oxazaborolidines (780), to the formation of new metal chelates of alkyl borates (781), of cyclic amine(758). Copper has been substituted boranes and their catechol derivatives by iron in some chelates by reaction (782), and of ethylthioborane (783) within the mass spectrometer (759). have been reported. Other elements A number of mechanistic and enerof group 111-4 which have been studied getic studies have been reported. are various organoaluminum compounds There is evidence that elimination of (784), aluminum isopropoxides (785, odd-electron fragments from even-elec786), and cyclopentadienylthallium tron ions is accompanied by a change (787). in the oxidation state of the chelated Group IVA. Pentafluorophenyl metal (760, 761). The ionization po(788) and vinyltriphenyl (789) derivatentials of tris(p-diketonato)metal(III) tives of group IV elements have been complexes are close to that of the free used for mass spectrometry. Ionizap-diketone, in conflict with predictions tion potentials of (CH3)JI radicals based on application of Koopman’s hade been determined (790). Pertheorem to the usual N O theory of phenylcyclosilanes and -germanes and these complexes (762). Metastable a perphenylcyclostannane have been peaks in the first and second field-free investigated (791). Rearrangements regions of a double-focusing mass spechave been observed in the mass spectra trometer suggest that an apparent oneof acyloxysilanes and -germanes (792). step loss of a ligand in the mass specMass spectra are reported for tetratrum of tris( 1,1,1,5,5,5-hexafluoropenmethylsilane (793), tetraphenylsilane tane-2,4-dionato)aluminum occurs, rather, in t\vo steps (763). Ion in(7941, tetrakis(3,3,3-trifluoropropynyl)tensities and ionization potentials of silane (795), compounds of the type (CH3).Si(OCH3)4_, (796), trispentasome copper(I1) chelates have been fluorophenylsilane and its silyl-metal rationalized on the basis of electronic 178R
ANALYTICAL CHEMISTRY, VOL. 42,
NO. 5, APRIL 1970
derivatives (797), and trimethylalkoxysilanes (798). The mass spectra of organogermanes have been described (799). Also studied are alkyl digermanium compounds (800) and some substituted germacyclopentanes and -pentenes (801). The dominant reactions of the digermanium compounds are explained in terms of the valency of the metal atom (800). Spark-source mass-spectrometric analysis of some chlorogermanium alkoxides has been undertaken for determination of the atomic ratio of halogen to metal as main components (802). Fragmentations have been described for tetraalkyltins (803). trialkyltin halides (804), aryltrimethyltins (805), trimethylstannanes (806), and tetrabutylstannane (807). Organotin ketones and esters have been studied by mass spectrometry (808). Group VA. N a s s spectra of fluoroalkylphosphorus compounds containing two or more phosphorus atoms (809) and of trifluoromethylphosphines and -halogenophosphines (810) have been presented. Other mass-spectrometric studies have been made on some substituted phosphines (811) and organic derivatives of phosphines and diphosphines (812). The energy of the phosphorus-phosphorus bond in diphosphines has been determined (813). Phenylphosphinidene and group VX analogs have been generated by electron impact (814). Other reports involving phosphorus are: some dialkylphosphinic acids and their alkyl esters (815), diarylphosphinates (816), analysis of insecticidal tetraethylpyrophosphate preparations (817 ) tris(trimethylsily1)phosphate (818), hexachlorotriphosphonitrite and octachlorotetraphosphonitrile (819), trimeric chlorobromophosphonitriles (820), phosphonitrilic chlorides (821), and phosphonitrilic fluorides (822). Organometallic compounds containing phosphorus have been studied, e.g., metal carbonyl complexes of tris(dimethy1amino)phosphine (823, 824) and trifluorophosphinecarbonylcobalt hydrides (825). Xass spectra of bis(diphenylamino, diphenylphosphino, and dipheny1arsino)methane have been published (826). Group VIA. T h e mass spectra of benzo [blselenophenes have been compared with those of benzo[b]thiophenes (827). The mass spectra of of methyl p-bromo- and p-chlorobenzene selenoate (828) and of the antioxidant 2,2’ - selenobis(4 -methyl- 6-t- butylphenol) (829) have been discussed. Methyl-substituted 3-selenocyanatopyrroles (830) and selenium derivatives in the indole series (831-835) have been investigated. Other studies involved alkyl-substituted 4-aminophenyl selenocyanates (836), simple aromatic
se1enocyan:ites and the corresponding diselenides (837), and others (838-840). Organometallic Compounds. T h e effect of metal-metal bonds on mass spectra has been noted (841-843). The energetics of the ionization and dissociation of a number of metal carbonyls have been determined (844). The mass spectra and appearance potentials of acetylacetonates of trivalent metals (845) and of bis(acety1acetonate)metal (11) complexes (846) of the first transition series have been reported. Mass spectra have been obtained from cobalt, yttrium, zirconium, and rare-earth chelates (847). Metal complexes of the following organic compounds have been studied: corrins (754), fluorocarbons (848-850), 3,3,3-trifluoroprop-l-yne (85f), pyridylazophenols and -naphthols (852), and polyfluorobenzenes (853). Transition metal complexes with 5-, 6-, and 7-membered aromatic-ring ligands (854), indenylmetal derivatives (855), and cyclopentadienylmetal complexes (856, 857) have been investigated. Mass spectra of metal oxinates (858) and organometallic halides (859) have been reported. The following metal carbonyl derivatives have been studied by mass spectrometry: organosulfur (860), organonitrogen (861), mononuclear olefinic (862), cyclopentadienyl (863, 864), halide (864)cyclopentadienone (865), and cyclooctatetraenyl(842). Groups IB-VIB. Mass spectromet r y was used to study t h e reaction of organomercury compounds with isopropyl alcohol (866). I n the mass spectrometer, parent negative ions of group IVB dicyclopentadienylmetal dichlorides are formed a t low eV by direct capture (867)). Ionization potentials of substituted benzene-chromiumtricarbonyl complexes (868), of group VIB substituted transition-metal carbonyls (869) , and appearance potentials of ions from substituted acetylacetonates of trivalent chromium (870) and from (C6&)&r (871) have been reported. The mass spectra of binuclear copper (I) acetate and benzoate (872), of basic zinc carboxylates (873), of titanium cyclopentadienyl complexes (874), of dicyclopentadienyltitanium dichloride and -zirconium dichloride (875), of substituted benzenechromtricarbonyl complexes (876), of binuclear chromium complexes (877), and of pentacarbonylchromiumcarbene complexes (878) have been discussed. The mass spectra of the basic zinc carboxylates contain metastable peaks of doubly-charged ions (873). Groups VIIB and VIIIB. On t h e basis of mass spectrometry, structures have been suggested for hydridododecacarbonylirontricobalt and -rutheniumtricobalt, in which t h e hydrogen a t o m is located inside t h e metal-atom
cage (879). A new tricobalt enneacarbonyl carbon cluster has been reported (880). I n flash-vacuum pyrolysis, the formation and ionization of cyclopentadienyl, of cylopentadienyl nickel, and of dihydrofulvalene from nickelocene have been studied (881). Complexes containing porphyrinoid or corrinoid skeletons as the main ligands have been investigated by mass spectrometry (882). Mass spectra of the following compounds have also been reported: methyldifluorosilyltetracarbonylcobalt (883), methinyltricobalt ennecarbonyls (884), N-substituted salicylaldimine complexes of iron(II1) (885), derivatives of ferrocene (886,887), tricarbonylcyclohexa- 1,3-dieneiron derivatives (888), n-allylic rhodium and palladium complexes (889), 1,5-cyclooctadiene-rhodium complexes (890), cyclopentadienylrhodium-olefin complexes (891), manganese and rhenium pentacarbonyl halides (892), cis-1,Zethylenedithiolate derivatives of manganese carbonyl (893), and CSHSMn (CO)2PX3complexes (894). MASS SPECTRA OF INORGANIC COMPOUNDS
A book on mass spectrometry in inorganic chemistry illustrates the types of research being done in that area, although the papers therein were presented a t a symposium in September 1966 (32). Some of the papers describe: a research mass spectrometer utilized as a self-contained chemical laboratory to investigate decomposition reactions induced by the absorption of ionizing radiation and by a hot metal surface (895), a study of the photolysis of low pressure mixtures of diboraneoxygen (896), application of mass spectrometry to the study of unstable inorganic species, including boron hydrides and nickel tetracarbonyl (897). mass spectra of phosphorus hydrides (898), and a mass-spectrometric investigation of the conversion of metathioboric acid to boron sulfide (899). The mass spectra of P3Hs (goo), Br03F (901), c ~ c Z 0 - S ~and cyclo-Slo (902), xenon trioxide difluoride (903), krypton difluoride (god), stannane and stibine (905), Br3+hF6- (906), NS2 Clz+BCla- (907), and H B r 0 4 and HClO4 (908) have been presented. The gas-phase decomposition of the nitrous oxide ion has been studied in order to clarify disputed points about a diffuse metastable peak (909). Mercury(I1) chloride evaporates readily a t 200-500 " C and can be used to determine microgram quantities by mass spectrometry (910). The mass spectra of phosphorus and phosphorus oxides showed the mass spectrum of white phosphorus to be very similar to that of red phosphorus (911). Bridgebonded aluminum compounds (912),
chemically modified silicas (913 ) , and chlorides of the main subgroups of groups 111-V (914) have been studied by mass spectrometry. The Knudsen cell in a vacuum furnace combined with a mass spectrometer is used to identify components in vaporization studies of low-volatility inorganic systems. The use of mass spectrometry in the high-temperature chemist'ry of simple metallic oxides has been mentioned in an article, along with other techniques (915). The mass spect.rometry of inorganic halides, prior to 1967, has been reviewed (916). The mass-spectrometric investigation of vaporization (92 7) and interpretation of mass spectra in high-temperature thermodynamic studies (918) have been reviewed. An ion source for vaporizations in the 100-400 "C'range has been described and applied (919). Pulse heating and time-resolved mass spectrometry were applied to the Langmuir vaporization of rlg, Zr, ZrO,, and ZrO, N, a t temperatures up to 2100 OK (920). Some thermodynamic relations for two-component systems with a complex vapor composit'ion and their application in mass-spectrometric investigations have been described (921). High-temperature mass spectrometers have been calibrat'ed by a technique using a dual-cell system (922). Determination of the composition of vapor in mass-spect'rometric thermodynamic investigations has been discussed (923). The reaction of aluminum vapor with S2,Se2, and Te2 has been studied (924). I n an investigation of high-temperature vapors, S2,Se2,and Tez were studied by photoionization (g25). Vaporizations of potassiumchloride (926) and arsenic (927) single crystals have been investigated. Vaporization studies have been made on manganese (928), magnesium and calcium and their hydrides (969), aluminum phosphide (930), gallium sulfide, selenide, and telluride (931),germanium telluride (932), the germanium telluridelead telluride system (933), zinc chloride, bromide, and iodide (934), beryllium chloride (935), lathanum, europium, and lutetium chlorides (9S6),lead chloride-group 1A chloride systems (937, 938), scandium, yttrium, lanthanum, and rare-earth fluorides (939), NaSnF3 and KSnF3 (940), the rutheniumoxygen system (941), the beryllium oxide-beryllium fluoride system (942), the rhenium-oxygen system (943), the barium-oxygen system (944), chromium trioxide (945), BaMo04 and B a W 0 4 (94'4, molybdenum trioxide (947), some vanadium oxides (948), and potassium hydroxide (949). Thermodynamic studies have been reported for the urania-uranium system (950), holmium-carbon and dysprosium-carbon systems (951), the erbiumcarbon system (952), lanthanum hexa-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
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boride (953),the sodium fluoride-beryllium fluoride system (954),beryllium fluoride (956), W O Z I ~(956), gaseous monoxide-dioxide equilibria for cerium, praseodymium, and neodymium (957), trieuropium tetroxide (968)) and europium monoxide (969). The stabilities of tantalum pentafluoride and oxytrifluoride (960), of tungsten and molybdenum oxyfluorides (961), of gaseous germanium dichloride and dibromide (962), and of diatomic carbides of elect'ropositive transition metals (963) have been investigated by mass spectromet'ry a t high temperat'ure. The formation of polymeric (GeF2). has been det'ected in the vapor phase over GeF2 (964). Activities have been determined for the molten lead bromide-potassium bromide system (966),the gold-copper system (966),and in iron-aluminum and silver-aluminum liquid alloys (967). Dissociation energies have been determined for lanthanum monosulfide (968), SiiF and P b F (969), Pdz (970), Ti2 and V2 (971), and others of the first' transition period (97b), silicon monoxide (9729, TiO(,, (974),the molecules CeAu (975), h g M n (976), Fez (977), BeCl and BeC12 (978), Ce? (979), XuSi (980), BO (981),Aunln (982),gold monoboride (983),RuC, IrC, and P t B (984),AuNi, XuCo, and AuFe (985),and indium sulfides, selenides, and tellurides (986). The dissociation energy of CaF has been reevaluated (987). Mass-spectrometric determinations of the dissociation energies of the gaseous rare-earth monosulfides (988) and experimental and predicted bond energies of gaseous rare-earth aurides (989) have been published. Bonding in group IIh fluorides has been studied by mass spectrometry (990). A paper has appeared on the dissociation energy of zirconium mononitride (991) and of thorium mononitride and on the predicted energies of diatomic group 111-IV transition-metal nitrides (992). The enthalpy of sublimation of uranium (993) and of plutonium(II1) fluoride and the dissociation energy of plutonium fluoride have been reported (994). The enthalpy and free energy of formation of NaAlF4 in the gas phase have been calculated (995), and thermodynamic properties of the condensed phase in the NaF-X1F3 system have been reported (996). The determination of the dissociation energies of PN and P2 resulted from a mass-spectrometric study of the react'ion between diatomic nit'rogen and phosphorus vapor (997). SPARK SOURCE MASS SPECTROMETRY
h general review discusses the status of spark source mass spectrometry prior to 1967 (998). I n the same volume other aspects of this technique during 180R
the same time period are presented (23). Sources of error have been evaluated (999),and effects of source parameters on ion intensities have been determined (1000). Line profiles have been mathematically determined (1001), and the statistical distribution of sensitivity factors has been calculated (1009). Time-resolution spark source mass spectrometry has been discussed (1003). Some simple techniques for preparing powder samples have been devised (1004). A technique for ultrapurification of graphite uses preburning (1006), and graphite-pressed electrodes of powdered sample have been used (1006). The principal criteria of the spark method of analysis of liquids have been outlined (1007). A new method of determining impurity concentrations on some metals using spark source mass spectrometry has been presented (1008). Microsamples have been analyzed by single-exposure spark source mass spectrometry (1009). I n the mass-spectrometric analysis of layers, the analyzed materials have been atomized by a vacuum spark (1010), and impurities in sputtered layers have been determined by spark source mass spectrometry (1011). Spark source mass spectrometry has been used for analysis of high-purity iron (1012), radioactive metals (1013), sodium ( f O f 4 ) , oxygen and nitrogen in niobium (1015), trace impurities in molybdenum (1016), tobacco ash (1017), plant ash (1018), human hair (1019), mercury in apples (1020), etc. A spark-source auxiliary-electrode method has been used for the trace analysis of nonconducting materials (1022). Trace elements in platinum have been determined simultaneously by isotope dilution and spark source mass spectrometry (1022). The superiority of rotating electrodes compared to fixed electrodes is theoretically shown in a paper concerned with analysis of impurities in metallic surface deposits (1023). The analysis of surface films has been reviewed (1024). The elements hydrogen, carbon, nitrogen, and oxygen lead to the presence in the highfrequency spark source mass spectrum of thorium, of a score of interference peaks which correspond to molecular ions (1025). I n preparation for studying lunar rock, multielement analysis of basaltic rock has been preformed using spark source mass spectrometry (1026). The technique can be used for the general analysis of geological samples (1027), and has been used to analyze minerals and rocks (1028),to semiquantitatively study trace elements in pyrite (1029), and to determine the distribution of trace elements in Smithonia iron meteorite (1030).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
ANALYSIS
Recent advances in the applications of mass spectrometry t'o analysis have been listed (2031). Development of Auger electron emission analysis into a reliable analytical procedure has been described (1032). Rutherford scattering and channeling have been shown to be a useful combination for chemical analysis of surfaces (1033). I t appears that the atom-probe field-ion microscope will become a quite useful research tool for surface chemistry (1034). The ion-microprobe mass analyzer supplies information concerning the concentration and distribution of the mass isotopes of the elements and should be useful in the areas of isotope chemistry, surface chemistry, and trace-element chemistry (1035). Bombarding ions sputt'er atoms from a surface as ions and these sputtered ions are collected and analyzed in a mass spectrometer. Ion bombardment of solids has been covered in a book (1036). A modified time-of-flight mass spectrometer has been used to study the emission of positive ions and electrons from polymers bombarded by singly-charged rare-gas and nitrogen ions (202'7). Secondary-ion emission of alloys has been investigated by mass spectrometry (1038). h mass-spectrometric met,hod for measuring the sputtering rate and secondary-ion-emission coefficients of solids by thin layers (1039) and for layer-by-layer analysis of thin semiconducting films (2040) have been described. Ion implantation has been studied in silicon (IO41), and isotope separators have been used for ion implantation (1042). The laser-mass spectrometer microprobe is a very powerful tool for the microanalysis of solids and can be used for studying structures of solids and their vaporization processes. Laserproduced vapors have been compared with conventionally-produced vapors (1048). The vaporization of selenium has been studied (1044). The plasma formed in the interaction between laser radiation and matter can be investigated, and the data obtained include mass spectra, ion-charge multiplicity, energy and angular distribution of the plasma, and characteristics of the plasma expansion (1045). A laser was used to induce vaporization of antimony and tellurium; ionic species were studied with the filament off and neutrals with it on, a t 15 eV (1046). A capillary immersed in a liquid mixture to be analyzed has been used to obtain vapor entering the ionization zone continuously (1047-1050). Ions have been accumulated in vibrating packets for quantitative analysis with a t'imeof-flight mass spectrometer (1051). il. mathematical treatment has been given whereby the need for a priori knowledge
of the number and nature of the components of a mixture and their mass spectra may be overcome (1052). I n those cases where this method is applicable, it gives as a solution the number of components, their mass spectra, and their concentrations. A mathematical solution of the mass-spectrometric standard-mixture problem has been derived (1053). A technique for obtaining mass spectra of substances isolated by thinlayer chromatography has been described (105.4). Tautomeric equilibria of organic compounds can be investigated from changes in the mass spectra with changes in the inlet temperature (1055). Mass spectrometers have been used for sampling in shock-tube studies (1056, 1057). Mass spectrometers have been used for investigating the outgassing products of hydrogen-containing aluminum-silicon alloys (1058) and for determining gas permeability of thick plastic films (1059). Trace analysis by mass spectrometry has been discussed and reviewed (10601063). Determining microimpurities by using a mass spectrometer with a laser ion source is a possibility (1064). Neptunium-237 (1065) and lead (1066) have been determined in nanogramand subnanogram-size samples by massspectrometric methods. Trace-metal analysis has used heptafluorodimethyloctanedione chelates (1067), and chromium has been determined rapidly as chromium(II1) hexafluoroacetylacetonate (1068). The possibility of determining element microimpurities as readily volatile compounds has been discussed (1069). Mass spectrometry has been used for analyzing germanium and silicon tetrachlorides for microimpurities (1070),molybdenum additive in tantalum-oxide capacitor films (1071), high-purity carbon dioxide ( I O " / ) , low concentrations of carbon dioxide in nitrogen (1073), trace contaminants in high-purity potassium dichromate (1074), etc. The ion-beam-chopper technique gives improvement in the reproducibility of mass-spectrometric analysis of steel and aluminum (1075). Element impurities have been determined by the mass-spectrometric method of isotope dilution (1076-1079). Phenol and aniline derivatives, related fungicides, and metabolites, have been analyzed as 2,4,7-trinitrofluorenone complexes (1080). Aromatic fractions (1081, 1082), nitrogen and oxygen compound types (1083), and nitrogen compounds (1084) in petroleum have been analyzed by mass spectrometry. Crude-oil carboxylic acids have been determined by preparative thin-layer chromatography and high-resolution mass spectrometry (1085) and as the corresponding hydrocarbons (1086). The compositions
of pyridine extracts from reduced and untreated coal have been determined (1087). Soot has been studied (1088, 1089). Coal particles have been heated by laser in the source of a time-offlight mass spectrometer (1090), and carbon species generated by laser evaporation have been analyzed (1091). Mass spectrometry has been used to study many types of reactions. For example, rate constants of elementary reactions (1092), the high-temperature reaction of hydrogen with boron carbide (10957, the reaction of nitrogen-28 and nitrogen-30 in equilibrium with nitrogen-29 catalyzed by molybdenum (1094), the radiolysis of methane by reactor radiation (1095), and minor products in the radiolysis of liquid cyclopentane (1096) have been observed by mass spectrometry. Use of the mass spectrometer as a radiolytic and catalytic laboratory has been described (1097). Surface ionization has been used to study the high-temperature diffusion of uranium in tungsten and the allotropic phase transition in zirconium (1098). The low-pressure pyrolysis of triphosphine-5 has been analyzed by mass spectrometry (1099). The mass spectrum and ionization potential of condensed cyclobutadiene were reportedly obtained (1100) from reaction of cyclobutadieneiron tricarbonyl in a pyrolysis furnace mounted inside a cryogenicallycooled inlet system attached to a timeof-flight mass spectrometer (1101). However, on the basis of flash-vacuum pyrolysis in an oven coupled to a mass spectrometer, the only detected CaH4 thermal product from cyclobutadieneiron tricarbonyl was vinylacetylene (1102). A time-of-fllight mass spectrometer has been used to study the primary products of pulsed pyrolysis of ammonium perchlorate (1103). Thermogravimetric-mass spectrometric analysis can utilize an interfacing similar to ones used with gas chromatographymass spectrometry and can be applied to a variety of problems (110.4). Mass spectrometry has been used to analyze thermal degradation of polystyrene (1105), cellulose (1106), polytetrafluoroethylene (1107), and polyamide and polyvinyl acetate (1108). Polystyrene cross-linked with pure m- and p-divinylbenzene has been characterized by mass spectrometry (1109). ISOTOPIC RATIO MEASUREMENTS AND GEOAND COSMO-APPLICATIONS
Isotopic analysis by mass spectrometry can be improved b y use of an amplitude selector for pulse counting with a scintillation ion detector (1110). An ion source for the precision relative measurement of the isotopic ionization of solid-state samples has been described (1111). Precise determination
of isotopic ratios can be made in the case of ion-beam intensities varying with time (1112). A theoretical model has been developed for isotopic effects in solid source mass spectrometry, when there is imperfect mixing between the surface and subsurface layers of the sample (111s). The isotopic distribution in organic molecules has been studied with a field-type mass spectrometer (1114). T h e isotopic analysis of chlorine by the surface-ionization method (1115) and surface isotopic analysis by a laser source mass spectrometer (1116) have been described. Small samples have been analyzed using aluminum oxide as the substrate (1117). An ion emitter has been designed for measurement of the isotopic composition of micro amounts of lead (1118). A new method has been developed for the mass-spectrometric isotopic analysis of lead contained in rocks and minerals (1119). The joint isotopic analysis of silicon and germanium in small quantities has been described (1120). The absolute isotope ratio of a natural boron standard was determined; the bias factor of the mass-spectrometric measurements was constant over a wide range of boron isotopic ratios, and was accurately determined (1121). il mass spectrometer has been designed for the measurement of the isotopic l*O/leO-oxygen ratio of water without any previous chemical operations (1122). Seawater has been analyzed (1123), and the oxygen of water has been isolated for study by a guanidine method (1124). The 16LV-enrichment in nitrogen obtained by Dumas combustion has been measured (1125). An improved procedure for the measurement of 15N-content has been developed (1126). Sealed tubes have been used to convert hydrogen compounds to hydrogen for mass-spectrometric precision analysis of deuterium in the range of natural variation (1127). Mass spectrometry has been used to study 8decay (1128),to measure the half life of plutonium-214 (1129), and to examine numerous short-lived fission products (1130). The isotopic composition and the distribution of lithium have been studied in the Holbrook meteorite, in granite and in hornblende, using a sputtering ion source mass spectrometer of new design (1131). I n micrometeroid simulation experiments, an impact-ionization process led to the emission of positive ions from the projectile material; these ions were analyzed with a time-offlight mass spectrometer (1132). Massspectrometric elemental and isotopicabundance analysis in the investigation of the history of meteorites and the planetary system has been reviewed extensively (1133, 113.4). Organic analysis of the Pueblito de
ANALYTICAL CHEMISTRY, VOL. 42, NO; 5, APRIL 1970
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Allende meteorite by high resolution mass spectrometry has been reported (1135). Likewise, the organic constituents of the Murray and Holbrook chondrites have been determined (1136). Fatty acids isolated from the Colorado Green River Formation have been studied by high-resolution mass spectrometry (1137-1140). Oxocarboxylic and dicarboxylic acids in complex mixtures can be detected and identified by reductive silylation and computeraided analysis of high-resolution massspectrometric data (1141). Branchedand normal-chain alkanes have been identified in sedimentary rocks by a combined gas chromatographic-mass spectrometric method (1142). Similarly, steroids and triterpenes have been identified from Green River shale (1143).
By removing the oxygen in the atmosphere with phosphorus, it is possible to determine the carbon dioxide with an inaccuracy of less than 1% by mass spectrometry (1144). The thermosphere at high latitudes was measured by means of a monopole spectrometer flown aboard a rocket (1145). Rockets carrying magnetic mass spectrometers have measured the composition of the lower thermosphere, studying dayto-night variations (1146,1147). Three mass spectrometers are carried in each rocket, one with a n ion source exposed to the ambient atmosphere and two with ion sources connected to the ambient atmosphere through a telescoping cylinder (1147). Two quadrupole mass spectrometers in a rocket confirmed the existence of atomic nitrogen in the altitude range 120-180 k m (1148). An ion-energy spectrometer in a satellite measured the positive ions in the topside ionosphere, showing the ions O + and He+ to be the major massive components of the ionosphere (1149). Mass spectrometry of the ionized constituent of the upper atmosphere has been reported (1150). Metal ions and atoms occur in the Earth’s atmosphere; measurements of some metal-ion reactions with ozone have been made in a flowing afterglow reaction bystem, leading to the conclusion th.it ion chemistry doesn’t play a significant role in the deionization of sporadic E layers in the ionosphere (1151). The proceedings of the symposium on Laboratory Measurements of Aeronomic Interest have been published (1159). For example, electron-impact cross sections for NB,0 2 , and 0, used in aurora and dayglow studies, are summarized (1159). Also, the role of mass spectrometer ion source measurements in the elucidation of ionospheric negative-ion chemistry is expected to be limited because so many of the most critical reactions involve relatively unstable neutral reactants which have not 182 R
been successfully used in such sources (1164). Instrumentation and measurements performed prior to 1968 by means of rocket- or satellite-borne mass spectrometers have been summarized (1166). A sterilizable instrument weighing ten pounds and requiring ten watts has been developed for atmosphere studies of Mars and Venus (1156). Eightythree of the 142 principal investigators involved in the Apollo 11 program have been listed; twenty-two of those listed are using mass spectrometry in their work (1157). Techniques to be used in organic analysis of the returned lunar sample were summarized prior to the return of the samples (1158). COMBINED GAS CHROMATOGRAPHY-MASS SPECTROMETRY (GCIMS)
The coupling of a mass spectrometer with a gas chromatograph has been discussed as an analytical technique of great potential (1169-1161). Organic analysis by pyrolysis GC/AIS has been investigated as a possible experimental approach to the biological exploration of Mars (1162). The parameters for coupling the chromatograph to the spectrometer have been considered (1163).
A review has appeared which covers the various interfacial systems used in GC/MS, as of December 1968 (1164); the factors involved in a choice of types are outlined and the advantages and disadvantages of the most widely used types are given. A molecular separator of the Biemann-Watson type has been built into the handle of a direct-introduction probe of an AEI AIS9 (1165). A silver-membrane type carrier-gas separator has been used (1166), and silanized porous stainless steel has been used as interface material (1167). A membrane molecular separator can be used, also (1168). By in situ silanization, chemisorption on a fritted-glass molecular separator can be deactivated (1169). A splitter which may be used for open tubular columns has been described, which allows a wide choice of split ratio between mass spectrometer and flame-ionization detector without significant loss of GC resolution (1170). Micro-scale dry-column chromatography has been combined with mass spectrometry, with the microcolumns fitted directly into the probe of a n AEI LIS902 Mass Spectrometer (1171). A simple heated inlet system, in which the column of the chromatograph is replaced with a reservoir flask from which the sample vapor slowly leaks down the stainless-steel capillary and into the spectrometer, has been developed for use with GC/MS (1172). The performance of semi-permeable membrane and silver-palladium membrane gas separators has been theoretically in-
ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970
vestigated to establish their feasibility as sample-enrichment devices for GC/
MS (1173). Bleeding material can be trapped out
as it emerges from the column by absorption in an attached short column of high thermal stability (1174). Alternately, peaks can be collected from
a column with a large bleeding effect and subsequently analyzed by GC/MS with a column of low bleeding effect (1176). A technique for collecting and transferring packed-column GC fractions to a capillary column for fast-scan mass-spectral analysis has been described (1176). An improved technique has been used for transferring fractions from a gas chromatograph to a mass spectrometer in a study of possible toxic combustion products of building materials involved in fires (1177). Techniques for trapping the sample prior to mass-spectrometric analysis have been used (1178, 1179). Loss of chromatographic resolution in the vacuum line of a GC/MS system has been investigated (1180). Organic compounds eluted from a GC column have been studied by on-line highresolution mass spectrometry (1181). Thin-layer chromatography has been coupled directly with mass spectrometry (1182). The profiles of overlapped peaks obtained from incomplete separations of isotopic molecules can be recognized by using a capillary GCIMS system and an isotope-scan method (1183). A new technique, mass fragmentography, has been described and used to identify chlorpromazine and its metabolites in human blood (1184). The mass spectrometer is set to monitor continuously three mass numbers of compounds emerging from the gas chromatograph; compounds are identified by their retention times and the fact that all mass numbers are represented in characteristic relative intensities. Mixtures of diterpene hydrocarbons have been analyzed quantitatively by GC/bIS using a technique similar to mass fragmentography, but requiring no instrumental modification (687). Several applications of GC/MS have appeared in other sections of this review and many more were omitted. A few more will be given here to illustrate the scope of the technique. Trace amounts have been identified and quantitatively determined by GC/MS (1185). Complex mixtures of hydrocarbons have been analyzed by time-of-flight mass spectrometry coupled with opentube chromatography (1186). Paraffin and naphthalene hydrocarbons with the same retention times can be analyzed with GC/MS (1187). Complete isotopic fractionation of penta-0-trimethylsilyl-D-glucose and its trimethylsilyl-& analog has been achieved by GC/i\lS (1188). Deuterium-labeled
triniethylsilyl derivatives are useful in mass spectrometry (1189). Occurrence of Cz9-Czs isoprenoids in Bell Creek crude oil has been established by GC/hlS (1190). Volatile products from the room temperature autoxidation of cis,cis-6,9-octadecadiene have been analyzed (1191). Acetylcholine has been identified in fresh rat brain by GC/RIS (1192). Butylboronate esters can serve as derivatives for the GC/lIS of hop constituents (1195). MULTIPLY-CHARGED, METASTABLE, AND NEGATIVELY-CHARGED IONS
Multiply-Charged Ions. The structural significance of doublycharged-ion spectra of organic compounds has been discussed (1194). Doubly-charged ions could be used to differentiate between certain structural isomers of N,N’-alkyl substituted phenylenediamines which give practically identical singly-charged ions. Doubly-charged molecular ions of higher abundance than the corresponding singly-charged species have been reported for a dibromotetrahydroindenoindendione (1195), in the mass spectra of some phosphorus and arsenic substituted biphenyl systems (1196), and for I-(0-nitropheny1)anthra-9,lOquinone (1197). In the mass spectra of diphenylmethanes containing t-butyl groups, a spectrum of doubly-charged ion transitions of much greater intensity than their singly-charged counterparts is observed; the positive charges have tentatively been assigned to sites on each aromatic ring (1198). Kinetic-energy release obtained from “flat-topped” metastable-peak widths, when a doubly-charged ion decomposes to two singly-charged ions, has been interpreted for decompositions of 1,1, 4,4-tetraphenylbutatriene ions in terms of variable apparent charge separation (1199). Investigations of metastable ion transitions have shown that spontaneous charge separations and ion-pair formations may be important factors in formation of doubly-charged ions (1200). Multiply-charged ions are observed in the mass spectra of methylsiloxanes (1201). Kinetic-energy r e lease and metastable transitions have been discussed in relation to the decomposition of doubly-charged ions of aliphatic and alicyclic hydrocarbons (1202).
Relative abundances of multiplycharged ions formed by electron impact (2-16 eV) on some noble gases were measured with a charge analyzer of 100% transmission, and large discrepancies were found to exist in comparison to values obtained in low-transmission mass spectrometers (1203). Mercury ions of charge multiplicity up to n = 9 have been formed by repeated collisions
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8:
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