Chemical Ionization Mass Spectrometry:

Burnaby Munson Department of Chemistry University of Delaware Newark, Del. 19711

Chemical Ionization Mass Spectrometry: Chemical ionization mass spectrometry (CIMS) is an outgrowth of studies of gaseous ion-molecule reactions. Products of ion-molecule reactions were observed early in the history of mass spectrometry. Thomson reported the ionic species H3+ by 1913 (1). The rare gas hydride ions (HeH + , NeH+, ArH + ) were sufficiently common to be used for the precise determination of atomic masses (2). Dempster established in 1916 that H3+ was formed by secondary processes, H 2 + + H 2 — H3+ + H and this work may be considered as

(1)

the origin of ion-molecule reaction studies (3). With the advent of better vacuum technology, however, ion-molecule reactions were observed less frequently and received little attention in the development of mass spectrometry. There was perhaps enough work on ion-molecule reactions between 1916-1956 to provide a good genealogical tree; somewhat more realistically, the early work was nearly completely forgotten, and ion-molecule reactions were rediscovered in the 1950's. About 20 years ago, work was independently reported on ion-molecule reactions by Tal'roze and Lyubimova

Figure 1. GC/CIMS Top trace: reactant ion monitoring with C H 5 + , m/e = 17. Bottom trace: flame ionization detector. Peaks (left to right): solvent (diethyl ether), benzyl propionate, 1-phenyl-1-pentanone, 1-phenyl-1-heptanone, 1-phenyl-1-nonanone, methyl stéarate

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in the Soviet Union (4), by Stevenson and Schissler at the Shell Development Co. (5), and by Field et al. at the Humble Oil Co. (now Exxon) (6). All of these groups reported the completely unexpected reaction: CH 5 + + CH 3 (2) Coincidentally, CH 4 with its major product ions, CH 5 + and C 2 H 5 + , was the first reagent'gas used in CIMS. In 1966 as a direct outgrowth of work on gaseous ion-molecule reactions at high pressures, Munson and Field reported CIMS as an analytical application of ion-molecule reactions by showing the CH 4 CI mass spectra, or the distribution of product ions. from ion-molecule reactions of CHs + and C2H5+ with different compounds (7). Because of the need for new instruments or modifications to existing instruments, the only work during the next few years was done by these workers. However, by 1969 other instruments had been modified for highpressure operation, and other workers began to use the technique for analysis (8, 9). Since then, many more laboratories have developed CI capabilities, and the technique is now routinely used in analysis. Most of the early work was concerned with the development of the methodology and could be readily recognized by key words in the title. It is now no longer possible to determine the number of papers published annually which use CIMS because in an increasing number of instances the technique is only incidental to the work and is not mentioned even in the abstract of the paper. Much of the early work was to obtain chemical ionization mass spectra and to compare these spectra with the standard electron ionization (EI) mass spectra to show the utility of the technique. In many instances the CH 4 CI mass spectra were easier to interpret than the EI mass spectra and contained sufficient information for a confirmation of structure. Such studies are still useful and are being done today. For example, the CH 4 and iC 4 Hi 0 CI spectra of peracetylnitrate (PAN) provide excellent confirmation of the proposed structure and moleCH4+ + CH 4

Report

Ten Years Later cule weight, whereas the EI spectra are not useful (10). Other illustrations are the mass spectra of explosives for which the molecular weight and major structural features can be established by CI but not EI mass spectrometry (11). Of course, there are the many examples of the CI mass spectra of molecules of biomedical interest which show an enhancement of abundances of ions in the molecular weight region when compared with EI mass spectra (12). One of the advantages of chemical ionization mass spectrometry over electron impact mass spectrometry is the simplification of the spectra—a smaller number of ions per compound in CIMS. In many cases, CI reagent gases are chosen to produce only one or two peaks for each compound. The advantage of this technique for the analysis of mixture is obvious. As one illustration, we can consider the analysis of cholesterol esters of fatty acids. The EI mass spectra of these esters contain essentially no molecular ions, and the spectra of the higher esters are sufficiently complex to mask the characteristic low mass fragment ions of the lower esters; however, the CH 4 or Î-C4H10 CI spectra contain essentially only protonated acid ions and ions at 369 (M + H - RCOOH)+ (13). Consequently, an analysis of mixtures of these sterol esters can be performed readily by CIMS. The pyrolysis of polymers in the source of a mass spectrometer with a direct insertion probe produces EI spectra that are almost too complex to interpret, but the CI spectra allow ready identification of molecular species. The EI mass spectrum of the pyrolysis of polymethylmethacrylate at 250 °C directly into the source of the mass spectrometer contains essentially every mass from 29 to 250. The characteristic ions of methylmethacrylate are observed, and this compound can be identified in the spectrum. The CH4 CI mass spectrum obtained under similar conditions contains predominantly (M + H ) + and (2M + H ) + ions of methylmethacrylate, and the presence of the monomer as the dominant pyrolysis product is immediately obvious (14).

A simple kinetic analysis shows that the ionization of sample molecules by ion-molecule reactions, and therefore the sensitivity for a given compound, depends upon the time for reaction between the reactant ions and sample molecules, the concentration of sample molecules, and the rate constants for reactions of the reactant ions with the sample molecules (15). For CH 4 , the following result is obtained: Σ/,· = At(kl7In

+ k29I29)[M]

(3)

in which [M] = concentration of sam­ ple, ki = ki (I+, M) = rate constant for reaction of ion I+ with molecule M, At = reaction time, and 7, = ion current at m/e = i. Exothermic proton trans­ fer reactions have roughly the same rate constants (within a factor of two), independent of the nature of the reac­ tant ion or sample molecule (16-19). For high-energy reactant ions, like CH5 + and C2Hs + , which should react exothermically and rapidly with most organic molecules, one would not ex­ pect major variations in rate constants with changes in molecular weights of the compounds. Under similar condi­ tions, one would expect essentially equivalent CI sensitivities with meth­ ane for all compounds. In the early work in CI mass spectrometry, essen­ tially the same sensitivities were noted for a series of propionate esters, Ci to C7 (20). More recently, the same sensitivities were observed for the CI spectra of valerophenone with CH 4

(reactant ions = CHs + ι the reagent gas and with CH4/H2O mixtures (reactant ion = HsO + ) as the reagent gas (21 ). For a series of polyamines, similar sensitivities were ob­ served using CH 4 , N2, and i-C 4 Hio as reagent gases (22). The approximation that exothermic ion-molecule reactions have equal rate constants is only a crude approxima­ tion. The theories of ion-molecule re­ actions predict that the rate constants for reaction will increase with increas­ ing polarizability and increasing dipole moment of the sample molecules (23-27), and rate constants differing by factors of 2-3 have been observed for exothermic reactions. Under CI conditions the formation of each prod­ uct ion requires the loss of a reactant ion. If we monitor the signal of one of the reactant ions during a GC/MS ex­ periment, we can expect to observe a decrease in the ion current of the reac­ tant ions as the sample molecules pass through the source of the mass spec­ trometer. This decrease in signal de­ pends on the ionic reaction time in the source of the mass spectrometer, the concentration of sample molecules, and the rate constants for reaction. By continuously recording the ion current of one of the reactant ions during a GC/MS run, we can obtain a trace that is the equivalent of the total ion current vs. time in EIGC/MS or the flame ionization detector trace in GC analysis. This technique is called reac­ tant ion monitoring (28, 29). Figure 1 shows a comparison of a reactant ion trace with CHs + and the simultaneous trace of the flame ionization detector of the GC for a mixture of oxygenated compounds. It is possible to relate the peak areas in these measurements to the relative rate constants for reac­ tions of the sample ion with different samples to obtain the data shown in Table I (30). From these data it is possible to see the expected increase in rate constant and sensitivity with increasing molecular weight and dipole moment of the sample molecules. These data may be used for quantita­ tion in GC/CIMS studies. If ion-molecule reactions are ap­ proximately thermoneutral, then it is

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very likely that small differences in the structures of the sample molecules will have significant effects on the rate constants for ion-moleculé reactions. It has been reported, for example, in the acetylation of the norborneols with (CH 3 CO) 3 + that the rate constant for reaction with the less hindered exonorborneol is four times as large as the rate constant for reaction with endonorborneol (31 ) . We have also observed that the isobutane CI spectra of endo- and exonorborneol are significantly different in the relative abundances of (M — H ) + ions. The dominant ion in the i-C 4 Hi 0 spectra of both of these compounds is (M — OH) + , but endonorborneol (for which the α-hydrogen is in the exoconfiguration and therefore less hin­ dered for attack) shows about 5% (M — H ) + in its spectrum, whereas, exo­ norborneol for which the α-hydrogen is in the more hindered endo-position has only about 0.5% (M - H)+ in its spectrum (32). These small differ­ ences, which we attribute to steric ef­ fects, are accentuated by the use of larger, lower energy reactant ions, since the CH 4 CI mass spectra of endo- and exonorborneol are almost identical (32). In CI mass spectrometry, increasing the exothermicity of ion-molecule re­ actions (by changing the nature of the reactant ions) increases the extent of fragmentation (33, 34). Conversely, decreasing the exothermicity of the ion-molecule reactions will decrease the extent of fragmentation and en­ hance the abundances of ions in the molecular weight region of the CI mass spectra. When the interest is pri­ marily to determine the molecular weight of a compound or to confirm it as one of a small set of compounds, a low-energy reactant ion such as tC 4 H 9 + (from t-C 4 H 10 ) is frequently used. The t-C4Hio CI spectra have been used for identification in drug overdose cases (35) and also for the characterization of street drugs (36). The i-C 4 Hg + ion may react by O H transfer instead of proton transfer; therefore, the t-C 4 Hi 0 CI spectra of al­ cohols generally do not contain (M + H) + ions (37). However, if one uses an even weaker protonating agent like NH 4 + , it is possible to obtain (M + H ) + and (M + NH 4 ) + ions to charac­ terize polyhydroxy compounds like sugars (38). CI mass spectra can be used to give approximate values for proton affini­ ties of complex molecules. The CI spectra obtained with ammonia, methylamine, dimethylamine, and trimethylamine have been used to deter­ mine an approximate order of basici­ ties of several complex amines (39). Similarly, the basicities of some phosphine oxides have been estimated

from their i-C 4 H 10 , ammonia, pyri­ dine, and trimethylamine spectra (40). In these experiments, one ex­ pects and observes abundant (M + H) + ions of the sample if the sample is more basic than the reagent gas and abundant (M + XH)+, where XH+ = protonated reactant ion, if the sample is less basic than the reagent gas. Another type of reaction is observed if one uses labeled polar compounds as reagent gases in CIMS experiments. D2O, for example, has been used to determine the number of replaceable hydrogens in polar compounds by comparing the H2O CI spectra with D2O CI spectra. All of the active hy­ drogens are replaced (41). Similar dis­ crimination was demonstrated among primary, secondary, and tertiary amines with ND 3 as a reagent gas (42). Recently, GC/CIMS studies have been reported in which a complex mixture of amines was characterized for re­ placeable hydrogens by comparison of the CH3OH and CH3OD CI spectra (43). In the early work on CIMS, the re­ actions involved heavy particle (H + , H _ , or X - ) transfer. However, in the past few years, the definition has been expanded to include charge exchange (CE) or electron transfer reactions as well. The term "chemical ionization" is now used to include all types of ionmolecule reactions for the production of spectra of the sample molecules. A wide variety of charge exchange reac­ tant ions are available and have been used. Charge exchange spectra ob­ tained using Ar, N2, or CO are rela­ tively similar to conventional electron ionization mass spectra (22, 44). One advantage of this technique of ion pro­ duction is in GC/CIMS when one may use N2 as the carrier-reagent gas and obtain spectra of compounds that may be compared with the compilations of EI mass spectra. The charge exchange mass spectra (N2, CO) of many biological molecules contain little or no M + for the deter­ mination of molecular weights. Conse­ quently, there has been work using low-energy charge exchange ions to produce simplified spectra. The most common low-energy charge exchange reactant ion is NO + (44-46). NO has been used as a reagent gas, and its utility as a pure reagent gas is extend­ ed by using an electric discharge rath­ er than a conventional hot wire fila­ ment as the source of electrons (47). Dilute mixtures of NO in N2 can be readily used in conventional CI mass spectrometers, and these mixtures provide significant enhancement of M+ ions for many compounds, partic­ ularly for TMS ethers of biologically important compounds (46). NO + reacts with compounds by reactions other than charge exchange. Hydride

transfer and oxidative addition reac­ tions have been noted for aliphatic compounds for which charge exchange is endothermic (48, 49). Because of its use as a carrier gas in gas chromatography, He has been used as a reagent gas for charge ex­ change reactions. He + has a high re­ combination energy: RE(He + ) = 24.6 eV (50). The ionization potentials of most organic molecules will be about 8-10 eV (50). Hence, the M+ ions pro­ duced by charge exchange He+ + M — M+* + He (4) will contain a large amount of internal excitation energy and should decom­ pose almost completely. Experimen­ tally, spectra obtained with high pres­ sures of He still contain reasonable abundances of high mass ions (28, 51, 52). Direct ionization with low-energy electrons that are present in the high pressure of He, charge exchange reac­ tions of sample ions or impurity ions in He, or collisional stabilization of the excited-M+* ions may be occurring. Although the modes of formation of the sample ions have not been deter­ mined, spectra obtained with high pressures of He are analytically useful. One of the problems that is associ­ ated with charge exchange spectra is an incorrect isotope ratio for the mo­ lecular ion. In general, the ratio, (M + 1) + /M + , is always higher than predict­ ed for 13 C, etc., and sometimes (M + 1)+ is larger than M+ (44). The highpressure mass spectrum of the reagent gas contains varying abundances of hydrogen containing ions (HeH + , N 2 H+, or ArH+), and H 3 0+ is almost always present. Stable (M + H) + ions are formed by proton transfer from these impurity ions and/or proton transfer from sample ions to sample molecules. It is virtually impossible to remove all water and other hydrogen containing molecules from the reagent gas; therefore, isotope ratios should not be used from charge exchange mass spectra to estimate the elemen­ tal composition of molecular ions. Ex­ periments have been done using mixed charge exchange and proton transfer reagent gases by adding a small amount of H 2 0 to He or Ar (53, 54). Dissociative charge exchange oc­ curs from reactions of He + or Ar + , and (M + H)+ ions or (M + H 3 0)+ ions are produced from reactions of H 3 0 + . These spectra are routinely used for characterization of compounds. The CI spectra of polyfunctional compounds are often not directly pre­ dictable from the spectra of the monofunctional compounds. The CH 4 CI spectra of alcohols contain essentially no (M + H)+ ions (7, 55). The spectra of some steroidal diols in which the hydroxyl groups are widely separated by a rigid nucleus also show essential-

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ly no (M + H ) + ions (56). However, the C H 4 CI spectra of some α,ω-aliphatic diols show significant abun­ dances of (M + H)+ ions (57). These very obvious differences are attributed to t h e stabilization of (M + H ) + ions of t h e diols by internal hydrogen bonding; this internal hydrogen bond­ ing is possible in the case of the flexi­ ble aliphatic diols and impossible for the rigid 3,17-steroidal diols. We have used these internal hydro­ gen bonding effects of t h e unambigu­ ous determination of the stereochem­ istry of 2,5-diendoprotoadamantanediol. Of the four isomeric 2,5-protoadamantanediols, only this isomer can exhibit hydrogen bonding. For this isomer, (M + H ) + is the most a b u n d a n t ion in t h e Î-C4H40 mass spectrum. (M + H ) + is only a minor ion in the spectra of the other three isomers. These internal hydrogen bonding effects have been used for the differentiation of epimeric compounds, b u t there is not agreement about the necessary conditions for stabilization of (M + H ) + by internal hydrogen bonding (58-61). In some cases, internal hydrogen bonding leads to a decrease in the abundance of (M + H ) + ions by opening a new p a t h for decomposition. T h e C H 4 CI spectra of long chain aliphatic carboxylic acids and their methyl esters cortain (M + H ) + as the most a b u n d a n t ion; however, the CH4 CI spectra of t h e diacids and diesters of about the same molecular weight contain (M + H — H 2 0 ) + or (M + H - CH 3 OH)+ as t h e most a b u n d a n t ions (62). Much additional work is needed on t h e effects of intramolecular interactions on CI spectra. There has been much discussion about the relative sensitivities of E I and CI mass spectrometry, b u t few data have been presented on comparisons of the two techniques, and the comparisons are very difficult to make (63). Opinions of the sensitivities of the two techniques vary widely: I t has been reported t h a t CI with A r / H 2 0 mixtures is approximately 50 times more sensitive t h a n EI mass spectrometry (64), and also t h a t E I M S is approximately 4 times more sensitive than Î-C4C10 mass spectrometry (65). We have recently completed a comparison of EI vs. CI sensitivities in a modified CEC (Du Pont) 21-110B mass spectrometer. In this instrument we attempted to optimize the sensitivities of the two modes with different ionization sources. T h e CI sensitivity strongly depends upon experimental conditions. Our experiments indicated essentially equal sensitivities for t h e two techniques under the best conditions which we were able to obtain for each technique (66). For many complex molecules, however, t h e abundance of the molecular ion is almost

CIRCLE 94 ON READER SERVICE CARD 776 A · ANALYTICAL CHEMISTRY, VOL. 4 9 , NO. 9, AUGUST 1977

negligible, b u t (M + H ) + ions are a b u n d a n t in CI spectra. Selected ion monitoring of these high mass ions will be much more sensitive for CI t h a n for EI mass spectrometry. A somewhat less precise measure of sensitivity in CIMS is given by sample sizes t h a t are used for analyses or levels of impurities which may be detected. Detection limits of S F 6 in air with GC/charge exchange were estimated at 3 Χ 1 0 - 1 2 g of sample, a n d quanti­ tative analyses were obtained at t h e 20 p p b (v/v) level of SF6 in air, or on lO-io g 0 f sample. T h e sensitivity of this analysis was considered to be "substantially better" than t h e sensi­ tivity of electron ionization and "com­ parable t o " t h e sensitivity of electron capture (67). Also organic compounds in water may, be detected at the p p m level (or about 50 ng of sample) using water from the solution as the CI re­ agent gas (68). Our G C / M S is not a particularly sensitive one, and we are able t o detect approximately 1 0 - 1 0 g of sample with the selected ion moni­ toring technique (69). T h e kinetic analysis mentioned be­ fore indicates t h a t the extent of con­ version of reactant ions t o sample ions depends on the number of collisions the reactant ions make in the source of the mass spectrometer. One method of increasing the number of collisions be­ tween reactant ions and sample mole­ cules is t o increase the pressure within the source. Instrumentation has been developed in which the source of t h e mass spectrometer is operated a t 1 atm, atmospheric pressure ionization mass spectrometry (70, 71) I t has been reported t h a t 10~ 1 3 g, or lower, of basic compounds can be detected with this technique. With this extremely high sensitivity, trace levels of impuri­ ties are readily detected in almost every sample. At t h e other extreme, it is possible to use t h e "trapped ion m o d e " of ion cyclotron resonance mass spectrome­ try to keep the reactant ions within the source chamber for times as long as seconds (compared with millisec­ onds in high-pressure mass spectrom­ etry). Under these conditions it is pos­ sible t o observe extensive conversion of reactant ions t o sample ions even for very low-pressure ( 1 0 - 7 torr) sam­ ples. Even though few ions are pro­ duced at the low pressures of iCR ex­ periments, essentially all of these ions are detected. Hence, it may be possi­ ble t o analyze very low-pressure, ther­ mally unstable molecules with this technique (72). All of the preceding discussion has been directed toward t h e use of posi­ tive ions a n d their reactions for analy­ sis. Many instruments now have t h e capability of detecting either positive or negative ions, and there are many

i l l u s t r a t i o n s of n e g a t i v e c h e m i c a l i o n ­ i z a t i o n m a s s s p e c t r o m e t r y for a n a l y s i s (73, 74). L a c k of s p a c e p r e v e n t s a d i s ­ c u s s i o n of t h i s i m p o r t a n t a r e a of r e ­ s e a r c h in a n a l y t i c a l a p p l i c a t i o n s of i o n - m o l e c u l e r e a c t i o n s . O n e of t h e m o s t r e c e n t d e v e l o p m e n t s in t h i s a r e a , h o w e v e r , is t h e d e v e l o p m e n t of a q u a drupole mass spectrometer with the c a p a b i l i t y of s i m u l t a n e o u s d e t e c t i o n of b o t h p o s i t i v e a n d n e g a t i v e i o n s (75).

Acknowledgment T h e a u t h o r is g r a t e f u l t o B a r b a r a Jelus, Frank Hatch, and Charles Polley for t h e i r a s s i s t a n c e .

References (1) Sir J. J. Thomson, "Rays of Positive Electricity and Their Applications to Chemical Analysis", Longmans, Green, London, England, 1913. (2) F. W. Aston, "Mass Spectra and Iso­ topes", Longmans, Green, London, En­ gland, 1933. (3) A. J. Dempster, Philos. Mag., 31,438 (1916). (4) V. L. Tal'roze and A. K. Lyubimova, Dokl. Akad. Nauk SSSR, 86, 909 (1952). (5) D. P. Stevenson and D. O. Schissler, J. Chem. Phys., 23,1353 (1955); D. O. Schissler and D. P. Stevenson, ibid., 24, 926 (1956). (6) F. H. Field, J. L. Franklin, and F. W. Lampe, J. Am. Chem. Soc, 78, 5697 (1956). (7) M.S.B. Munson and F. H. Field, ibid., 88, 2621 (1966). (8) G. P. Arsenault, J. R. Althaus, and P. V. Divekar, Chem. Commun., 1414 (1969). (9) H. M. Fales, G.W.A. Milne, and M. L. Vestal, J. Am. Chem. Soc, 91, 3682 (1969). (10) C. T. Pate, J. L. Sprung, and J. N. Pitts, Jr., Org. Mass. Spectrom., 11, 552 (1976). (11) R. G. Gillis, M. J. Lacey, and J. S. Shannon, ibid., 9, 359 (1974). (12) R. L. Foltz, Chem. TechnoL, 5, 39 (1975). (13) Takeshi Murata, Seiji Takahashi, and Tsunezo Takeda, Anal. Chem., 47, 577 (1975). (14) Yukio Shimizu and Burnaby Munson, unpublished data. (15) Burnaby Munson, in "Interactions Between Ions and Molecules", Pierre Ausloos, Ed., ρ 505, Plenum, New York, N.Y., 1975. (16) D. K. Bohme, ibid., ρ 489. (17) D. P. Ridge, thesis, California Insti­ tute of Technology, Pasadena, Calif., 1972. (18) Timothy Su and M. T. Bowers, J. Am. Chem. Soc, 95, 7609 (1973). (19) Lucette Hellner and L. W. Sieck, J. Res. Nat. Bur. Stand., 75A, 487 (1971). (20) M.S.B. Munson and F. H. Field, J. Am. Chem. Soc, 88,4337 (1966). (21) J. Michnowicz and B. Munson, Org. Mass. Spectrom., 6, 283 (1972). (22) T. A. Whitney, L. P. Klemann, and F. H. Field, Anal. Chem., 43, 1048 (1971). (23) G. Gioumousis and D. P. Stevenson, J. Chem. Phys., 29, 294 (1958). (24) T. F. Moran and W. H. Hamill, ibid., 39,1413 (1963). (25) S. K. Gupta, A. G. Jones, A. G. Harri­ son, and J. J. Myher, Can. j . Chem., 45, 3107 (1967). (26) T. Su and M. T. Bowers, J. Chem. Phys., 58, 3027 (1973).

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B u r n a b y M u n s o n is p r o f e s s o r of c h e m i s t r y a t t h e U n i v e r s i t y of D e l a ­ ware. Dr. M u n s o n earned his BA, MA, a n d P h D d e g r e e s from t h e U n i v e r s i t y of T e x a s in 1 9 5 4 , 1 9 5 6 , a n d 1959, r e ­ spectively. H e was associated with t h e Esso Research and Engineering Co. f r o m 1959 u n t i l 1967 w h e n h e j o i n e d t h e f a c u l t y a t t h e U n i v e r s i t y of D e l a ­ ware. Dr. M u n s o n was coinventor with F r a n k F i e l d of t h e c h e m i c a l i o n i z a t i o n t e c h n i q u e while a t Esso. His c u r r e n t research interests are in electron a n d chemical ionization mass spectrome­ try, kinetics, and t h e t h e r m o c h e m i s t r y of i o n - m o l e c u l e r e a c t i o n s .