CHEMICAL IONIZATION MASS SPECTROMETRY BURNABY M U N S O N Department of Chemistry University of Delaware Newark, Del. 19711
Chemical ionization (CI) mass spectrometry is a relatively new technique for the production of mass spectra of compounds in which the ions from the molecules of interest are formed by ion-molecule reactions. The method of ion production is different from t h a t used in electron ionization ( E I ) , field ionization ( F I ) , or photoionization ( P I ) mass spectrometry, and the decomposing species and consequent C I mass spectra are very different from the spectra produced by the other techniques. T h e technique is the outgrowth of mass spectrometric studies of ion-molecule reactions. Since the introduction of the technique in 1966 (1), about 35 papers have been published in this area. At present, there are about 10 laboratories studying CI mass spectrometry. (The use of this technique is covered by U.S. P a t e n t 3,555,272 by F . H . Field and M . S. B . Munson, assigned to Esso Research and Engineering Co., with exclusive manufacturing rights to Scientific R e search Instruments Corp., Baltimore, Md.) Principles of Operation
T h e technique itself is a relatively simple one, but modifications are necessary on conventional mass spectrometers to allow the CI technique to be used. T h e majority of the instruments presently being used have been built or modified by the researchers (1—4), but modifica28 A .
tions are now commercially available for the major instruments (5). Since CI mass spectrometers must operate at source (or ionization) chamber pressures of the order of 1 torr (much higher than the 10~4 torr maximum of conventional mass spectrometers), the major modifications are the addition of diffusion pump capacity and the reduction in size of the electron entrance and ion exit slits to the source chamber. With these modifications, it is possible to maintain the pressures in the flight tube or analyzer section of the mass spectrometer a t less than 10~B torr. No modifications of the mass separation, mass measurement, recording, or data-handling equipment associated with conventional instruments are necessary. T h e basic technique requires a large amount of a reactant gas and a small amount of the sample to be analyzed. T h e ratio of reactant gas to sample should be of the order of 10 3 , although this ratio is not a critical parameter. Ions are produced in the mixture of these two gases which react with the neutral molecules to form a distribution of ions which is characteristic of both sample and reactant gases. T h e ions are generally formed initially with high-energy electrons (50—500 eV). Because of the very large excess of the reactant gas, ions from the reactant gas are essentially the only ones produced by electron ionization, and it is the subsequent reactions of these ions t h a t are impor-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
t a n t and produce the C I spectra of the samples. Systematic studies have been made of several classes of compounds: alkanes (6), cycloalkanes (7), alkylbenzenes (8), alkenes and alkynes (9), alkanols (10), esters of monocarboxylic acids (11), esters of di- and tricarboxylic acids (12), alkaloids (13), amino acids (14), barbiturates (15), substituted benzophenones (2), aryl ketones (16), steroids (16), and dimeric cyclic ketones (17). Somewhat less extensive studies have been made on several other systems: amines (1), C 7 H 8 isomers (18), R X and R C O R ' (X = CI, Br, I ; R = C„H 2 „ + , ) (19), R C O O C H 2 O C H 3 (20), R C O O C H 2 SCH ; ! (21), borazine (2£), pristane (23), and botryodiplodin (24). Preliminary results have been given of the sequencing of peptides as simple peptides (25, 26) and as the phenylthiohydantoin derivatives formed by the E d m a n degradation (27). Nucleosides also appear to be susceptible to analysis by chemical ionization (28). T h e basic principles of CI mass spectrometry have been reviewed previously elsewhere (4, 19, 29). T h e most common reactant gas for CI studies has been methane, because it was the first reactant gas tried and because it generally gives information about molecular weight and molecular structure from (M -f- 1)+ and fragment ions (1, 6—9, 11, 13-15, 17, 23-26). Isobutane,
REPORT FOR ANALYTICAL CHEMISTS
Chemical ionization mass spectra of compounds are produced by ions formed by ion-molecule reactions. In the basic technique, a small amount of sample is used with a large amount of reactant gas. Conventional mass spectrometers can be modified to use this relatively new technique and the spectra produced are very different from the spectra in other types of mass spectrometry
as a r e a c t a n t gas, produces less fragmentation of t h e sample t h a n methane and has been used for several studies (10, 12, 20, 21, 27). P r o p a n e is intermediate between methane and isobutane in the a b u n dance of fragment ions produced from the samples (2). Preliminary results have been reported for basic compounds with N H 3 and with C D 4 as the reactant gases (28). Combinations of gas chromatogr a p h y and chemical ionization mass spectrometry h a v e been made in which the carrier gas for the chrom a t o g r a p h has been used as the rea c t a n t gas for the C I technique (SO, 31). Such a combination is now commercially available (5). High-resolution C I mass spectrometry and precise mass measurements for the determination of elemental compositions have been r e ported. D a t a have been obtained in high-pressure operation (CI) a t resolutions of 10,000-50,000 (2, 3, 13a, 24, 32, 33). D e g r a d a t i o n of resolution from t h a t obtained in low-pressure operation occurs only with i n adequate pumping a t high pressures when the pressure in t h e analyzer section becomes sufficiently high t o cause collisional broadening of t h e peaks (2). Mixtures of perfluorinated or perdeuterated hydrocarbons have been found successful as reference materials for precise mass measurement (33). As long as the ions of the r e a c t a n t gases are unaffected by instrumental changes, t h e C I mass spectra
should be independent of these changes. Recently, it h a s been shown t h a t the C I mass spectra a r e independent of electron energy, electron current, a n d accelerating voltage (16). If t h e pumping is adequate such t h a t t h e pressure in the region outside the source is sufficiently low, then t h e spectra a r e independent of t h e pressure of r e actant gas within the source chamber (2, 16). As t h e size of t h e sample increases, the concentration of sample within the source chamber increases and collisions of sample ions with sample molecules occur frequently to form (2 M -\- 1) + ions from reactions of the (M -f- 1 ) + ions with polar samples (16, 34). However, the spectra are independent of sample size for small samples when the (2 M -\- 1) + ions are less t h a n a few percent of t h e sample ionization (16). T e m p e r a t u r e and repeller voltage within t h e source have noticeable effects on t h e C I spectra. A n i n crease in fragmentation is noted with an increase in t e m p e r a t u r e (6, 11, 20, 21, 34, 35). T h e temperature effects are more pronounced with isobutane t h a n with methane (34, 35). Although t h e C I spectra are substantially independent of repeller for low voltages (0—10 V) (16), appreciable changes are produced in C I spectra a t high-repeller voltages ( > 3 0 V ) , particularly with isobutane (2). T h e high-repeller field probably gives the sample ions
sufficient translational energy t h a t some decompose on collisions within the source. T h e analytical possibilities of this technique have been mentioned (2). There are n o t y e t enough d a t a available for t h e same compounds on different instruments to make valid comparisons of spectra o b tained with different instruments. N o r is there y e t agreement on a standard set of conditions for comparison of spectra among different workers. T h e few d a t a available suggest reasonably good agreement (2, 16) ; however, all instrumental parameters have n o t y e t been thoroughly investigated. A rigorous comparison of the sensitivities of C I a n d ET mass spectrometry cannot be made a t present. T h e sensitivities appear roughly comparable, however, and useful C I spectra have been obtained on a few tenths t o a few micrograms of sample (16, 25, 26, 28). Since the n u m ber of ion-molecule collisions increases rapidly with increasing pressure, the fraction of sample ionization can be expected t o increase with increasing pressure. However, because of ion scattering and other loss phenomena, t h e absolute ion current per unit of sample can be expected to maximize on any instrument in CI operation as the pressure is increased. Very little work has been reported on q u a n t i t a t i v e analyses of mixtures. There should n o t be major difficulties in t h e development of
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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Report for Analytical Chemists
quantitative analyses, however. I n the first report of this technique, an analysis was made of simple mix tures of n-alkanes using a known mixture to obtain the relative sensi tivities of the compounds under the experimental conditions (1). Quan titative analyses have been reported on samples of phenylthiohydantoin derivatives of amino acids from pro teins; in these experiments, an in ternal standard was necessary since the materials were introduced into the source chamber with a directinsertion probe (27). Since the most common reactant gas for chemical ionization studies has been methane, a description of this system will be used to illustrate the C I technique.
Figure 1.
Typical high-pressure mass spectrum of methane
Reactant Gas, Methane
The direct ionization of methane with high-energy electrons (E > 50 eV) gives several ions: CH4 + e- -* CH 4 + , CH3+, CH2+, . . . .
(1)
However, C H 4 + and C H 8 + are by far the most a b u n d a n t of these ions, being formed in approximately equal amounts and comprising about 9 0 % of the total ionization. Since the s a m p l e / m e t h a n e ratio is of the order of 10 - 3 , only a very small fraction of the ions are pro duced by direct ionization of the sample. CH4+ and C H 3 + , the major ions produced by direct ionization of the reactant gas, will collide with the methane molecules, which are by far the most a b u n d a n t species in the mixture, to produce other ions by well-established, very rapid reac tions (36) : CH4 + + CH4 -* CH5+ + CH3 (2) CH3+ + CH4 — 02Ηϋ + + H2 (3) Some collisions occur between C H 4 + or CH,s+ and the sample molecules. These reactive collisions will produce ions dependent upon the sample. The rate constants for reactions of C H 4 + or C H 3 + with methane and the rate constants for reactions of C H 4 + or C H 3 + with the sample are of the same order of magnitude. Consequently, because of the factor of 10 3 between the con centrations of C H 4 and sample, the major reactions of C H 4 + and C H 3 + are those indicated with methane, Reactions 2 and 3. A small fraction 30 A ·
of ions from the sample m a y be at tributed to reactions of CH4+ and C H 3 + with the sample. These two product ions of reac tion with methane, C H s + and C 2 H 5 + , are unreactive in methane. Consequently, C H B + and C 2 H 5 + are the major ions in the high-pres sure mass spectrum of methane, be ing present in approximately equal amounts and comprising about 9 0 % of the total ionization. Smaller amounts of other ions are present, C 2 H 8 + , C 2 H 4 + , C 3 H B + , C 3 H 7 + , and C4H 8 + which are formed from other ions and ion-molecule reactions (36). Figure 1 shows a typical highpressure mass spectrum of methane. T h e distribution of ions is substan tially constant for pressures greater than 0.2 torr. T h e amounts of C3H7+ and the C4 and C5 species will depend on the purity of the methane, because these ions are formed primarily by ionic reactions with impurities. Their abundances and those of the other major ionic impurity, m/e = 19, H 3 0 + from proton transfer to water, will v a r y with pressure as well. At pressures of 0.2—2 torr, ions will undergo m a n y collisions within the source of a mass spectrometer. T h e majority of these collisions will be with the major component, meth ane, but some of the collisions will be between C H 5 + or C 2 H 5 + and the sample. T h e collisions between C H 3 + or C 2 H 5 + and the sample are
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
highly reactive and will involve pre dominantly proton transfer, CH5+ + Χ Η - Χ Η ΐ + + CH4
(4)
C2H5+ + X H ^ XH2 + + C2H4
(5)
or hydride transfer, C2II5 + + XII — X + + C2II6
(6)
These rapid proton and hydride transfer reactions are exothermic and the excess energy will be dis tributed between the ionic and neu tral products. Enough energy m a y remain in X H 2 + or X + for the ions to decompose to produce a set of ions characteristic X H : XH.+ - X + + H ,
(7a)
Xa+^ii+ift
(7b)
X
+
-*BÏ
+
+ D,·
(8)
T h e ions produced, X H 2 + , X + , Α 4 +, Bi+, comprise the chemical ioniza tion mass spectrum of X H . E t h y l addition ions, (M -f- 2 9 ) + , are ob served for some compounds. T h e chemical ionization mass spectrum of X H is certainly charac teristic of the compound, but it also depends upon the reactant gas. The major reactions which occur are proton and hydride transfer in which the reactant ions are acting as Bronsted and Lewis acids. T h e extent of fragmentation, Reaction 7 or 8, depends on the acid strength of the reactant ion. Bronsted acid strength is ex pressed in terms of the proton af-
Report for Analytical Chemists
finity of PA(X):
the
conjugate
CHa+ —CH 4 + H +
base, (9)
AH, = PA(CH4) = Atf,(CH4) + AH,(li+) - Δ# / (ΟΗ ί +) (10) Lewis acid strength can be ex pressed in terms of t h e hydride af finity of the ion, H " A ( B + ), C 2 H 6 -*C 2 H r ,++ H -
(11)
AHn = H - A ( C 2 H Ô + ) = ΔίΓ/(02Η3+) +
ΔΗ/(Η-) - iff/tCH.)
(12)
F o r Reaction 4, AH, = ΔίΓ/(ΧΗ2+) + AHfiCHi) Ai/,(XH) - AHf(CB:,+) (13a) AH, = Ai//(CH4) + ΔΗ/(Η+) ΔΗ/(ΟΗ5+) - ΔΗ,(ΧΗ) + ΔίΓ/(Η+) - ΔΑ/(ΧΗ2+) (13b) AHt = PA(CH4) - PA(XH) (13c) T h e heat of reaction for a proton transfer reaction is determined b y the proton affinities of t h e sample and t h e conjugate base of t h e react a n t ion. A positive heat of reaction im plies an activation energy a n d an activation energy implies a slow r e action." I n general, endothermic proton and hydride transfer reac tions a n d endothermic dissociations, Reactions 7 a n d 8, do n o t occur in chemical ionization mass spec t r o m e t r y . Consequently, b y v a r y ing t h e acid strength of the reacting ion, it is possible to v a r y t h e exothermicity of t h e proton transfer reaction a n d hence the fragmenta tion. I n principle, it should be pos sible to find a reagent which will r e act with one functional group and not another. As a trivial example, H 3 0 + , P A ( H 2 0 ) = 164 ± 4 k c a l / mol (37) will transfer a proton to
an aliphatic alcohol but not to an alkane. Table I shows the hydride and proton affinities for the predominant ions of the more common CI reac tant gases. Only for hydrogen and methane are the protonated reac tant gas molecules observed. Hy dride transfer for H 3 + or CH 5 + is essentially identical to dissociative proton transfer to give 2H 2 or CH 4 + H 2 . A subtle mechanistic differ ence may exist. It is necessary to recall that as the proton affinities of the molecules increase, the strengths of the con jugate Bronsted acids decrease. On the other hand, as the hydride af finities of the ions increase, their strengths as Lewis acids increase. Of the reactant ions listed in Table I, H 3 + will give the most exothermic reactions for both hydride and pro ton transfer, and hence the most fragmentation. The tert-CiH
