Anal. Chem. 1985, 57, 680-683
800
Comparison of Isopentane and Isobutane as Chemical Ionization Reagent Gases Jeffrey M. M c G u i d and Burnaby Munson* Department of Chemistry, University of Delaware, Newark, Delaware 19716
The C I spectra obtained with /-C5H,2 as the reagent gas are frequently very simple and provide abundant (M H)' Ions for compounds more basic than /-C5H10. The /-C5H12spectra are similar to /-C4H10 C I spectra but may contain somewhat fewer fragment ions and frequently more molecular ions,,'M from charge exchange by the relatively abundant /-C5H10+ Ion. I n addition, /-C5H12 may be somewhat more selectlve than /*C4H1om The two reagents can prbbably be used lnterchangeably. One advantage of using /-C5Hl2 Is the ease by whlch the vapor may be Introduced without a complex vacuum manifold.
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Perhaps the most commonly used reagent gas in chemical ionization mass spectrometry (CIMS) is the first reagent gas used, CHI (I). Methane has the advantage that the two reagent ions, CH5+and C2H5+, react with virtually every organic compound with similar rate constants (2)since CHI and C2H4 are weaker bases than practically all other organic compounds. The spectra obtained with CHI as the reagent gas generally contain structurally useful fragment ions and frequently contain (M H)+ and/or (M - H)+ ions for the determination of molecular weights. In analytical problems for which fragmentation is not necessary, weaker acids are used. One of the most common reagent gases for simplified spectra is isobutane (3,4). The tert-butyl ion is a much weaker acid than CH5+ or C2H5+and i-C4H10 CI spectra show less fragmentation than CHI CI spectra. However, (M + H)+ ions are not always observed in isobutane CI spectra (31, and the absence of these ions may result from X- transfer to give (M - X)+ and t-C,HgX rather than from dissociative proton transfer (5). In earlier work on thermally labile compounds with in-beam CI, surface CI, direct exposure (or any of the other names suggested for the technique), we routinely used imC5H12 as the reagent gas (6). We wish to report a comparison of i-C5H12 and i-C4H10 as general CI reagents.
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EXPERIMENTAL SECTION The experiments were performed with a Du Pont 21-llOB mass spectrometer, modified for high-pressure operation. The data were acquired with a Data General Nova 1200 computer (6-8). The source was a collinear design in which the electrons enter and the ions leave the source along the same axis (7). The electron energy was nominally 630 eV and the ion exit plate was maintained at a potential of -2 V with respect to the source block. Some samples were introduced from a glass probe at temperatures of 35-170 "C, depending on the volatilities of the compounds. Other samples were introduced with a gas chromatograph through an interface which has been described previously (7). The source temperatures were 100-150 "C for these experiments. The i-C4HI0 (99.5%,Matheson) was introduced through a conventional glass gas-handling manifold with a low-pressure gas regulator (Matheson) to maintain a constant pressure. "he i-CsHlz(%+%, Fisher) was introduced from a small bulb of degassed liquid. A constant source pressure of i-CSH12 was easily maintained if this bulb was 'Present address: Hercules, Inc., Research Center, Wilmington,
DE 19894.
0003-2700/85/0357-0880$01.50/0
immersed in a beaker of water to maintain a constant temperature of the liquid i-CSH12. The samples which were analyzed were obtained from several commercial sources and were of the highest purity available. The pesticides were NBS standards.
RESULTS AND DISCUSSION The ion/molecule reactions in i-C5H12 have been reported previously to give m/z 71+, t-C5H11+,as the major product (9). Figure 1shows plots of relative abundances of the major ions in i-C4H10 and i-CbH12 as functions of pressure. The dominant product ions for these two compounds at high pressures are the alkyl ions, C4Hg+ in i-C4H10, and C5H1,+ in i-C5H12. Alkene ions, C4Hs+ in i-C4H10 and C5Hlo' in i-CSH12, are also present as reagent ions a t high pressures. The extent of conversion of Cz and Ca ions to C5Hlo' and C5Hll' with i-C5H12 is greater than the extent of conversion of Cz and C3 ions to C4Hs+and C4Hg+ with i-C4H10 at comparable pressures. Similar pressure plots will be observed in other high pressure sources, but the relative abundances of the ions at a given pressure will depend on the source geometry (ion path length) and the field strength within the source (repeller or ion exit plate voltage). The relative abundances of the ionic species can be used to check the pressure of the reagent gas on a day-to-day basis after such a plot has been obtained with a given mass spectrometer. A kinetic method for estimating source pressures from the ion/molecule reactions in i-C4H10 and other reagent gases has been reported (10). Plots of total ion current vs. pressure pass through a sharp maximum slightly below 0.1 torr for both i-C4H10 and i-CSH12. Figure 2 shows plots of sample ion current, as (M H)+ ions for a constant amount of decanophenone, C6H6COC9Hl9,for each of these two gases. The maxima in the plots of sample ion current vs. pressure occur at pressures that are slightly higher than the pressures for the maxima in the plots for total ion current vs. pressure. Pressure plots such as these will be observed for these gases with all mass spectrometer sources. The decreases in ion current at higher pressures result from the loss of ions by scattering outside the source and from a decreased penetration of the electrons into the source. Similar plots which show maxima in sample ion current vs. pressure are observed with all CI reagent gases. The pressures of the maxima in sample ion current are inversely related to the molecular weights of the reagent gases. That is, one can achieve a good sensitivity in CI operation a t a significantly higher pressure of CHI or NH3 than of i-C4HI0or i-C5H12. These experiments were performed with CBH5COCSHIg introduced with the glass probe using the same values of source and probe temperatures, electron current, and electron multiplier voltage for all pressures of i-C4Hlo and i-CSH12. Comparisons between reagent gases of sample ion currents at the same reagent gas pressure indicate a slightly lower sensitivity for i-C5H12 than for i-C4H,p However, there are comparable sensitivities for the two reagent gases at all pressures and the pressures for the maximum sensitivities are approximately the same. Maximum sensitivity occurs at pressures of about 0.1 torr for both gases. However, at 0.1 torr, the ratio, C4H9+/C3H7+, is about 1.7 for i-C4H10 and the C5Hll+/C3H7+ratio for i-C5H12
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0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985 I
I
I
1
1
I
1
881
tions. Both ions are weak Bronsted acids for proton transfer reactions
+
CxH2x+l+ B
+
BH+ + CxH2x
(1)
The heats of these proton transfer reactions are calculated from the proton affinities (or basicities) of the neutral molecules AH1 = PA(CXH2,) - PA(B) (2) The proton affinity is defined as PA(X) = AHf(X,g) + M f ( H + , g ) - m f ( X H + , g ) (3) The tert-butyl ion is a slightly stronger Bronsted acid than the tert-pentyl ion, since PA(i-C4H8) = 195.9 kcal/mol and PA(i-CSHlo) = 196.4 kcal/mol (12);that is, i-C4H8is a slightly weaker base than i-CSHlO.For comparison, PA(C3He) = 179.5 kcal/mol (12); consequently, s-C3H7+is a much stronger Bronsted acid than t-C4Hg+or t-C5H11+. Both are relatively weak Lewis acids for hydride transfer reactions
PRESSURE, TORR
CxH2x+1++ AH
8
40
-
CxH2x+2+ A+
(4)
The heats of these hydride transfer reactions can be calculated from the hydride affinities (or the acid strengths) of the ions
t/
AH(4) = H-A(A+) - H-A(CXH2,+1+)
(5)
The hydride affinity of the ion is defined as H-A(A+) = Mf(A+,g) 0.1
-
0.2 0.3 PRESSURE, TORR
0.4
Figure 1. Pressure varlatlon of relative abundances of major ions in
isobutane (top) and isopentane (bottom).
+ Mf(H-,g) - MAAH,g)
The tert-butyl ion is a slightly stronger Lewis acid than the tert-pentyl ion: H-A(t-C4Hg+)- H-A(t-C5Hll+) = 3.2 kcal/mol (13-15). One would expect similar small differences in Lewis acid strengths of these two ions for the abstraction of C1- or OH- groups. The hydride affinity of s-C3H7+is about 16 kcal/mol greater than the hydride afffinity of t-C4H9+;consequently, s-C3H7+is a significantly stronger Lewis acid than t-C4Hg+or t-CsHll+. CI spectra obtained with imC4H10 at low pressures (-0.1 torr) include significant contributions to sample ionization from reactions of s-C3H7+as well as from the expected reactions of t-C4H9+. CI spectra obtained with i-CSH12 at comparable pressures will contain much smaller contributions from reactions of s-C3H7+and will result primarily from reactions of t-C5H11+. The present set of experiments was designed primarily to study the reactions of t-C4Hg+ and t-CSHll+; consequently, the experiments were performed at reagent gas pressures greater than 0.25 torr. In geheral, the i-C4H10 and i-CSH12CI spectra are very similar. Simple ketones and amines give (M H)+ ions as the only significant species. For aliphatic alcohols, the dominant ions are (M - OH)+ with small amounts of (M - HI+. For simple esters, the dominant species are the (M H)+ions. Low sensitivities are noted with branched hydrocarbons to give (M - H)+ ions with little fragmentation; i-CSHl2discriminates against aliphatic hydrocarbons more strongly than i-C4H10. For some substituted aromatic compounds, the iC5H12 spectra contain significantly more M+- ions than the i-C4H10 spectra, presumably from charge exchange reactions of C5Hlo+. Spectra of several pesticides with these two reagent gases (Table 11) were very simple and very similar. For ronnel (see Table I for structures), both spectra contain (M + H)+ ions as the base peak and only two minor fragment ions whose abundances are the same for the two reagent gases: (M + H - CH30H)+, 1%, and 111+, 3%, of the base peak. For dimethoate, (M + H)+ is the base peak and the only fragment ion is 125+,probably (CH30)2PS+,14% of the base peak with both reagent gases. The spectra of CIPC are shown in Table
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0,2
0,l PRESSURE,
TORR
Figure 2. Pressure variatlon of sample ion current for MH+ ions from
decanophenone with isobutane and isopsntane.
is about 8. There are few data for direct comparison, but such data indicate that C3H7+reacts more rapidly than t-C4&+ and that t-C4Hg+reacts more rapidly than t-CSHll+for exothermic reactions (11). In addition, s-C3H7+reacts rapidly with many compounds with which t-C4Hg+and t-C5H11+do not react because the reactions of t-C4H9+and t-CSHll+with the compounds are endothermic and reactions of s-C3H7+with the same compounds are exothermic. At 150 "C a reaction with an activation energy of 3 kcal/mol will occur a t only 3% of the rate for the reaction without an activation energy. One may expect very similar reactivities for t-C4Hg+and t-C5H11+as CI reagent ions from thermochemical considera-
(6)
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table I. Structures of Pesticides and Steroids
Table 111. Comparison of i-CdHI0and i-C6HI2CI Spectra of Steroids
ronnel dimethoate
% base Desk"
i-C4Hlo
ion
r
Testosterone methoaychlor
(M+ 57)'
CIPC
3.6 f 0.4 2.9 f 0.9 100 50.5 2.2 f u.4 3 f l 2.5 f 0.4 4.1 f 0.7
(M + 57 - 18)' (M + H)+ M+ (M - H)+ (M - OH)+ (M - OH - HZO)+ (M - H - 2HzO)'
neburon
6
(M + 57 - 18)' (M + H)+ M+.
testosterone
3.4 f 0.4 100 3.6 f 0.2 3.5 f 0.4 90 f 3
(M - H)+ (M - OH)+ 0 N
Table 11. Comparison of i-C4HI0and i-C6HI2CI Spectra of Pesticides 9i base peak"
i-C4H10
i-C5H12
CIPC (M + H)+ M+. (M + H - C3HS)' CICGH~NH~'
100 1.7 f 0.7 54 f 5 2.2 f 0.5
100 20 f 2 79 f 5 50.5
Methoxychlor (M + H)+ M+. (M - C1)+ (M - CC13)+
(M- C&OCH3)+
61 f 10 5f2 86 f 14 4f2 100
7f3 50.5
8fl 5 f l 100
OAverage value f standard deviation of 10 measurements for each compound. All C1 isotopes are included. P(i-C4H10) 2 P(iC,H,.J = 0.3 torr: tkource) = 135 OC.
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50.5 50.5 100 2.5 f 0.9 5f2 12 f 3 6f2 4.1 f 0.4
Pregnenolone
pregnenolone
ion
i-C6H12
11. Again, both reagents give (M H)+ as the base peak and the dominant fragment ion is the loss of C3H6from (M H)+, 55% of the base peak for i-C4H10 and 80% for i-C5Hiz. Significantly more molecular ions are present in the i-C5H12 CI spectrum than in the i-C4H10 spectrum, presumably because of the greater abundance of C5Hlo' from i-C5H12 than C4H8+ from i-C4H,o. Also perhaps some of the M+. ions of CIPC formed by the higher energy charge exchange reaction with C4H8+have decomposed. If we assume that C4&+ is the isobutene ion, IP(i-C4H8) = 9.2 eV (16),and that C5H10' is the lower energy isopentene ion, IP(CH3CH=C(CH3)2)= 8.7 eV (16),then the presence of an abundant M+. ion in the i-C5H12 CI spectrum of CIPC indicates that CIPC has an ionization energy less than 8.7 eV. The spectra of neburon contain two major ions, (M + H)+ and 88': there is a difference in the ratio of the two major peaks, 88+/(M + H)+ = 0.8 for i-C4H10and 1.2 for i-C5H12. The ion at m / z = 88+ is perhaps CH3(C4Hg)NH2+ formed in a cyclic rearrangement decomposition of (M + H)+. The spectra of methoxychlor are listed in Table 11. The CH4 CI spectrum of methoxychlor is also relatively simple: (M + HI+, 39%; (M - Cl)', 100%; (M - CC13)+,5%; and (M - C6H40CH3)+,93%. The expectation was, of course, that decreasing the acid strength of the reagent ions would decrease
+
50.5 100 23 f 2 50.1 97 f 4
Average f standard deviation of 10 measurements. P(i-C,Hlo) P(i-C5HI2)= 0.3-0.4 torr; t(source) = 130 "C.
the extent of fragmentation and increase the abundance of (M + H)+ ions. This expectation was achieved to some extent by changing from CH, to i-C4H10: (M + H)+ = 39% of base peak or 17% of sample ionization with methane and 61% of base peak or 24% of sample ionization with i-C4H10. The weaker Bronsted acid, t-C5H11+, gives only small amounts of (M + H)+ ions, 7% of base peak or 6% of sample ionization. Perhaps, then, the proton affinity of methoxychlor is less than that of isopentene. If this is the case (and no data are available on the proton affinity of methoxychlor), then additional reactions must be postulated to explain the base peak in the spectrum, (M - C6H40CH3)+.Indeed, it is surprising that this ion, (M - C6H40CH3)+,is the most abundant ion in the i-C4H10 CI spectrum of methoxychlor. A reaction which is possible, although it cannot be established from these data, is the attack by t-C4Hg+or t-C5H11+ on the aromatic ring to alkylate the species, followed by the loss of an alkylated anisole so that the (M - C6H40CH3)+ion could be represented as (M + R - RC6H40CH3)+.This type of reaction is an extenstion of the X- transfer reaction observed with t-C4Hg+and alkyl halides or alcohols (and indicated by the (M -Cl)+ ions in the spectra of methoxychlor in Table 11). Since the i-C4Hio CI spectrum of methoxychlor contains 5% M+ and the i-C5H12 spectrum contains very little M+,
