Anal. Chem. lg86, 58,2903-2907
2903
Effect of Ammonia Partial Pressure on the Sensitivities for Oxygenated Compounds in Ammonia Chemical Ionization Mass Spectrometry P a t r i c k Rudewicz' a n d B u r n a b y Munson* Department of Chemistry, University of Delaware, Newark, Delaware 19716 The sensitivities for monofuncflonal oxygenated compounds with proton affinities of 180-204 kcal/mol (alcohols, ketones, ethers, and esters) are very low under chemical ionization conditions with pure NH, at 0.5 torr and 180 O C . The sensitivities of these compounds increase with decreasing partial pressure of NH, at a constant temperature and total pressure. The sensitivities of bifunctional compounds having proton affinities similar to that of NH, are not affected by the partial pressure of NH,. These observations may be explained by a ligand-switching reaction for the monofunctional compounds and a solvation reaction for the bifunctional species.
yield more useful results than pure NH,. For example, it was reported that the pure NH, CI spectra of various biomolecules were weak and variable but that a CH4/NH3 mixture gave abundant (M NH4)+ions for the same compounds (24). Similarly, it was reported that a 1% NH3 in CHI reagent gas mixture gave analytically useful spectra for many compounds that gave no sample ionization with NH3 (25). A reagent gas mixture of NH3 in i-C4HIOproved more successful than pure NH3 in the analyses of carbohydrates (26). Another group obtained useful NH3 CI spectra of prostaglandins and other compounds of biological interest by placing ammonium chloride in a well probe with methane as the reagent gas (27). Despite these observations, no systematic studies have been done on the effect that changing the NH3 partial pressure has on sensitivities in NH3 CIMS.
+
Ammonia chemical ionization mass spectrometry (CIMS) has been used extensively for the analyses of many classes of organic compounds. Because of the relatively high proton affinity of NH, (204 kcal/mol) and the consequent weak B r ~ n s t e dacidity of the ammonium ion, NH3 CI spectra of many biomolecules often contain ions indicative of their molecular weights. Ammonia CIMS has been used for the analyses of carbohydrates (1-3), steroids and other lipids (4-7), peptides (8,9), nucleosides ( l o ) ,choline esters ( l l ) ,and also the animal and plant metabolites of drugs and other xenobiotic substances (12-14). Isotope exchange reactions with ND3 have been used for structural studies (15, 16) and for the determination of the number of active hydrogens in biological compounds (17). Ammonia CIMS has also proved useful for the study of the stereochemistry of various compounds (5,18, 19). Despite widespead use of NH3 CIMS, there are certain difficulties associated with NH, as a reagent gas. Both the reagent ion and sample ion spectra are very sensitive functions of experimental parameters (20);consequently, reproducibility of spectra among different laboratories is sometimes poor. In addition, the mechanisms of sample ionization and reasons for the marked differences in sensitivities for different classes of compounds have not been clearly established. In early work on NH3 CIMS it was reported that amines, amides, and a,P-unsaturated ketones were basic enough to be protonated by the ammonium ion to give MH+ ions, and these compounds, with the exception of tertiary amines, also formed (M + NH,)' adduct ions (21,22). Alkanes, aromatic hydrocarbons, ethers, monofunctional alcohols, and nitro compounds gave essentially no sample ions under NH3 CI conditions. Subsequent studies, however, have indicated that analytically useful NH, CI spectra may be obtained for these classes of compounds. For example, it was reported that ethyl ether, tert-butyl alcohol, and isopropyl alcohol formed abundant (M + NH,)' adduct ions (23). Additionally, NH3 CI spectra of steroidal alcohols and olefins have been reported (4). These conflicting reports suggest that NH, CI sensitivities may be dependent upon the NH3 pressure, which is generally not directly measured. Furthermore, studies have shown that dilute mixtures of NH3 in isobutane or methane often times
EXPERIMENTAL SECTION These experiments were done with a Du Pont 492B mass spectrometer and a Varian 2740 gas chromatograph with a jet separator interface. A Hewlett-Packard 21MX computer was employed for data acquisition (Du Pont data system). The source pressure was measured with a capacitance manometer (MKS Instruments, Burlington, MA) connected to the source through a hollow glass probe. The NH, was obtained from Matheson (anhydrous 99.99% minimum). The electron energy was 75 eV, and the emission current was 250 fiA. The accelerating voltage was approximately 1750 V. The temperatures were measured with a thermocouple attached to the CI source block. The sensitivity studies were done by observing the changes in sample ionization for each component in several equimolar mixtures as the partial pressure of NH3 in CHI was varied at a total source pressure of 0.5 torr and a source temperature of 180 OC. The mixtures consisted of four to six compounds of similar molecular weights and boiling points. Each mixture was introduced into the source of the mass spectrometer via the gas chromatograph (6 f t X 1/4 in., 3% SP-2100 packed column) in temperature-programmed experiments (50-150 "C at 8 OC/min). The composition of the reagent gas mixture was changed by using CH4 as the GC carrier gas and introducing NH, in through the CI reagent gas line. For example, a 50% NH, in CH, mixture was made with 0.25 torr of CHI from the GC and 0.25 torr of NH, from the reagent gas line. All reagent gas mixtures were made in this manner except for 1% NH3 in CHI, which was obtained from MG Scientific Gases (1.1%NH, in CH,). For a given mixture, the chromatographic conditions were chosen such that all components were well-resolved and each peak was sufficiently broad for the acquisition of 3-8 mass spectra (12-550 amu in 6.2 s). Ammonia CI sensitivity was determined from the total sample ionization for each component in a mixture. Sample ions for all the compounds in this study were (M + NHJ+ and/or MH+ ions. The data system was used to integrate the total sample ionization across each GC peak. For each mixture, the total sample ionization for each component was calculated relative to a very basic compound, pyridine or N-ethylaniline. Absolute NH3 CI sensitivities could not be determined using the GC for sample introduction because of the difficulty in introducing the same amount of the mixture at the different partial pressures of "3.
Present address: Smith Kline and French Laboratories, Department of Drug Metabolism, 709 Swedeland Rd., Swedeland, PA 19479.
RESULTS AND DISCUSSION Sensitivity in chemical ionization may be defined as the ion current of sample per unit sample pressure (28). Relative
0003-2700/86/0358-2903$01.50/00 1986 American Chemical Society
2904
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER
1986
a)--@/
A m
g 0.6
Tetrahydropyran
>_
f
A
I
0
20
+
In
8%
CH4
Figure 2. Sensitivities of monofunctional oxygenated compounds relative to pyridine (=1.O) as functions of the partial pressure of NH, in CH,.
Figure 1. Relative NH, CI sensitivities of (1) cyclohexanol, (2) tetrahydropyran, (3) 2-octanone, (4) hexyl acetate, (5) cyclohexanone, (6) ethylene glycol monobutyl ether, (7) amyl ether, (8) aniline, (9) mtoluidine, (10) pyridine, (11) N-ethylaniline, and (12) tripropylamine as functions of their proton affinities: pressure, 0.42 torr NH,, 0.08 torr He:source temperature, (170 i 10) OC. Proton affinity was estimated for 1, 3, 4, and 6 from proton affinities of homologues found in ref 29. sensitivity for two different compounds is simply the ratio of ion currents for the two compounds obtained under as nearly identical conditions as possible. The relative molar sensitivities of 12 compounds are plotted against their proton affinities in Figure 1, using NH3 as the reagent gas and N ethylaniline as the reference compound. (All proton affinities are taken from the Lias, Liebman, and Levin compilation (29).) The sensitivities of compounds with proton affinities less than 205 kcal/mol (alcohols, esters, ketones, aldehydes), ethers) are quite low with "pure" NH3 under these conditions (0.42 torr of NH,, 0.08 torr of He a t (170 f 10) "C) in agreement with earlier work with pure NH3 under similar conditions (30, 31). Under these conditions, compounds having proton affinities less than the proton affinity of NH,, 204 kcal/mol, give only (M + NH,)+ ions in their CI spectra. Compounds with proton affinities greater than that of NH, give both MH+ and (M + NHJ+ ions in their NH, CI spectra, except for tertiary amines, like tripropylamine, which show only MH+ ions. The increase in relative sensitivity with increasing proton affinity is in agreement with previous observations that the absolute sensitivities of the (M + NH4)+ions for oxygenated compounds with proton affinities less than 204 kcal/mol increased with increasing proton affinity (23). These previous experiments were done with NH, diluted with sufficient He to reduce the abundance of the NH,+.NH3 to less than 20% of the abundance of the NH4+ions at a source temperature of approximately 200 "C and 0.5-1.0 torr total pressure (23). We also note that with a mixture of 1%NH, in CH4 at 0.5 torr and 180 "C ([NH,+.NH,]/[NH,+] < 0.04) the sensitivities (as M NH4)+ions) of oxygenated compounds of low proton affinities (190-204 kcal/mol) increase with increasing proton affinities. A curve that was similar in shape in Figure 1 was noted previously (23) and is also obtained with the dilute mixture of 1% NH, in CH, as the reagent gas. Monofunctional Oxygenated Compounds. Although the NH, CI sensitivities for monofunctional oxygenated compounds are low with pure NH,, their sensitivities are strikingly dependent upon the total pressure of NH, in the system. Figure 2 shows the changes in sensitivities of 2-octanone, hexyl acetate, tetrahydopyran, and cyclohexanol relative to pyridine as functions of the partial pressure of NH, in CH,. As the composition of the reagent gas is changed from 100% NH, to 1% NH,3 in CH,, the total sample ionization for each compound increases relative to the sample ionization for pyridine. Furthermore, a t all compositions. the sensitivities
Cyclohexanol
60
40 %NH3
Proton Af fin1t y (kcal /mole)
2-Octonane Hexyl acetate
A
08
35,";
"3
%,NH+,
(NH3)2
+
0 5 torr, 1 8 0 ' ~
; LI:
-
041
0
02
i
,
A-* /*/A
!
1
40
20
60
80
%NH3 in CH,
Figure 3. Relative abundances of the ammonium ion and the solvated ammonium ions as functions of the percent of NH, in CH,. for the alcohol and the ether are nearly the same and much lower than those for the ketone and the ester. Sample ionization results predominantly from the reactions of NH4+and/or the solvated ammonium ions, NH4+.NH3and NH,+s(NH,)~.The relative abundances of these reactant ions are shown in Figure 3 as functions of the composition of the mixture at a total pressure of 0.5 torr and 180 "C. The major reagent ion in pure NH3 is the solvated ammonium ion, with equal amounts of NH,' and NH,+.(NH3)2. As the concentration of NH, in CH, is decreased the relative abundances of the solvated ammonium ions decrease, and with 1% NH,, NH4+ is the dominant reagent ion, [NH,+.NH3]/ [NH,'] < 0.04. With a 1 % NH, in CH, mixture some of the methane reagent ions, CH5+, CzH5+,and C3H5+,are also present (not shown in Figure 3). These ions, however, represent only 6% of the total reagent ionization; and even with 1% NH3 in CHI as the reagent gas, the sample ions result primarily from the reactions of NH4+ions. The values of the relative abundances of the reagent ions in this dilute mixture depend on the temperature, pressure, and ionic reaction time. An explanation for the decrease in sensitivities of these oxygenated compounds with increasing NH, pressure at the same total pressure is the occurrence of a ligand-switching reaction. The (M + NH4)+adduct ions may be formed by collisionally stabilized ammonium ion attachment (reaction 1)and/or by ammonium ion transfer from NH,+.NH3 (reaction 2). Once the adduct ion is formed, however, NH3 may
,+ ,"e,"
NH,+
+M
- + - + - + (M
(M + NH4)*+
+ NH4)*+ + CH, NH,+-NH? + M
(M
NH4)*+
(la)
NH4+ + M
(M
(M
NHJ+
NHJ+
(1b)
+ CH4* (IC)
+ NH?
(2)
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
is significantly greater than k,[M] under these conditions, and one can easily understand the low sensitivities of polar, but not strongly basic, compounds with pure NH3 as the CI reagent gas. One can use eq 6 to estimate a steady-state concentration of (M NH4)+
Table I. Enthalpies for the Reactions of NH,+ (1) and NH4+ NHs (2) with Monofunctional Oxygenated Compoundsa oxygenated compd
+
tetra-
reaction 1 2
PA
hydro-
hexyl acetate
2-octanone
pyran
cyclohexanol
-27.8 -3.0 202c
-27.6 -2.8 201'
-27.3 -2.5 199.7*
-25.5 -0.7 192?
"All enthalpies and proton affinities f 1.0 kcal/mol. *Proton affinity from ref 29. ?Proton affinity estimated from proton affinities of homologues found in ref 29. displace M in a ligand-switching reaction as shown in reaction 3. This ligand-switching reaction is simply the reverse of
(M + "4)'
+ NH3
+
NH4+*NH3
+M
(3)
reaction 2, the ammonium ion transfer from NH4+.NH3. Ligand-switching reactions have been studied in detail and have been reported as important processes in determining the composition of atmospheric ions (32). The degree to which the ligand-switching reaction occurs depends upon the solvation energy of NH4+ by M, (the NH4+-M bond strength) and the concentration of NH, in the source. The standard enthalpy of reaction 1 is simply the solvation energy of NH4+ by M, or the negative of the dissociation energy, D(NH4+-M). AH for reaction 2 is the difference in bond dissociation energies of NH4+with M and with NH3 as shown in the following:
AH1 = AHs(NH4+-M) = -D(NH,+-M)
(4)
A H 2 = D(NH4+-NH3) - D(NH,+-M)
(5)
where D(NH4+-NH3)is equal to 24.8 kcal/mol(33). The heats of solvation, AHs(NH4+-M), are estimated from linear correlations of ionic hydrogen bond dissociation energies for N-H+-.O bonds with the proton affinity difference, APA, between the proton donors and the proton acceptors (33,34). Direct measurements of the solvation energies of NH4+ions with polar oxygenated compounds are not available. The standard enthalpy changes, AH, for reactions 1and 2 for hexyl acetate, 2-octanone, tctrahydopyran, and cyclohexanol are listed in Table I. The slightly negative values for A",with these species results from the the slightly greater solvation of NH4+by oxygenated compounds than by NH,. Since the ligand-switching reaction 3 is the reverse of reaction 2, the ligand-switching reaction is slightly endothermic for each of the oxygenated compounds. Given that the proton affinities of hexyl acetate, 2-octanone, and cyclohexanol are estimated and that the uncertainty in the values of D(NH4+-M) is fl kcal/mol, the enthalpies for reaction 2 are roughly the same. Consequently, the differences in sensitivities of the oxygenated compounds shown in Figure 2 cannot be explained quantitatively by the thermochemical quantities listed in Table I. At high concentrations of NH,, the adduct ions, (M NH4)+,are formed predominantly by reaction 2, and the approximate differential rate expression can be written as follows:
+
d[(M
2905
+ NH4)+]/dt =
k2["4+*"31 [MI - k3[(M + ",)+I ["I, (6) If the ligand-switching reaction, which consumes (M + NH4)+ to form NH4+.NH3,is approximately 3 kcal/mol endothermic, then k 3 / k 2 = 0.03 a t 170 "C if one assumes equal collision efficiencies for the two reactions and that k3/kz = e x p ( - A H / R n . The concentration of M, [MI, is not known, but may be approximately 0.1% of [NH,]. Consequently, k,[NH3]
= (~Z[MI/~~~~[NH~+.",I/[NH,IJ (7) [(M + "4)'Iss From Figure 3, one can show that [NH,+.NH,]/[NH,] decreases somewhat, but not to a large extent, from pure NH, to 50% NH,. In Figure 2, one observes a noticeable increase in the relative sensitivities of the compounds over this range of NH, concentrations. At low concentrations of NH,, the relative sensitivities of these oxygenated compounds are much higher than they are at high concentrations of NH,, where the switching reaction 3 is much faster. At low concentrations of NH,, the (M + NH4)+adduct ions are formed predominantly by reaction 1 rather than reaction 2 because of the much lower concentration of NH4+.NH3ions. The approximate differential rate expression from reactions 1 and 3 is the following: d[(M
+ NH,)+]/dt
= kl,klc[NH4+][CH,] X - h,[(M + "4)+1["31
[Ml/(ki, + ki,[CH,I)
(8)
If one assumes that klc[CH4]> klb at high pressures of CH4, then eq 8 reduces to d[(M
+ NH4)+] =
kl,[",+l [MI - k3[(M + "4)+1 I",[ (9) With a mixture of 1%NH, in CH4, k,[NH,] may be 10-30% of kl,[M] for a system in which the switching reaction is endothermic by 3 kcal/mol. An increase in [NH,] will cause significant decreases in the abundances of the (M NH4)+ ions and, therefore, the sensitivities of these polar oxygenated compounds. Evidence for the ligand-switching reaction is shown in Figure 4. In this figure, the ion currents for NH4+ and NH4+.NH3are displayed in a GC/CIMS experiment with an equimolar mixture of 2-octanone, hexyl acetate, tetrahydropyran, cyclohexanol, and pyridine. In this experiment a reagent gas mixture of 1.1%NH, in CH4 at 0.5 torr and 170 OC was used. As each compound elutes from the GC, the ion current of the NH4+ion decreases and (M + NH4)+adduct ions are formed; consequently, NH4+reacts with each compound. In addition, as the oxygenated compounds pass through the mass spectrometer, the ion current for the NH4+.NH3increases. When pyridine elutes, the intensity of NH4+.NH3decreases. The ligand-switching reaction (reaction 3) occurs with the oxygenated compounds with proton affinities less than that of NH, but does not occur with the more basic pyridine. Ammonium ion transfer from NH4+.NH3 to pyridine is exothermic by 11.0 kcal/mol; hence, the reaction of NH4+-NH, with pyridine is thermochemically allowed. Some of this decrease in the ion current of NH4+-NH3in Figure 4, however, may also be attributed to a decrease in the intensity of NH4+, which reacts with NH, to form NH4+.NH,. The switching reaction with not occur readily with pyridine since it is endothermic by 11.0 kcal/mol and therefore k,/k, = e-"IRT = 4 X at 1 7 0 "C. Since NH,+.NH3 reacts more efficiently with pyridine than with the oxygenated compounds, the sensitivities of the oxygenated compounds decrease relative to pyridine as the partial pressure of NH3 is increased, i.e., ratio is increased. as the [NH4+.NH3]/ [",+I Bifunctional Oxygenated Compounds. In contrast to monofunctional oxygenated compounds, the NH, CI sensitivities for bifunctional oxygenated compounds do not decrease with increasing NH, concentration. The relative sensitivities for ethylene glycol dimethyl ether (PA = 204.9 kcal/mol)
+
2906
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986 Bose 21482
Bose 9 6 6 9 ( M + N H ~ ) Pyrldine,MH+ 2 - 0 c t a n o n e Hexyl acetate
Cyclohexanol 50
0
100
Base 3629 e
NH4(NH3)
dimethyl ether adduct relative to the dissociation energy of the c-C6H11NH3+,(C2H&0adduct has been reported (33). With this additional 4 kcal/mol for the second solvation step, the solvation energies of ethylene glycol dimethyl ether and ethylene glycol monobutyl ether with NH4+are estimated to be -32 kcal/mol in contrast with -28 kcal/mol for the solvation energy of NH4+ with isopropyl ether. The ammonium ion transfer reaction from NH,+-NH, (reaction 2) to the two bifunctional ethers is exothermic by approximately 7.2 kcal/mol. The switching reaction 3, therefore, is endothermic by 7.2 kcal/mol and does not occur to any significant extent even in 100% NH,, since an activation energy of 7.2 kcal/mol will reduce the reverse rate constant to a value of 3 x of the collision rate a t 170 "C. This invariance of the relative sensitivities of the bifunctional compounds with NH, pressure probably also results from other factors, since the mechanism for the formation of the (M + NHJ+ ions is different with 1% NH, in CH4 and with pure NH, as the reagent gases. With the dilute mixture as the reagent gas, the (M + NHJ+ ions must be formed by a process involving collisional stabilization
NH4++ M
Scan N u m b e r
+
Figure 4. Single ion traces of (a) NH,', (b) (M NH,)' for the monofunctional oxygenated compounds and MN' for pyridine, and (c) NH,+.NH, for a GC run of an equimolar mixture of tetrahydropyran (THP), pyridine, cyclohexanol, P-octanone, and hexyl acetate: reagent pressure, 0.5 torr: temperature, 170 'C. gas, 1% NH,/CH,;
MH'
+ NH3
(MHNH3)*++ CH,
(MHNH,)*+
i,,
lsopropvl Ether
I
20
40 60 %NH, In CH,
A
80
Figure 5. Sensitivities of isopropyl ether, ethylene glycol monobutyl ether (EGMBE), and ethylene glycol dimethyl ether (EGDME) relative to pyridine (=1.0) as functions of the partial pressure of NH, in CH,
ethylene glycol monobutyl ether (PA estimated to be 205 kcal/mol from homologues of known PA found in ref 29), and isopropyl ether (PA = 205.6 kcal/mol) are plotted in Figure 5 as functions of concentration of NH, in CH4 (pyridine, PA = 220.8 kcal/mol = 1). At all compositions of NH, in CH, the CI mass spectra of bifunctional ethers contain only (M + NH4)+ions (except at 100% NH, where they contain a small amount of (M + NH4.NH3)+ ions). Isopropyl ether and pyridine give predominantly MH' ions with 1%NH, and (M + NH4)+ ions with pure NH,. As shown in Figure 5, the sensitivities of the bifunctional ethers remain fairly constant (0.45 f 0.08) relative to pyridine as the partial pressure of NH, is increased. The relative sensitivity of the monofunctional ether, isopropyl ether, is about the same as the bifunctional ethers at 1% NH, in CH4 but it decreases significantly as the NH, pressure is increased. The different pressure dependences on NH, pressure displayed by the bifunctional ethers compared with the monofunctional isopropyl ether may be explained by hydrogen bonding between each oxygen in the bifunctional ether and the ammonium ion in the (M + NH4)+adduct ion complex. An increase of 4 kcal/mol in the dissociation energy of the ionic hydrogen bond for the c-C6HIINH1+.ethylene glycol
+ CH,
(MHNH3)*+
(M + NHJ+
+M
-
MH+ + NH3
-
-
(MHNH,)*+-
I
0.2
-
or more likely NH,+
rK
-
+ CH,*
(MHNH3)*+
NH,+
(M + NH,)'
+M + CH4*
(104
(lob) (10c) (Ila) (Ilb) (Ilc)
The latter reaction scheme appears more likely, since virtually no MH+ ions are observed for these bifunctional compounds under these or somewhat different conditions of NH, pressure. The lower sensitivities for these bifunctional ethers compared with pyridine using 1% NH, in CH4 as the reagent gas can be explained by the lower efficiency of the collisionally stabilized formation of (M + NH4)+with these compounds than of the simple proton transfer reaction from NH4+ to pyridine. The effective second-order rate constants for the formation of the adduct ions through reactions lla-c, keff = k~l,(k~,,[CH,l/(kll~ + k11,[CH4])},for small molecules are much smaller than the simple proton transfer reactions for the same molecules (28). Registry No. NH3, 7664-41-7;cyclohexanol, 108-93-0;tetrahydropyran, 142-68-7;2-octanone, 111-13-7;hexyl acetate, 14292-7; cyclohexanone, 108-94-1;ethylene glycol monobutyl ether, 111-76-2;amyl ether, 693-65-2;aniline, 62-53-3;rn-toluidine, 9553-4; pyridine, 110-86-1;tripropylamine, 102-69-2;N-ethylaniline, 103-69-5;methane, 74-82-8. LITERATURE CITED Hogg. A. M.; Nagabhushan, T. L. Tetrahedron Lett. 1972, 4 7 , 4827-4a30. Horton, D.; Wander, J. D.; Foltz, R. L. Carbohydr. Res. 1974, 3 6 , 75-96. Carr, S. A.; Reinhoid, V. N. Mass Spectrom. Rev. 1983, 2 , 153-221. Lin, Y. Y.; Smith, L. L. Biomed. Mass Spectrom. 1978, 5 , 604-611. Djerassi, C.: Tecon, P.; Hirona, Y. O r g , Mass Spectrom. 1982, 6 , 277-285. Ayanoglu. E.; Wegman, A.; Pilet, 0; Marbury, D.: Hass. J. R.; Djerassi, C. J . Am. Chem. SOC. 1984, 106, 5246-5251. Lin, Y. Y.; Smith, L. L.; Mass Spectrom. Rev. 1984, 3 , 319-355. Bjorkman S. Biomed. Mass Spectrom. 1982, 9 , 315-322. Baldwin, M. A . ; McLafferty, F. W. Org. Mass Spectrom. 1973, 7 , 1353- 1356. Esmans, E. L.; Freyne, E. J.; Vanbroeckhoven, J. H.; Alderweireldt, F. C. Biomed. Mass Spectrom. 1980, 7 , 377-380. Hasegawa, Y.; Maruyarna, Y.; Weintraub, S. T. Biomed. Mass Spectrom. 1984, 1 1 , 315-319. Cairns, T.; Siegrnund, E. G. Biomed. Mass Spectrom. 1984, 1 7 . 589-593.
Anal. Chem. 1980, 58,2907-2912 (13) Cairns, T.; Sigmund, E. G.; Rodney, L. B. Anal. Chem. 1984, 5 6 , 2547-2552. (14) Straub, K.; Levandowski, P. Biomed. Mass Specfrom., in press. (15) Hunt, D. F.; McEwen, C. N.; Upham, R. A. Tetrahedron Left. 1971, 4 6 , 4539-4542. (16)Buchanan, M. V. Anal. Chem. 1982, 5 4 , 570-574. (17) Lin, Y. Y.; Smith, L. L. Biomed. Mass Spectrom. 1979, I , 15-18. (18) Winkier, F. J.; Stahl, P. J . Am. Chem. SOC. I97B, 101, 3685-3687. (19) Tabet, J. C.; Tondeur, Y.; Hlrano, Y.; Wegmann, A,; Tecon; P.; Djerassi, C. Org. Mass Spectrom. 1984, 473-481. (20) Rudewlcz, P.; Munson, B., paper presented at the 32nd Annual Conference on Mass Spectrometry and Allied Topics, San Antonio TX, May 27-June 1, 1984. (21) Hunt, D. F. I n Advances in Mass Spectrometry; West, A. R., Ed.; Applied Science: London, 1974; Vol. 6. (22) Hunt, D. F. Prog. Anal. Chem. 1973, 6 , 359-376. (23) Keough, T.; DeStefano. A. J. Org. Mass. Spectrom. 1981, 16, 527-533. (24) Horning, E. C.; Stillwell, R. N.; Nowlin, J. G.; Carroll, D. I.Anal. Chem. 1981, 5 3 , 2007-2013. (25) Smith, D. E.; Smith, J. S.;Jerolamon, D.; Weston, A. F.; Richton, D.; Brozowski. E. J.. paper presented at the 26th Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, MO, May 28-June 2, 1978.
2907
(26) Dougherty, R. C.; Roberts, J. D.; Binkley, W. W.; Chizhov, 0. S.;Kadentsev, V. I.; Solov'yov, A. A. J . Org. Chem. 1974, 3 9 , 451-455. (27) Bose, A. K.; Fujlwara, H.; Pramanik, B. N.; Spillert. C. R.; Lazaro, E. Anal. Biochem. 1978, 8 9 , 284-291. (26) Harrison, A. G. I n Chemical Ionization Mass Spectrometry; CRC: Boca Raton, FL, 1983; Chapter 2. (29) Lias, S . G.;Llebman. J. F.; Levin, R. D. J . Phys. Chem. Ref. Data 1984, 13, 659-808. (30) Weinkam, R. J.; Toren, P. C., paper presented at the 31st Annual Conference on Mass Spectrometry and Allied Topics, Boston, MA, May 8-13, 1983. (31) Suzuki, M.; Tatematsu, A.; Takeda, N.; Konishi, H.; Nakata, H. Mass Specfrosc. (Tokyo) 1963, 4 , 275-279. (32) Ferguson, E. E. I n Kinetics of Zon Molecule Reactions; Plenum: New York; 1978; pp 377-403. (33) Meot-Ner, M. J . Am. Chem. SOC. 1984, 106, 1257-1264. (34) Davidson, W. R.; Sunner, J.; Kebarle, P. J . Am. Chem. SOC. 1979, 107. 1675-1680.
RECEIVED for review May 19, 1986. Accepted August 4,1986. This work was supported by a grant from the National Science Foundation. CHE-8312954.
Electron Capture Negative Ion Chemical Ionization Mass Spectrometry of 1,2,3,4-Tetrachlorodibenzo-p -dioxin J. A. Larameg, B. C. Arbogast, and M. L. Deinzer* Department of Agricultural Chemistry and Environmental Health Sciences Center, Oregon State University, Corvallis, Oregon 97331
The conditions necessary for reproducible electron capture negative ion chemlcal lonlzatlon (ECNICI) mass spectrometry of polychlorodlbenzo-p-dloxlnsare dependent on the nature and pressure of the reagent gas In the lonlzatlon source, as well as on other controllable parameters. A comparison of argon, xenon, sulfur hexafluoride, hydrogen, and helium reagent gases for production of molecular Ion Me-,[M a]-, and the CI- Ions from 1,2,3,4-tetrachlorodibenzo-p-dloxln [1,2,3,4-TCDD] shows that Intensities of these ions are hlghly pressure-dependent. These results are rattonallzed on the basis of pressure-dependent electron thermalization as It affects relative cross sectlons for resonance and dlssoclatlve electron capture. The results from this study show that heavier mass gases give best sensitlvltles at lower lonlzatlon source pressures. Helium as reagent gas glves the most intense molecular Ion, and the most llnear Ion abundance over a large pressure range.
-
Electron attachment negative ion chemical ionization mass spectrometry has become an important technique for the analysis of chlorinated aromatic compounds in environmental samples ( 1 ) largely because of its inherent sensitivity and specificity. However, this method is plagued by variable and irreproducible results and the appearance of artifact peaks in the mass spectra (2). A systematic investigation of some variables affecting negative ion mass spectrometry results has now been carried out and we wish to report some of our findings. For compounds with large electron affmities and/or thermal electron capture sites, the technique of negative chemical ionization (NCI) yields detection limits as much as 3 orders of magnitude lower than the positive ion mode. Thus, NCI is the preferred method for polyhalogenated aromatic analytes,
particularly for trace analyses of pesticides, wood preservatives, herbicides, and environmental toxins generated as byproducts from the manufacture of these halogenated compounds. We report investigations using six different reagent gases: methane, hydrogen, helium, sulfur hexafluoride, xenon, and argon. Enhancements of molecular anion or fragment ion currents provide useful a priori data which are of value for organic analytical applications. Negative ion mass spectra are the result of three ion-formation processes: (1) resonance capture, (2) dissociative resonance capture, and (3) ion-pair formation ( 3 ) . The abundance of products from these processes is critically dependent upon, among other factors, sample pressure, reagent gas pressure and nature, ionizing electron energy and current, and ion source temperature ( 4 ) . Of the three negative-ion-formation mechanisms only ionpair production is independent of the ionizing electron's energy although it is dependent upon Frank-Condon factors for excitation. Both resonance and dissociative resonance capture cross sections depend upon, among other considerations, the nature and pressure of the reagent gas and the ionizing electron energy. Resonance capture is the primary mechanism responsible for the formation of molecular radical anions. The cross sections for these modes of ion formation, each with their own dependence on reagent gas pressure, account for the poor reproducibility commonly experienced in negative chemical ionization.
EXPERIMENTAL SECTION All measurements were performed on a Finnigan 4500 quadrupole mass spectrometer,equipped with a negative ion detection kit ( 5 ) . The instrumental conditions used were 70 eV electron energy, 0.30 MA electron emission, and an axial ion energy of 5 f 1 eV. The source temperature was 90 "C. All spectra were recorded by a NOVA 3 minicomputer operating under the INCOS software.
0003-2700/86/0358-2907$01.50/0 0 1986 American Chemical Society
