Anal. Chem. 1989, 61, 2523-2528
TLC-LMS a potentially very useful analytical technique. LITERATURE CITED McFaddsn' w' TenhnlquesOf Combhsd chrome@FWhy'Mess Specirescopy: AppVcedkns In olgank Ana&ls; Wlley: New York, . . . e
iaia.
Modem RaCilCes ofchrrwnatosplsphy; Kkland, J. J., Ed.; Wlley: New York, 1971. Arplno, P. J.; Gukchon, 0.Anal. Chem. 1981, 57(7), 882A-70lA. Covey, T. R.; Lee, E. D.; Bruins, A. P.; Henlon, J. D. Anal. Chem. 1988, 58, 1451-148lA. Jounel of chrometCgrephy-WnC H@ Pwfomance Thin L a p chrometoprephy; Ziatkle, A., Kaiser, R. E., Eds.; Elsevfer Sclentlfic: Amsterdam, 1977; Vol. 9. Kalser, R. Chem. Brly. 1989, 5 , 54-81. Jacob. J. J . chrometog. Scl. 1975, 13, 415-422. Dwden, D. A.; Jwrlo, A. V.; Davls, B. A. Anal. Chem. 1980, 52, 1815-1820. ScheHers, S. M.; Verma, S.; Cooks, R. G. Anal. Chem. 1983, 55, 2280-2288. Warner. M. Anal. Chem. 1967, 59, 47-48A. Unger, S. E.; Vlncze, A.; Cooks, R. 0.Anal. Chem. 1981, 53, 978-981. DiDonato, G. C.; Busch, K. L. Anal. Chem. 1988. 58, 3231-3232. Chang, T. T.; Ley, J. O., Jr.; Francel, R. J. Anal. Chem. 1984, 58, 109-1 1 1. Stanley, M. S.; Busch, K. L. Anal. Chlm. Acta 1987. 194, 199-209.
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(15) Fide, J. W.; DIDonato, G. C.; Busch, K. L. Rev. Scl. Instrum. 1986, 57(9), 2294-2302. (18) Novak, F. P.; W L , Z. A.; Hercules, D. M. J . Trace Mlcroprok, Tech. 1985, 3(3), 149-183. (17) Novak, F. P.; Hercules, D. M. Anal. Lett. 1965, 18(A4), 503-518. (16) . . Helnen. H. J.: Meler. S.: Voat. H.: Wechsuna. -. R. Int. J . Mess Smctrom. Ion phvs. 1S83,~47.-19-22. (19) Kraft, R.; Butmer, D.; Franke, P.; Etzold, 0. Bkmed. EnvWn. Mess specbwn.1987, 14, 5-7. (20) Kraft, R.; Otto, A.; Zopfl, H. J.; Etzold, G. B&med. EnvWn. Mew Spectrom. 1987, 14, 1-4. (21) Cheng. W. C.; Lee, M. L.; Chou, C. K.; Lee, S. C. Anal. Bkchem. 1983, 132, 342-344. (22) Jost, W.; Heuck, H. E. Anal. Blochem. 1983, 135, 120-127. (23) Armstrong, D. W.; McNeeiy, M. Anal. Lett. 1979, 12(A12), 1285-1291. (24) Hercules, D. M.; Novak, F. P.; Vlswanadham, S. K.; WIk, 2. A. Anal. Chim. ACte 1967, 195, 61-71. (25) Kubs, A. J. M.S. Thesis, University of Pbburgh, 1988. (28) Karas, M.; Bachman, D.; Bahr, U.; Hlllenkamp, F. Int. J . Mess Spectrm. IOn Processes 1987, 78, 53-88.
RECEIVED for review June 26,1989. Accepted July 14,1989. This work was supported, in part, by the National Science Foundation under Grant CHE-84-11835.
Elimination of Unexpected Ions in Electron Capture Mass Spectrometry Using Carbon Dioxide Buffer Gas L. J. Sears and E. P. Grimsrud*
Department of Chemistry, Montana State University, Bozeman, Montana 5971 7
The hlgh-preswre electron capture (HPEC) ma88 spectra of tetracyanoethyiene (TCNE), tetracyanoquinodimethane (TCNO), wrfluoro-p-benzoquinone (fluoranil), perchloro-pbenzoquinone (chloranll), and perchioro-5,l-bis( cyciopentadbne) (p" are )drown to be greatly shrpll(kd when carbon dloxlde, rather than methane, is used as the buffer gas. Them compounds have previously been shown to be partkularly rutrc.ptibie to reaction with gabphase or surface-bound radkal specks, which are prevalent in an Ion source contalnlng methane. Through these secondary processes and rubaeqwni electron capture (EC) reactions, u r t e x p e ~ ~knr W of m a w Intenstty are observed wWh the use of CH, buffer gas. w#h COObuffer gas these unusual kns are eliminated, and only ions that can be expialned in terms of slmple resonance and dissoclatlve EC processes are observed. The hlgh k v d of sendtlvity normally expected of HPEC mrr# spectrometry k also maintained wtth CO, buffer gas. Other non-hydrocarbon buffer gases, including helium, argon, xenon, and nitrogen, are found to yield greatly diminished senrltlvlty relative to that observed with CH, and CO,.
INTRODUCTION The high-pressure electron capture (HPEC) ion source has demonstrated extraordinarily high chemical specificity and sensitivity in the mass spectrometric (MS) analysis of numerous molecules of environmental and biomedical importance (1-3). In HPECMS negative ions are generally formed in thermal electron capture (EC) reactions by either the dissociative or resonance EC mechanisms (4,5), shown in re0003-2700/89/0381-2523$01.50/0
-
actions l a and lb, respectively. e-
+
N
MX
A considerable body of M
+
MX-
X-
(la) (1b)
knowledge exists concerning these two EC mechanisms, and HPEC mass spectra can be readily interpreted or predicted for a given molecule when these reactions,alone, are operative. An inspection of the literature reveals, however, that HPEC spectra are often not explainable in terms of the simple dissociative and resonance EC reactions alone (6-11). In order to account for the HPECMS spectra reported in several studies, it has been necessary to suggest the occurrence of various secondary processes often involving the participation of gas-phase (6) or surface-bound (10) free radicals. These reactive species originate from the e-beam irradiation of the buffer gas and are thought to react with the analyte molecule prior to electron capture. The terminal negative ions thereby formed often include all or part of the reactive free radicals along with all or part of the anal@ molecule. Thw secondary reactions invariably complicate the spectra of molecules that undergo these reactions and can greatly confuse their identification by HPECMS. In this paper we report recent attempts in our laboratory to eliminate secondary reactions of free radicals in a highpressure ion source while maintaining the high level of sensitivity normally achieved by HPECMS. Since previous studies have indicated the participation of hydrocarbon-derived free radicals in secondary ion source processes (6-IO), we will focus here on the use of several non-hydrocarbon buffer gases-helium, argon, xenon, nitrogen, and carbon dioxide; and we will compare the results obtained with those observed 0 I989 Amerlcan Chemical Society
2524
ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989
A
B
(M+CHB+H-PCN).
1
(M+CH&CN)' 117
I I
(M+2H-ZCN).
H4
204
128
M'
C
D
M'
b
CN\,
CNHcN
CN
co2
CN
,CN
CN "CN
(M-CN)' 102
I,
1.
mlz Figure 1. HPECMS spectra of tetracyanoethylene (A and C) and tetracyanoquinodimethane (B and D) obtained by using methane (A and B ) and carbon dioxide (C and D) buffer gases. The ion source pressure was 0.5 Torr, and its temperature was 150 OC.
in methane. In view of several previous studies in which carbon dioxide has been reported to be particularly effective for thermalizing low-energy electrons (12-16), special consideration will be given to the use of this buffer gas. The EC-active molecules selected for study here have been previously shown to be particularly sensitive to prior reactions of gas-phase and surface-bound free radicals in a HPECMS ion source. These include tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNE), which McEwen and Rudat (6) have shown to react rapidly with many gas-phase hydrocarbon-derived free radicals prior to EC; and perfluoro-pbenzoquinone (fluoranil), perchloro-p-benzoquinone (chloranil), and perchloro-5,5'-bis(cyclopentadiene) (pentac), which Seam et al. (IO)have shown to react rapidly with surface-bound hydrogen atoms, but not gas-phase radicals, prior to EC. The relatively welldocumented behavior of these selected molecules in a HPECMS ion source undoubtedly also accounts for unusual and unexpected ions observed in the HPEC spectra of numerous other molecules; many such examples can be noted by careful inspection of the recent compilation of 361 HPEC mass spectra determined in methane buffer gas by Stemmler and Hites (17). EXPERIMENTAL SECTION All spectra reported here were obtained by using a doublefocusing, medium-dution mass spectrometer (VG Model 7070 E-HF).The pressure within the ion source was measured with a capacitance manometer and was maintained at 0.5 Torr. The temperature of the ion source was 150 OC. The f i i e n t emission current was 200 4, and the electron energy was 150 eV. Fluoranil, chloranil, and pentac (about 1 ng of each per analysis) were introduced to the ion source by a capillary gas chromatographic column, which was threaded through the GC/MS interface and
into the ion source block. The flow rate of helium carrier gas was 1-2 atm cm3min-', and the flow rate of the buffer gas was about 10 times greater. Tetracyanoethylene and tetracyanoquinodimethane (about 50 ng of each per analysis) were introduced to the ion source by a temperature-programmed direct insertion probe. All assignments of ion identity were verified by accurate mass measurements to within 1.0 millimass unit. The buffer gases used in this study were obtained as follows: COz (99.99%), CH, (99.97%), and Xe (99.9%) from Matheson; N2(Prepurified Grade), Ar (Prepurified Grade), and He (High Purity Grade) from Linde Specialty Gases. Nitrogen, argon, and helium were passed through an oxygen-removing trap (Alltech Associates) and molecular sieve filter (Alltech) prior to their introduction to the ion source. Methane was passed through an oxygen-removing trap, only, and xenon and carbon dioxide were used as received. In addition to the experiments to be reported here, each was also performed by using an ion source block in which a layer of gold had been electroplated on all interior surfaces. The mass spectral results obtained with this ion source using methane buffer gas, as well as the non-hydrocarbon gases, were essentially identical with those obtained with the normal stainless steel ion source.
RESULTS AND DISCUSSION Of the five non-hydrocarbon gases examined here for their potential use as buffer gases in HPECMS, only carbon dioxide was found to provide detection sensitivities comparable to those obtained in methane. The results obtained with carbon dioxide will be presented first, followed by a discussion of results and problems associated with use of the other nonhydrocarbon gases. Carbon Dioxide as Buffer Gas. The HPEC mass spectra of tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) obtained with methane buffer gas are shown in Figure
ANALYTICAL CHEMISTRY, VOL. 61, NO. 22, NOVEMBER 15, 1989 180
A
I
II..
I ,I.
180 M.
C
2525
D
M 244
0 I1
co2
::?!I:: II
ii
120
140
180
160
200
220
240
L
m/z Figure 2. HPECMS spectra of fluoranil (A and C) and chloranil (B and D) obtained by using methane (A and B) and carbon dioxide (C and D) buffer gases.
1A,B. McEwen and Rudat (6) have shown that in hydrocarbon buffer gases these two molecules react rapidly with gas-phase free radicals prior to electron capture, as shown in generalized form in reactions 2 and 3 where M is TCNE or
R' + M
(R + M) + e-
-
-
(R + M)
(R + M - CN)- + CN
(2)
(3)
TCNQ, R' is one of several free radicals formed by the continuous e-beam irradiation of the buffer gas, and (R + M CN)- is a negative ion that contains R' and a major fragment of TCNE (or TCNQ). In spectra A and B of Figure 1, the Occurrence of reactions 2 and 3 is evident, where the radical species involved are primarily IT,CHi, and C2H
