Anal. Chem. 1991, 63, 721-725
(2) Charles, M. J.; Been, B.; Tondeur, Y.; Hass, J. R. Chemosphere 1989, 79 (I-6),51-57. (3) Charles, M. J.; Tondeur, Y. Environ. Sei. Technol. 1991, 24 (12),
1856-1860. (4) Busch. K. L.; Glish, G. L.; McCluckey, S. A. Mass SpectrometrylMass Spectrometry: Techniques and Applications of Tandem Mass Spec trometry; VCH Publishers, Inc.: New York, 1988; Chapter 3. (5) Dawson, P. H.; Douglas, D. J. Tandem Mass Spectrometry; McLafferty. F. W., Ed.; John Wiley & Sons: New York, 1983; Chapter 6. (6) Huang, L.; Tomer, K. 6.; McGown, S.; Moore, C. Presentation at the 10th International Conference on Organohalogen Compounds, Sept. 10-14. 1990, Bayreuth. Germany. (7) Scheiienberg. D. H.; Bobbie, 8. A.; Reiner, E. J.; Taguchi, V. Y. Proceedings of the 38th American Society of Mass Spectrometry (ASMS) Conference on Mass Spectrometry and Allied Topics; 1988. (8) Martinez, R . I . Rapid Commun. Mass Spectrum. 1988, 2 (I), 8-13.
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(9) Reiner, E. J.; Schelienberg, D. H.; Taguchi, V. Y.; McCurvin, D. M.; Clement, R. E. Proceedings of the 37th ASMA Conference on Mass Spectrometry and Allied Topics, Miami Beach, FL, 1989. (IO) Samson, J. A. R. Techniques of Vacuum Ultraviolet Spectroscopy; John Wiiey and Sons: New York, 1987. (11) Weast, R. C., Astle, M. J., Eds. CRC Handbook of Chemistry and Physics; CRC Press Inc.: Boca Raton. FL, 1982-1983. (12) Pauling, L. General Chemism; W. H. Freeman and Co.: San Francisco, 1970. (13) Analysis of polychlorinated dibenzo-pdioxins and dibenzofurans. VG Analytical Organic Mass Spectrometry; VG Analytical Limited: Wythenshawe. Manchester, M23 9LE England, 1987.
RECEIVED for review July 2,1990. Revised manuscript received November 21, 1990. Accepted December 17, 1990.
Quantitation Using Benzene in Gas Chromatography/Chemical Ionization Mass Spectrometry Charles Allgood,l Yee Chung Ma, and Burnaby Mumon* Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716
Dllute mixtures of benzene in helium provlde abundant C,H,'+ Ions, which selectlvely react by charge transfer under chemlcal ionlzatlon ( C I ) condltlons with unsaturated compounds in complex hydrocarbon mlxtures and do not react with alkanes or cycloalkanes. The charge-transfer spectra from the ion/molecule reactions of C,H~*+are very simple, containing essentially only M" Ions from the samples. Relative molar sensitivities for olefins and alkylbenzenes In benzene chemical ionizationmass spectrometry (CIMS) are essentlally constant within f10-15 %, Independent of molecular welght In the range of 100-200 Da, molecular structure, the degree of substitution, or Ionization energy. The relative molar sensitlvltles In benzene CIMS show much loss varlatlon with molecular structure than relative molar sensnlvltles In low-voltage electron ionization mass spectrometry (LV-EIMS). GCKIMS with benzene as the charge-transfer reagent gas allows quantitative analyses of the aromatlc and olefinic components In gasolines or other complex hydrocarbon mlxtures without the necesslty of calibration curves for the individual components.
INTRODUCTION Benzene has several advantages that make it an attractive reagent gas for the analysis of complex mixtures using gas chromatography/chemical ionization mass spectrometry (GC/CIMS). The ionization energy of benzene is 9.25 eV, about in the middle of the range of ionization energies for most organic compounds (1-3). Thus, benzene ions should undergo charge-transfer reactions with unsaturated hydrocarbons that have ionization energies lower than 9.25 eV. This selectivity for unsaturated compounds using benzene CI has been demonstrated previously (4-6). Since the proton affinity of the phenyl radical, 208 f 7 kcal/mol (3, 7) or 212 f 2 kcal/mol (3,8),is larger than the
* To w h o m correspondence should be addressed.
Present address: Organic Analytical Research Division, NIST, Gaithersburg, MD 20889.
proton affinities of most hydrocarbons ( 3 ) ,proton transfer from C&'+ to most hydrocarbons will be endothermic and consequently will not be observed. It has also been reported recently, for a small number of examples, that if both proton transfer and charge transfer from C6H6'+ions are exothermic, only charge transfer is observed (9). The appearance potentials of most fragment ions from alkylbenzenes and olefins are larger than 9.25 eV, the ionization potential of benzene (2); therefore, dissociative charge-transfer reactions of C6H6+ to give fragment ions are endothermic and should not occur. Consequently, benzene CI mass spectra of aromatic and olefinic hydrocarbons are predominantly one-peak spectra, containing essentially only M'+ ions. Quantitation requires sensitivity factors for each compound being analyzed: a tedious or impossible task for complex mixtures. I t was noted previously that the relative rate constants for reactions of C6H6'+ with several alkanes were essentially zero and that the rate constants for charge-transfer of C6H6'+ with a few aromatic hydrocarbons (or the relative molar CI sensitivities of the compounds) were independent of the exothermicity of reaction, molecular structure, and molecular weight-essentially constant (10). It has also been reported that the molar sensitivities of aromatic hydrocarbons in chlorobenzene CI are independent of molecular weight and molecular structure (11). Consequently, one may use C6H6*+ as a CI reactant ion in GC/CIMS with little danger of mass interference from isobaric compounds (except for isomers) and with minimal need for calibration data. Low-voltage electron ionization mass spectrometry (LVEIMS) was developed many years ago for the quantitation of unsaturated hydrocarbons in complex mixtures (12-16). Early experiments showed that the relative sensitivities per gram for a homologous series (alkylbenzenes, for example) decreased with increasing length of the aliphatic side chain (13). Other experiments showed that the relative molar sensitivities for alkylbenzenes were independent of the length of the side chain and increased significantly with increasing number of alkyl substituents on the benzene ring (15). Consequently, in an analysis from a batch inlet system without separation of the individual compounds, the deter-
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mination of the molecular weight distribution by LV-EIMS requires some assumptions about the length and number of side chains for each class of alkylaromatic compounds in the mixture. Even if one uses high-resolution capillary column gas chromatography for separation prior to quantitation by LV-EIMS, each component must be identified because the relative sensitivity factors for the individual isomers at each carbon number vary widely.
EXPERIMENTAL SECTION The experiments were done with a Du Pont 492B double-focusing mass spectrometer, modified for high-pressure operation, which has been described recently (17). Data were acquired with an IBM X T computer controlled by a Teknivent interface and software, using a Hall probe sensor. The samples were introduced from the gas chromatograph with the helium carrier gas on one side of the source, and the benzene reagent gas was introduced through a manifold of 1/4-in. stainless steel tubing, ultrafine metering valves, and toggle on/off valves on the other side of the source. Source pressures were measured with a capacitance manometer (MKS Baratron) inserted into the source through the probe inlet. The source temperature was approximately 200 "C. The mass spectrometer was connected to a Varian 2740 gas chromatograph with a heated glass interface (no separator). For most experiments a 3% SP-2100 Supelcoport packed column in. by 6 ft.) was used with temperature programming for the separations. The compounds used in these experiments were obtained from several commercial sources and gave no chromatographic or mass spectrometric evidence for significant amounts of impurities. Relative molar sensitivities were obtained from the ratios of integrated areas of ion currents across the chromatographic peaks. As noted previously, the reactant ions in these benzene/helium mixtures were predominantly benzene molecular ions, C6H6'+, approximately 80% of the total ion current, with small amounts (510%)of protonated benzene, C&+, and much s m d e r amounts of a few other ions (9). Pressure studies were performed on the benzene/helium reagent gas system to establish the optimal partial pressure of benzene and total source pressure. It is desirable to have a sufficiently high pressure that one can be reasonably certain the sample ions are produced by reactions of the reactant ions, C6H6*+,and not by electron ionization or reactions of He'+. However, sensitivity in CIMS passes through a maximum with increasing source pressure. Figure 1 shows plots of total ion current vs benzene composition of the mixture at constant total pressure and total ion current vs total pressure at constant composition. The maxima are relatively broad, and the sensitivity of the technique is not critically dependent on either the composition of the mixture or the total pressure. Values of 20 mol % benzene and 0.300 Torr were chosen for these experiments. Similar but somewhat different values are likely to be optimal with other instruments. The C6H6CI spectra of aromatic and olefinic hydrocarbons were essentially one-peak spectra, M'+, in these experiments. The [M + l]+/[M]'+ ratios were larger than expected from the 13C contributions, and these abnormally high ratios were independent of sample size, total pressure, and electron multiplier voltage. The [M + l]+/[M]*+ratios in the C6D6+CI spectra of these hydrocarbons were within the experimental precision (*10-15%) of the correct values for the 13C isotope ratios. The [MD]+/[M]'+ ratios in the C6Dsspectra of aromatic hydrocarbons were 0.05-0.10. Small (a few percent) amounts of fragment ions were observed in some of these spectra (highly branched alkylbenzenes), as noted previously, which are attributed to dissociative electron ionization and dissociative proton transfer (9). The present results are consistent with charge-exchange reactions of the dominant reaction ions, C6H6'+ (or C6Ds'+), and proton transfer from the small amounts (!%-lo%of total reagent ionization) of C6H7+(or c6D7+) present as reactant ions in these mixtures. Relative sensitivities were obtained from the integrated areas of the 12C isotopes of the M'+ ions, excluding all other isotopic contributions, because of the presence of MH+ ions (9). The relative sensitivities can be measured by both reactant ion monitoring, RIM, (IO,18-20) and product monitoring (9,17), and the same results should be obtained, since the formation of each sample ion requires the loss of one reactant ion. Product ion
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 7, APRIL 1, 1991 Table I. Relative Molar Sensitivities for Hydrocarbons
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Flgure 2. GCKIMS Analyses of a mixture of alkanes and alkylbenzenes. (a, top) Conditions: normalized ion current 1 35 vs scan number; He as carrierheagent gas; 0.30 Torr He; 200 "C. Key: A = n-heptane; B = toluene; C = noctane; D = ethylbenzene; E = n-decane; F = p-tert-butyltoluene; G = nhexylbenzene. (b, bottom) Conditions: normalized ion current 2 89 vs scan number; 20% benzene in He; 0.30 Torr: 200 OC. Key: B = toluene: D = ethylbenzene; F = p-terf-butyltoluene; G = n-hexylbenzene.
ion current, AI(C6H6'+),is proportional to the sample concentration. Other experiments have shown that the integrated areas across chromatographic peaks are proportional to amount of sample injected into the gas chromatograph (10, 20). Approximately 0.01 FL samples of to IO4 M solutions were injected into the gas chromatograph. A fixed, but unknown, fraction of this sample was taken into the mass spectrometer. The range of sample sizes which could be analyzed in an experiment was limited to a factor of approximately 100: the maximum ion current from the data system is 63 X lo4units, and the peak areas become very irreproducible if the peak height is less than approximately 600 units. Approximately equimolar mixtures were used to obtain the relative sensitivities of the compounds with the greatest accuracy. Figure 2 shows data for a mixture of paraffins and alkylbenzenes. Figure 2a shows the total ion current for m / z ? 35 (normalized to 100) for this mixture with only He as the carrier/reagent gas. The sample ions are produced by charge-transfer reactions of He+ and/or by electron ionization: all seven components are detected in this mixture. Figure 2b shows the ion current for m / z I 89 with He/C6Ds as the carrier reagent gas. In this figure the total ion current has been normalized to 100 and the background ionization due to the other reactant ions in C6D6 (C9D7+,CIODO+, CI2Dll+,etc.) has been subtracted. Only four peaks for the aromatic hydrocarbons are observed.
RESULTS AND DISCUSSION Table I shows the CI sensitivities for benzene charge transfer with several hydrocarbons (relative to ethylbenzene = 1.00) from this work and from earlier work on CI sensitivities determined by reactant ion monitoring (10). T h e relative molar sensitivities for all of these aromatic and olefinic hydrocarbons are essentially the same by either technique. Although there may be a slight increase with increasing molecular weight, within the 10-15% uncertainty of the measurements, there is no obvious variation in relative molar sensitivity with molecular weight, molecular structure, degree of substitution, or ionization energy in the data of these experiments or in the previous work. In addition, the average values for the relative molar sensitivities for aromatic hydrocarbons from the reactant ion monitoring technique (10) and from product ion monitoring (this work) do not differ by an amount that is larger than the standard deviations of the measurements. T h e relative molar sensitivities of the paraffins obtained by product ion monitoring in these experiments are zero within experimental error. T h e value of
