Anal. Chem. 1983, 55, 2170-2174
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(7) Giddlngs, J. C.; Karalskakls, G.; Caldwell, K. D. Sep. Sci. Techno/. 1981, 16, 607. (8) Yau, W. W.; Kirkland, J. J. Sep. Sci. Techno/. 1981, 16, 577. (9) US. Patent, 4285810, Aug 25, 1981. (IO) Myers, M. N.; Glddlngs, J. C. Anal. Chem. 1982, 5 4 , 2284. (1 1) Yau, W. W.; Kirkland, J. J., manuscript In preparatlon. (12) Kirkland, J. J.; Dilks, C. H., Jr.; Yau, W. W. J. Chromatogr. 1983, 255, 255. (13) "Du Pont Products Book"; E. I . du pont de Nemours & co.: Wilming-
ton, DE, 1981; p 119.
(14) Kirkland, J. J.; Yau, W. W.; Szoka, F. C. Science 1982, 275, 296.
RECEIVEDfor review June 21,1983. Accepted August 15,1983. Presented in part during the Stephen Dal Nogare Award Symposium at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 8, 1983, paper no. 220.
Elemental Detection with a Microwave- Induced Plasma/Gas Chromatograph-Mass Spectrometer System Richard A. Heppner
University of Wyoming Research Corporation, P.O. Box 3395, Laramie, Wyoming 82071
Development of a technique that uses a low-pressure mlcrowaveinduced plasma (MIP) for selective elemental detectlon In a combined gas chromatograph/mass spectrometer (GCMS) system Is described. Wlth this technique, complex organic molecules are converted Into a few slmple neutral specles by passage through the MIP unit. The elements present in the orlglnal molecules determlne whlch species wlll be formed in the MIP. I n a hydrogen-rlch plasma, oxygen forms CO and H,O, sulfur forms CS,, nitrogen forms HCN, chlorine forms HCI, and carbon forms, CH,, C,H,, C,H,, and C,H,. Identlflcatlon and quantlflcatlonof these slmple neutral specles enable elemental composition information for the orlglnal molecules to be determlned. Detectlon of elemental C, N, 0, S, and CI and measurement of elemental C/O and C/N ratlos in a varlety of compound types and structures by the MIP/GC-MS technique are discussed. Wlth the relatively unsophlstlcated mode of operatlon used, the dynamlc range for the technlque Is 400, and the sensitlvity limn for elemental carbon passlng through the MIP Is 0.3 pg/s.
Because of their simple design, low operating cost, and energetic discharge, low-pressure microwave-induced plasmas (MIP) have been used in a variety of analytical applications (1-15). In optical spectroscopy the MIP serves as an excitation source and is combined with an optical spectrometer for selective elemental detection by observation of characteristic atomic emission line spectra (1-9). This instrumental combination has been successfully used as an element-specific gas chromatographic (GC) detector (4-6). MIP operation a t atmospheric pressure has had similar application and has also been used in combination with a nebulizer for measurement of elemental concentrations in liquid samples (7-9). Recently, the MIP has been used as a mass spectrometer ion source (10, 11). Molecules entering the MIP are atomized and ionized and are immediately extracted from the plasma for analysis by a quadrupole mass selector. Inductively coupled plasmas (ICP) have also been used as ion sources (16),and both the ICP and the MIP techniques have been used for the analysis of metallic elements in solutions. The MIP/GC-MS system discussed here can be used for the determination of elemental composition information and also as an element-selective detector for GC or other gas streams. In the present system, the MIP unit is located in
the transfer line between the GC and the mass spectrometer. The mass spectrometer operates in a conventional manner, sampling products of the MIP. Modification to either the GC or the mass spectrometer is minimal. EXPERIMENTAL SECTION Gas Chromatograph. Sample material is introduced into the MIP with a Hewlett-Packard 5700A GC with hydrogen carrier gas flowing at approximately 3 cm3/min (STP). Sample material passes through a 15.2-m (50-ft) Dexsil300 capillary column (0.5 mm id.) and a 60-cm length of fused silica transfer line before entering the MIP. The transfer line, GC injection port, and oven are maintained at a temperature of 200 OC. The analytical separation capabilitiesof the GC were not used for these experiments; rather the GC served as a convenient means for mixing sample material into a gas stream. Other devices (nebulizer or probe) might also have been used to introduce liquid or solid sample materials into the MIP. MIP Unit. The M P interface unit is shown in detail in Figure 1. The main tube is fabricated from quartz and is approximately 22 cm long with an inside diameter of 1.3 cm. The two quartz legs on the bottom of the main tube connect to the buffer gas handling system. Buffer gas is fed in through the left leg and is pumped out through the right leg. The two quartz arms on the ends of the main tube are used for bringing sample material into the MIP region and for extracting sampled material from the discharge. The left arm is thick wall having a bore diameter of 0.38 mm, slightly larger than the outer diameter of the fused silica tubing that passes through it. The right side arm supports the sampling tube assembly. The limbs are approximately 7.5 cm long with an outer diameter of 6.6 mm and, except for the left side arm, an inner diameter of approximately 5 mm. Stainless steel Swagelok fittings with graphite-impregnated Vespel ferrules connect the fused silica transfer lines to the interface unit. The fused silica transfer line from the GC ends approximately 1 cm upstream of the visible MIP and directs the GC effluent into the center of the plasma region. The buffer gas flow is pardel to this fused silica line and is monitored at standard conditions with a Brooks flowmeter. The buffer gas flow is laminar, with a Reynolds number of approximately 2 and a Knudsen number for helium gas at 200 "C (17). Laminar of approximately 2 X flow aids in concentrating the GC effluent in the center of the main tube and in preventing products from wall reactions on the main quartz tube from reaching the sampling volume. The buffer gas also provides a medium for sustaining the plasma and, in the case of hydrogen, to serve as a reactant in quantitative excess for the high-temperature reactions that occur in the region immediately following the MIP. MIP operating pressure can be controlled both with the flow of buffer gas into the plasma tube and with the throttle valve on the buffer gas pump. Pressure is
This article not subject to US. Copyrlght. Published 1983 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
tf
Microwave Cavitv
Buffer Gas In
Flgure 1. Sketch nf the M I P unit.
Table I. Typical Operating Conditions for the MIP/GC-MS System pressure buffer gas buffer flow rate power frequency temp (electron) temp (plasma)
1.3-26 kPa (10-200 torr) helium or hydrogen 0.1-10 cm3/s at 100 1cPa 30-150 W ( < l o W reflected) 2.45 GHz (2-5) x l o 4 K (2.2-5.6 eV) (4-5) X l o 3 K (13.4-0.5 eV) -~
monitored by a Wallace-Tiernan Model FA-145 gauge. Both hydrogen and helium were used as buffer gases for this work. Other investigators have wed nitrogen and oxygen as buffer gas species (15). Molecular species produced by the MIP are sampled by a 1-mm i.d., 10-cm long quartz tube running down the center of the main tube. The end of this tube connects to a 75-cm length of fused silica, maintained at a temperature of 200 “C, which leads directky into the ion source of the mass spectrometer. Sampling of the plasma products iEi performed most efficiently when the gas flow velocity in the sampling tube, us, is greater than that of the surrounding buffer gas, ub. MIP operation a t high pressure with low buffer gas flow rate improves the u , / q , ratio. Species leaving the region of the MIP react rapidly among themselves and with hydrogen present in thke buffer and carrier gas flows. Simple modeling was done by assuming only a single bimolecular reaction is neeessary to go frorn the very energetic species in the plasma to ,ground-state neutral molecules. An expression describing the decay in the concentration of unreacted species as a function of distance from the plasma, adapted from work with ion/molecule reactions in flowing afterglows, was applied (18). Assuming a reaction rate constant of 1 X cm3/8, then 1cm from the plasma zone the ratio of unreacted to reacted species is essentially zero. Even with the several reactions necessary to arrive at ground-state neutral molecules, all reactive processes should be completed before samipling occurs. The microwave cavity is made of brass (19). A Holoday Industries Mcdel HI.2450-PLW microwave power supply drives the cavity with powers up t o 300 W at 2.45 GHz:. Cavity tuning can be optimized so that less than 10% of the incident power is reflected. ‘The m,ain tube is heat traced except where the microwave cavity surrounds the tube. Heating is necessary to prevent condensation (especially of H20) from occurring. Compressed air is used to cool the quartz tube in the immediate area of the cavity. Microwave power of approximately 100 W is necessary to completely dissociate the n-butane in a 190 by volume mixture with hydrogen. This concentration and power level are comparable to those observed in other investigations (5) using optical detection. Table I summarizes the typical range of operating conditions used with the MIP/GC-MS system. The electron and plasma temperatures are deduced by comparison with previous work by other investigator3 (12,13) who estimated plasma temperature from emission line intensities. Note that the “electron temperature” is approximately ten times higher than the “plasma temperature” because of nonequilibrium energy distribution in the low-pressure MIP. Mass Spectrometer. Mass spectra were obtained with a Kratos AEI MS-12 mass spectrometer coupled to a Finnigan-Incos
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Model 2300 data system. Two different scan ranges were used for acquiring mass spectral data. The first scanned from m / z 5 to 35 in approximately 2 s with a total cycle time of 4 s; the second scanned from n / z 12 to 125 in approximately 10 s and had a total cycle time of 1 2 s. Operating resolution was approximately 600 and an ionizing voltage of 70 V was used, with an accelerating voltage of 2 kV. Gas Analysis. Mass spectra were analyzed by using a pattern recognition technique (20) that was modified to include individual compound sensitivity factors. This analysis routine compares the mass spectrum of an unknown with mass spectra stored in a reference library collection and uses a least-squares procedure (other optimizing procedures are also available) for obtaining the best match between the “unknown” spectrum and an admixture of library spectra. Each compound in the admixture is weighted by a sensitivity factor to obtain its partial pressure contribution to the total sample. For this work, library spectra were obtained for CHI, NH,, H20, C2H2,C2H4,HCN, CO, and C2H,. (Spectra for HCN and NH, were obtained from the American Petroleum Institute collection of mass spectra. The other compounds were individually run on the MS-12.) Intensities at m / z ratios of 12 through 18 and 23 through 30 were used in the pattern recognition analysis. The relative sensitivity factors were assigned to each library entry by considering, for each compound, the absolute intensity of its mass spectrum, the ion source operating pressure, the ion gauge sensitivity factor, and the source operating conditions. An absolute pressure measuring device was not available for this work; gas pressure in the ion source was estimated with an ion gauge by using sensitivity corrections for each gas species (21). A Cyber-760 computer system was used for running the pattern recognition analysis programs. Chemicals. All chemicals used for this work were quantitative-grade purity and were supplied by Polyscience Corp.
RESULTS AND DISCUSSION During operation of the MIP/GC-MS system, effluent from the GC is carried into the MIP by a buffer gas flow and is rapidly dissociated into highly excited and ionized species. Some experimental work supports molecular dissociation to the atomic level (22),while other spectroscopic evidence indicates that hydrocarbon species are reduced to CH units (23). Species leaving the MIP react to yield a mixture of simple neutral molecules. This mixture is sampled by a mass spectrometer and its mass spectrum is obtained with electron impact ionization. An identification of the compounds present in this mass speckrum indicates which elements existed in the more complex molecules in the original GC effluent. For example, HCN indicates the presence of nitrogen; CH4,C2H2, C2H4,and C& indicate hydrocarbon-type material is present. Several of these compounds have also been produced in experiments where an electrical discharge is passed through an organic vapor (22-25). The compounds produced by the MIP depend only upon the atomic constituents available and are nearly independent of the structure or functionality present in the original molecule. This characteristic enables limited empirical formula information for the original sample molecules to be determined from a measurement of the relative concentrations of the gaseous compounds leaving the plasma. For the low resolution mass spectrometer used in this work, a pattern recognition technique (20) is used to identify and quantify the compounds produced in the MIP. However, with a medium- or high-resolution mass spectrometer, selective elemental detection could be performed by monitoring the exact mass(es) of the compound(s) containing the element of interest. For instance, to identify nitrogen-containing species, m / z 27.1019 (HCN) would be monitored. Mass Spectra. Figure 2A shows the effect of the MIP on the mass spectrum of n-hexane. (To obtain the spectra shown in this and the next figure, helium was used as the buffer gas a t a flow rate of 1 cm3/s and hydrogen was used as the GC carrier gas. The MIP pressure was 13 kPa (100 torr), microwave power was 90 W, and the sample size was 0.3 pL.)
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983 100.0
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Figure 3. Effect of the MIP on two nitrogen-containing compounds, isobutylamine (A) and 3,4-dimethylpyridine (B).
A comparison of Figure 2A with the standard spectrum of n-hexane indicates that essentially no n-hexane survives passage through the MIP. An analysis of the spectrum (by the pattern recognition technique) in Figure 2A indicates that the three major components produced by the MIP are acetylene, methane, and ethylene, present at 80, 14, and 5% concentrations, respectively. Figure 2B illustrates the effect of the MIP on toluene. Comparison of the spectra in Figure 2A and Figure 2B shows that they are nearly identical, while the two compounds, n-hexane and toluene, are structurally very dissimilar. This indicates that the methane, acetylene, and ethylene observed in these spectra are not merely molecular fragments from the original hexane and toluene molecules but are produced from reactions among the highly dissociated species generated by the MIP. Figure 3 compares the mass spectra obtained for two nitrogen-containing molecules, isobutylamine and 3,4-dimethylpyridine. Again, the spectra are quite insensitive to molecular structure. Pattern recognition analysis of these spectra inicates that, in addition to acetylene, methane, and ethylene, hydrogen cyanide (mlz 27) is also present. Compared to the pyridine derivative, the amine shows a relatively higher concentration of HCN. This is expected from the higher nitrogen concentration in the amine. Another difference between the two spectra is in the amount of methane produced from the two compounds during exposure to the MIP. Appreciably more methane exists in the spectrum of the 3,4-dimethylpyridine. This increased production of methane from higher molecular weight compounds (3,4-dimethylpyridine at m / z 107 and isobutylamine at m / z 73) is seen in all the data acquired to date. The cause or mechanism for this phenomenon is not yet understood. The MIP did not produce ammonia from nitrogen-containing compounds. In addition to nitrogen and carbon, the MIP/GC-MS technique has been applied to compounds containing oxygen, chlorine, and sulfur. For the conditions used in this work, these elements are converted into characteristic compounds: oxygen forms CO and HzO, chlorine forms HC1, and sulfur forms CSz. It is interesting that even in the reducing atmosphere of the MIP, H,S and NH3 were not observed from sulfur- and nitrogen-containing compounds, respectively.
Modeling of the thermochemical equilibrium in the plasma and freezing of the plasma products as they leave the energetic plasma have explained the lack of NH3 production (22). Basic functional groupings, such as -NOz and -COOH, are also dissociated by passage through the MIP. For instance, the nitrogen and oxygen in the -NOz group in nitrobenzene is converted into HCN, GO, and H20.As might be expected, buffer gas composition strongly influences the species produced in the plasma. Increased hydrogen concentration in the buffer gas results in the increased production of more saturated species. Ion intensities in the mass spectra of the gases produced by the MIP exhibited a linear response with sample size for n-CsHI8over a concentration range of 0.1 to 4% by volume in hydrogen and helium buffer gases. Operating conditions were a pressure of 4 kPa, buffer gas flow rate of 0.5 cm3/s, and a microwave power of 90 W. The minimum usable concentration was estimated to be 0.01% corresponding to a dynamic range for these conditions of 400. Under these same conditions, a mimimum concentration of 0.01 % n-C8HI8 corresponds to 0.3 kg/s of carbon passing through the MIP unit. This level is approximately lo3 times higher than required for carbon detection using optical spectroscopic techniques (7). However, by operating the mass spectrometer in the multiple ion detection mode (MID), instrumental sensitivity could be increased by up to a factor of lo3 (26),and thus, the ultimate detection limits of the two techniques would be comparable. MID operation would also increase the dynamic range of the MIP/GC-MS technique accordingly. Products from the exposure of organic materials to high power laser induced plasmas (27-33) are very similar to those produced in the MIP. The major hydrocarbon species produced by laser irradiation of coal were Hz, CH4, CzH4, and CzH,. Except for hydrogen, whose detection was not possible in the present work, these same species are also produced by exposure of organic compounds to the MIP. The principal oxygen-containing species observed in the laser pyrolysis of coal is CO. Slight amounts of COz were also observed; no H 2 0 was reported. Nitrogen was found in the form of HCN. No sulfur-containing species were observed, but both CSz and H2S were predicted in theoretical modeling work (24). Other investigators doing laser pyrolysis of pure sulfur in various
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983 2173
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Figure 4. Measured vs. actual values of the C/O ratio for a variety
of oxygen-containing compounds. Dashed line indicates Meal behavior. Solid line indicates quadratic fit to the data. Compounds used for obtaining these data were alcohols (methanol, ethanol, 2-propanol, 1-pentanol, 3-methyl-1-butanol,1-octanol),ketones (acetone, 2-butanone, 3-pentanone, 3-methylcyclohexanone, 2-octanone),aldehydes (1-butanal, 1-pentanal, 1-ocianal),and acids (tiutanoic acid). hydrocarbon atmospheres (32)found CS2 to be the predominant sulfur-containing species produced by the plasma. Elemental Analysis. Mass spectra obtained with the MIPIGC-INS system result from the linear superposition of the mass spectra of a few simple compounds: CH4,H20, C2H2, C2H4,CO, HCN, CS2, and C2H6. Analysis of the observed spectra with the aForementioned pattern recognition technique yields the relative molar concentration for each gas in the mixture. [deal gas behavior is assumed where the molar concentration of a gas is proportional to its partial pressure. From the molar concentration of each gahi the relative molar concentration of each element in the mixture can be determined. The elemental molar concentrations in the mixture will be the same as those in the original compound, Le., prior to application of the MIP. (This is true for all elements except hydrogen, which was used as the carrier and sometimes the buffer gas.) Ratios between the measuresd elemental molar concentrations can be used to determine limited empirical formula information about the original compound. The result of applying this procedure to oxygen-containing organic compounds is shown in Figure 4. Data for this figure were obtained by using an operating prestiure of 13 kPa (100 torr) with a buffer gas flow rate of 1cm3/sI. Both helium and hydrogen were used as buffer gases in separate experiments. In Figure 4 the measured C/O ratio is plotted vs. the actual C/O ratio for several oxygen-containing organic compounds: alcohols, ketones, aldehydes, and butanoic acid. Repeated measurements a t several C/O ratios indicate typical uncertainties. The dashed line in Figure 4 indicates the one-to-one correspondence expected for ideal measurements. The solid line results from the application of a quadratic regression analysis to the data. Because of difficulty in obtaining a reliable sensitivity factor for H20, this value was determined by a least-squares fit of the measured to the actual C/O ratios. It was the only adjustable parameter used. Admittedly, the data cover only a small carbon number range, but the results are encouragingly good except for a deviation a t the higheir C/O ratios. A comparison of measured and actual C/N ratios is presented in Figure 5. Again, ,% variety of functional and structural types of nitrogen-containing compounds was used in obtaining the data for this figure. Each data point corresponds to the average of several repeated measurements; measurement
I' 8
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Actual C/N Ratio
Figure 5. Measured vs. actual values of the C/N ratio for a variety of nitrogen-containing compounds. Dashed line indicates ideal measurements. Solid line indicates quadratic fit to the data. Compounds used for obtaining these data were normal and isobranched amines (isobutylamine, 1-hexylamine,di-n-propylamine, 1-octylamine,tri-npropylamine), cyclic/aromatic amines (benzylamine,dicyciohexylamine), and pyridlnes (pyrldine, 3,4-dimethylpyridine, 2,4,6-trimethylpyridine).
uncertainty is shown for fl standard deviation. The dashed and solid lines again correspond to ideal measurements and the result of applying a quadratic regression analysis to the data, respectively. The only adjustable parameter in these data was the sensitivity factor for HCN, which had to be determined by doing a least-squares fit of measured to actual C/N ratios because of the lack of a standard HCN gas sample. Agreement between measured and actual C/N values is poorer than observed for the C/O ratio data. Similar to the behavior observed with the C/O ratio measurements, the measured C/N ratios at higher values exhibit a significant deivation from the actual values. While the cause of the deviation at larger C/O and C/N values has not been positively identified, one possible explanation is that the relatively larger amount of methane produced in the MIP by higher molecular weight compounds (those with higher C / N and C/O ratios) is being weighted too heavily by an incorrect value for the methane sensitivity coefficient. The published methane ion gauge sensitivity coefficient (21) may not be appropriate for the geometry and voltages used in the ionization gauges of the MS-12. Future work will use standard gas mixtures to establish the relative sensitivity for each compound in the pattern recognition library. The measurernent of other elemental ratios could also be done. As was shown earlier, the MIP converts sulfur in organic compounds into CS2and chlorine into HC1. Thus, an analysis for S and C1 heteroatoms would require obtaining the spectra for CS2 and HC1 under standardized operating conditions and adding them to the reference compound library. Other halides would probably also be converted into their hydrides by the MIP.
CONCLUSION The combined MIP/GC-MS system discussed here can be used as a selective element detector and also as a means for determining eleinental ratio information. The technique is applicable to the simultaneous detection of a number of elemental species, is capable of analyzing very small size samlples, typically a few micrograms, and can be adapted to existing mass spectrometer systems. Atomic species investigated to date have been C, 0, N, S, and C1. Other investigators have detected 14C and Br (15). Other atomic species may also be detectable, but only those that form simple compounds existing totally in the vapor phase at transfer line temperatures (200-300 "C). This requirement precludes detection of most
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metallic elements but enables the detection of atomic species commonly found in organic compounds. The elemental information available with the MIP technique could be invaluable when performing GC/MS analyses on complex sample materials. Polar materials, in particular, can be difficult to analyze because of the variety of heteroatomic species they can contain. Quantitative elemental information on each GC peak is possible because GC resolution is not destroyed by the MIP system. Selective elemental detection and elemental analysis need not be restricted to GC applications. Gases from process streams, pyrolyzers, etc., could also be analyzed with this technique to directly provide elemental composition as a function of process conditions. Future development could extend ita utility to both liquid and solid samples. Preliminary work in analyzing solid sample materials has already been done; liquid samples might be analyzed by use of a nebulizer arrangement. While the MIP/GC-MS system has not been optimized to the extent that MIP/optical spectroscopic techniques have, future refinement should considerably improve its operation.
ACKNOWLEDGMENT Donald W. Fausett assisted in the modification and use of the pattern recognition programs, and Frank D. Guffey and Francis P. Miknis supplied useful contributions and suggestions to this work. The loan of a microwave power supply and cavity by Arthur Denison of the University of Wyoming Physics Department is also appreciated.
LITERATURE CITED (1) Llchte, F. E.; Skogerboe, R. K. Anal. Chem. 1972, 4 4 , 1321-1323. (2) Luippoid, D. A,; Beauchamp, J. L. Anal. Chem. 1970, 4 2 , 1374-1381. (3) Houpt, P. M. Anal. Chlm. Acta 1976, 8 6 , 129-138. (4) McCormack, A. J.; Tong, 5. C.; Cooke, W. D. Anal. Chem. 1965, 3 7 , 1470-1 476. (5) McLean, W. R. I n "Recent Analyticai Developments In the Petroleum Industry"; Hodges, D. R., Ed.; Wlley: New York, 1974; Chapter 9. (6) van D a h , J. P. J.; de Lezenne Coulander, P. A.; de Galan, L. Anal. Chlm. Acta 1977, 9 4 , 1-19. (7) Beenakker, C. I. M. Spectrochim. Acta, Part8 1977, 3 2 8 , 173-187. (8) Beenakker, C. I.M. Spectrochim. Acta, Part8 1976, 3 1 8 , 483-486. (9) Beenakker, C. I.M.; Bosman, Bieneke; Boumans, P. W. J. M. Spectrochim. Acta, Part8 1978, 3 3 8 , 373-381.
(10) Douglas, D. J.; French, J. B. Anal. Chem. 1981, 5 3 , 37-41. (11) Douglas, D. J.; Quan, E. S. K.; Smith, R. G. Presented at the 30th Annual Conference on Mass spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982. 12) Busch, K. W.; Vlckers, T. J. Spectrochim. Acta, Part 8 1973, 2 8 8 , 85-104. 13) Brassem, P.; Maessen, F. J. M. J. Spectrochlm. Acta, Part 8 1974, 2 9 8 , 203-210. 14) Heppner, Richard A. Presented at the 30th Annual Conference on Mass Spectrometry and Allied Topics, Honolulu, HI, June 6-11, 1982. 15) Markey, Sanford P.; Abramson, Fred P. Anal. Chem. 1982, 5 4 , 2375-2376. 16) Houk, Robert S.;Fassel, Velmer A.; Flesch, Gerald D.; Svec, Harry J.; Gray, Alan L.; Taylor, Charles E. Anal. Chem. 1980, 5 2 , 2283-2289. 17) Dushman, Saul, Lafferty, J. M., Ed. "Sclentiflc Foundatlons of Vacuum Technique," 2nd ed.; Wiley: New York, 1962. 18) Ferguson, E. E.; Fehsenfeid, F. C.; Schmeltekopf, A. L. I n "Advances in Atornlc and Molecular Physics"; Academlc Press: New York, 1969; Vol. 5, Chapter 1. (19) Fehsenfeld, F. C.; Evenson, K. M.; Broida, H. P. Rev. Sci. Instrum. 1965, 3 6 , 294-298. Type 5 cavity was used. (20) Fausett, Donald W.; Weber, James H. Anal. Chem. 1978, 5 0 , 722-731. (21) Nakao, F. Vacuum 1975, 2 5 , 431-435. (22) Bronfln, Barry R. I n "Chemical Reactions In Electrical Discharges"; Blaustein, Bernard D., Ed.; American Chemical Society: Washington, DC, 1969; Advances In Chemlstry Series 80, Chapter 35. (23) Vastoia, F. J.; Wightman, J. P. J. Appl. Chem. 1964, 74, 69-73. (24) Masayuki, Kawahata I n "Chemical Reactions in Electrical Discharges"; Blaustein, Bernard D., Ed.; American Chemical Society: Washington, DC, 1969; Advances in Chemistry Series 80, Chapter 26. (25) Wightman, J. P.; Johnston, N. J. I n "Chemical Reactions in Electrical Discharges"; Blaustein, Bernard D., Ed.; American Chemical Society: Washlngton, DC, 1969; Advances in Chemlstry Series 80, Chapter 27. (26) Millard, Brlan J. "Quantitative Mass Spectrometry"; Heyden and Son: Phlladelphla, PA, 1978; Chapter 2. (27) Karn, F. S.;Friedel, R. A.; Sharkey, A. G., Jr. Fuel 1972, 5 7 , 113-1 15. (28) Hanson, Ray L.; Vanderborgh, N. E. I n "Analytical Methods for Coal and Coal Products"; Academic Press: New York, 1979; Voi. 3, Chapter 40. (29) Karn, F. S.;Frledel, R. A.; Sharkey, A. G., Jr. Chem. I n d . (London) 1970, 239-240. (30) Karn, F. S.; Friedel, R. A,; Sharkey, A. G., Jr. Fuel 1969, 4 8 , 297-303. (31) Karn, F. S.;Sharkey, A. G., Jr. Fuel 1968, 47, 193-195. (32) Miknls, F. P.; Biscar, J. P. H@h Temp. Sci. 1972, 4 , 49-51. (33) Howe, J. A. J. Chem. Phys. 1963, 3 9 , 1362-1363.
Received for review March 31,1983. Accepted August 8,1983. Mention of specific brand names or models of equipment is for information only and does not imply endorsement by the University of Wyoming Research Corp.
