910
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
materials was the same as that used for iodine-chlorine materials (17). The bromine was determined by burning a portion of the sample in an oxygen-filled flask, the liberated bromine being absorbed in alkali with the bromine being determined by an iodometric method (18). Again each sample was analyzed 5 times using 5 different aliquots of sample. The standard deviation was based on the 5 results without rejections. For bromine-iodine-containing materials, the bromine analysis is carried out by integrating 0.618- and 0.666-MeV photopeaks. Again, a daily standard monitor containing both iodine and bromine in the sample capsule was run under the established conditions to correct for 252Cfdecay and day to day instrumental changes. Using the calibration curves, one compound was analyzed. The results of the analysis along with the results obtained by a microchemical analysis for the same compounds are given in Table VI. The microchemical method used for bromine-iodine materials involved a Schoniger-flask combustion, absorption in acidic NaN02, carbon tetrachloride separation of the iodine, and bromide titration potentiometrically with silver nitrate (19). The iodine was determined as previously described for the iodine materials (15). The results indicate that chlorine can be determined in the presence of bromine and iodine. Neutron absorption curves for chlorine, bromine, and iodine show that most of the neutron absorption for chlorine occurs in the thermal region, while bromine and iodine absorption nears zero in this region. For bromine and iodine, however, strong neutron absorption occurs in partially overlapping areas of the epithermal region. Hence, the presence of iodine can influence the bromine analysis and vice versa. However, by using the procedure developed, the amount of iodine and bromine present was kept to a minimum and/or the error was reduced to a negligible level. T h e method developed should be applicable to halogens in all organic matrices. For inorganic matrices, the method should be generally applicable. Consultation of tables of photopeak energies or cross-section tables for the inorganic elements in a sample can readily indicate those cases where line interferences occur or neutron absorption problems exist. With the CFX, activities are great enough to permit (when
line interferences occur) the analysis to be carried out on a Ge(Li) semiconductor detector. Under the conditions established for the analysis, approximately 3 to 5 samples can be analyzed for chlorine, 1 2 samples for bromine, or 15 samples for iodine per hour of instrument time. The precision and accuracy generally are better than 1% of the amount present (1 u ) for major levels. The analysis can be performed down to 100 ppm for chlorine, 20 ppm for bromine, and 10 ppm for iodine if the analysis is optimized for sensitivity.
ACKNOWLEDGMENT T h e authors express their appreciation to R. L. Griffith, without whose help and efforts, as Department Head, we would not have the CFX, and to G. N. Meyer for providing the comparative analyses using established classical and instrumental procedures of analysis. Technical assistance was given by L. J. VerWeire and C. C. Swanson.
LITERATURE CITED (1) Californium-252 Progr., 12, 41 (1972). (2) Californium-252 Progr.. 20,31 (1976). (3) S.Ghsstone and M. D. Edlund, "The Elements of Nuclear Reactor Theory", Macmiiian and Co. Ltd., London, 1953. (4) J. B. Hoag, "Nuclear Reactor Experiments", D. VanNostrand Co., New York, N.Y., 1958,Chapter 4. (5) Californium-252 Progr., 12, 52 (1972). (6) Californium-252 Progr., 13, 42 (1972). (7) Californium-252 Progr., 16, 26 (1973). (8) Californium-252 Progr., 18, 27 (1975). (9) H. R. Lukens, Jr., "Estimated Photopeak Specific Activities in Reactor Irradiations", General Atomic Division of General Dynamics, GA-5073,
1964. (10)J. H. Lauff, E. R. Champiin, and E. P. Przybyiowicz, Anal. Chem., 45, 52 (1973). (11) G. J. Lutz. R. J. Boreni, R. S. Maddock, and J. Wing, "Activation Analysis: A Bibliography Through 1971,"Natl. Bur. Stand. (US.) Tech. Note, 467, U.S. Government Printing Office, Washington, D.C., 1972. (12) T. S.Ma, Anal. Chem., 48, lOlR (1976). (13) C. M. Lederer, J. M. Hollander, and I . Periman, "Table of the Isotopes". 6th ed., John Wiiey and Sons, New York, N.Y., 1968. (14) E. C. Olson and A. F. Krivis, Mlcrochem. J . , 4, 181 (1960). (15) W. Schoniger, Mikrochim. Acta, 1, 123 (1955). (16) L. Haiiett, Ind. Eng. Chem., 10, 1 1 1 (1938). (17) L. Nebbia and B. Pagani, Chlm. Ind. (Milan), 36, 14 (1954). (18) I. Koithoff, and H. Yutzy, Ind. Eng. Chem., 9, 75 (1937). (19) S.S.M. Hasson and M. B. Eisayes, Microchim. Acta, 1, 115 (1972).
RECEIVED for review February 11, 1977. Accepted February 27, 1978.
Gas Chromat ography/ Photoionizat ion Mass Spectrometry . Nobuaki Washida," Hajime Akimoto, Hiroo Takagi, and Michio Okuda The National Institute for Environmental
Sfudies, P.O. Yatabe, Tsukuba, Ibaraki, 300-2 7 Japan
A gas chromatograph (GC)/photoionlzation mass spectrometer (PIMS) was constructed by attaching a light source and pulse counting electronics to a commercial gas chromatography/ mass spectrometer (GC/MS) (NEVA, Model TE-600). The sensitivities and the characteristics of fragmentations for the GC/PIMS were investigated for 18 organic compounds. The measured detection sensitivity for the organic compounds studied was a hundred to several hundred ng for a SIN -3, which was lower than the GC/MS by electron Impact with 70 eV. However, the absence of fragmentation for mass spectra shows that the GCIPIMS is a useful Instrument for the analysis of compounds.
I t is well known that molecules can be ionized by heat, 0003-2700/78/0350-09 10$01.OO/O
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chemical reactions, electron bombardment, field desorption, and photons. A specific characteristic in the photoionization process, in comparison with other ionization methods, is the so-called "threshold law". According to the theories by Wigner ( I ) , Wannier (2)and Geltman (3),the probability for ionization for multiple (n-fold) ionization processes just above the threshold follows the forms P ( E ) a ( V - V,)" for ionization by electron impact and P(E) 0: (V - Vc)"-'for photoionization, where V is the ionizing electron or photon energy and V , is the ionization potential. For the single ionization process ( n = l),the threshold law gives a first-order function for ionization by electron impact and a zero-order function or a step function for photoionization. Therefore, in the case of photoionization, molecules can be ionized effectivelyjust above the threshold, where most molecules can be ionized without fragmentation.
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0 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 M SF
Figure 1. Schematic diagram of ionization chamber. G.C.C. = GC column, G.P. = glass capillary, D.P. = oil diffusion pump, W = lamp window, L = V.U.V. lamp, M.S.F. = quadrupole mass filter I C L 7
1
r
--
DP5
Lc-. Flgure 2. Schematic diagram of GC/PIMS. I.C. = ionization chamber, L = V.U V. lamp, I.G. = ionization pressure gauge, B = buffle, D.P. = oil diffusion pump, T.C.G. = thermocouple pressure gauge, R.P. = rotary pump, D.P.S. = differential pumping system, P.C. = channeltron,
L.N.T. = liquid nitrogen trap
Utilizing these characteristics, the photoionization mass spectrometer (PIMS) has been used for the study of dhemical reactions (4-8) and the analysis of ppm levels of gases (9). The combination of a gas chromatograph (GC) a n d a mass spectrometer (MS) has been shown t o be a very powerful analytical tool, and many combinations of different types of instruments have been reported (10, 1 1 ) . Recently a photoionization detector (PID) for gas chromatography has been developed (12). However there has been no attempt to combine a GC with a PIMS, although a strong parent peak without fragmentation is often desired for the identification of compounds by a GC/MS. In this study, the combination of a PIMS with GC was attempted, a n d sensitivities and characteristics of fragmentations were investigated for a variety of organic compounds.
EXPERIMENTAL A commercial GC/MS (NEVA, Model TE-600) was modified by attaching a vacuum ultraviolet (VUV) light source, a channeltron, and pulse counting electronics. This GC/MS is characterized by a direct coupling between the GC and MS, that is, the quadrupole mass spectrometer and GC are connected directly by a glass capillary without a separator and all of the GC effluent is taken into the mass spectrometer through the capillary. The housing of the mass spectrometer is pumped by an oil diffusion pump with a pumping speed of 3200 L/s for He and the pressure of the analyzer is kept at about Torr. The ionization chamber was remodeled in order to attach the light source. A hole of 12-mm diameter was bored on the side wall of the ion repeller and a light beam was introduced into the ionization chamber through this hole. The light source was placed on the side opposite the glass capillary (Figure 1). Microwave powered (2450 MHz) argon and krypton resonance lamps were used to photoionize the molecules. The energy of the resonance lines are 11.83 and 11.62 eV for an argon lamp with a LiF window and 10.64 and 10.03 eV for a krypton lamp with
911
a MgF2 window. The design and the procedure for making these lamps were the same as reported by Gorden et al. (13). Ions formed by photoionization were accelerated and focused into a quadrupole mass filter by several lens elements and then guided to a channeltron (Galileo, CEM4028). Since the channeltron is sensitive to the VUV light, it was placed off-axis to the center of the quadrupole (see Figure 2). Since the channeltron requires higher dc voltage and lower ambient pressure than the electron multiplier, the chamber of the channeltron was pumped differentially by another oil diffusion pump (280 L/s). Typical pressures of the ionization chamber and the channeltron chamber were and lo4 Torr, respectively. The ion signals were pulse counted because the number of ions formed by photoionization was much less than that for the conventional electron impact ionization with 70 V electrons. The output of the channeltron was amplified and discriminated against radiofrequency pickup from the quadrupole. Signals were counted on a System 150 minicomputer (System Industries), which is a commercial attachment for the GC/MS. Data acquisition was done also by a SI-System 150 minicomputer, which scans from mass 10 to mass 300 by a step function and counts at every mass peak. The scanning time was 3 s (10 ms/l amu), and 0.5 amu around the top of mass peak was pulse counted in 5 ms. The data were displayed on a CRT display (Tektronix 4010-1) and plotted on an X-Y plotter. The column packing materials used for the GC were silicone GE SE-30 supported by Shimalite W (2070,mesh 60-80) from Wako Pure Chemical Ind. and Porapak-T (mesh 80-100) from Waters Associates Inc. Helium was used as a carrier gas. The solution of organic compounds, whose concentration is shown in weight percent in the text, was injected onto the GC column by a micro syringe.
RESULTS AND DISCUSSION Measurements were made for aromatic hydrocarbons, phenols, nitro compounds, aldehydes, n- and cyclohexane, acetone, glyoxal, biacetyl, and nitrates. Figures 3 and 4 show t h e results for a mixture of benzene (mass 7 8 ) ,toluene (mass 92), and m-xylene (mass 106) in a cyclohexane (mass 84) solution. T h e krypton lamp (MgF2 window) was used to photoionize the molecules. The column oven temperature was raised from 80 t o 200 "C a t t h e rate of 5 OC/min. Figure 3 shows t h e results when 0.2 fiL of the cyclohexane solution containing 10% benzene, 10% toluene, and 10% m-xylene was injected. Total ion current (TIC) chromatograms (A), mass fragmentograms for m / e = 78 (B), m / e = 84 (D), m / e = 92 (F), and m / e = 106 (H) and mass spectra for peaks c, e, g, and i on the TIC chromatogram are shown (Figure 3, (C), (E), ( G ) ,and (I), respectively). T h e mass spectra show t h a t only t h e parent ion was obtained for every compound by photoionization. T h e weak M 1 peak appearing in every mass spectrum is thought t o be t h e isotope peak due to I3C. I t should be noted that even if two peaks were overlapped such as the case of peak c, benzene and cyclohexane could be easily identified from t h e two parent ions, mass 78 and mass 84. Apparently, cyclohexane seems t o have less sensitivity in comparison with t h e aromatic compounds. T h e ionization potential of cyclohexane 9.88 eV, is higher than those of the aromatic compounds (see Table I) and t h e photon energies (10.64 and 10.03 eV) were very close t o the threshold. T h e examination (14) of photoionization efficiency vs. photon energy curves show t h a t alkanes do not have as sharp a rise in ionization efficiency as benzene, although theoretically molecules can be ionized effectively just above the threshold. For example, the ionization potential of benzene is 9.2 eV and t h e maximum efficiency is reached a t approximately 9.5 eV. On t h e other hand, t h e ionization potential of n-heptane is 9.9 eV but t h e maximum efficiency is reached a t approximately 10.8 eV. A similar fact was pointed out for n-butane by Chupka and Berkowitz (15). Therefore the smaller ion-
+
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912 * ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
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TIC chromatogram, mass fragmentogram, and mass spectrum for the mixture of benzene (loyo), toluene ( l o % ) ,and m-xylene (lO%)/cyclohexane. (A) TIC chromatogram. (B), (D), (F), and (H) are mass fragmentograms for m / e = 78, 84, 92, and 106, respectively. (C), (E), (G), and (I) are mass spectrum for peaks c, e, g, and i in (A), respectively. Column: SE-30 ( 2 m). Column temp.: 80-200 OC, 5 OC/min. Lamp: Kr lamp (MgF,). Injection: 0.2 pL. Mass scanning range: 60-120 Figure 3.
ization cross section for cyclohexane just above the ionization threshold yielded t h e lower sensitivity in cyclohexane in comparison with t h e aromatic compounds when using the krypton resonance lines. Figure 4 shows t h e results when 0.1 pL of a cyclohexane ' 0 toluene, and 0.1 % solution containing 0.1% benzene, 0.1 7 m-xylene was injected. TIC chromatogram (A), mass fragmentograms (B), (C), (E),and (G), and mass spectra (D), (F), a n d (H) are shown. I n this case, the peaks for benzene, toluene, and m-xylene are hidden under the noise of the TIC chromatogram and only the peak for cyclohexane can be seen. However, the mass fragmentograms for m/e = 78,m / e = 92, and m / e = 106 show t h e peaks for benzene, toluene, and m-xylene. T h e ratios of signal to noise of the mass fragmentograms for m / e = 78,92, and 106 are all about 3. Mass spectra for toluene (F) and m-xylene (H) show the parent peaks, which also give S / N -3. From these results, the
calculated sensitivities t o S / N -3 for these three aromatic compounds are about 100 ng. Figure 5 shows t h e results when 0.2 pL of an acetone solution containing l % each of benzaldehyde, m-cresol, nitrobenzene, 2,4-xylenol, and m-nitrotoluene was injected. The conditions for the GC and the lamp for photoionization were the same as t h e conditions of Figures 2 and 3. Since t h e scanning range for the mass spectrometer was set from mass 90 to mass 150, t h e signal for acetone did not appear on the T I C chromatogram. T h e mass spectrum for each GC peak shows that only the parent ions are obtained for benzaldehyde ( m / e = 106), m-cresol ( m / e = log), m-nitrobenzene ( m / e = 123), 2,4-xylenol (m/e = 122), and m-nitrotoluene (m/e = 137). Figure 6 shows t h e results of biacetyl ( m / e = 86),acetaldehyde (m/e = 44), propionaldehyde (m/e = 58),and nitromethane ( m / e = 61). T h e GC column used was SE-30 (2 m in length), and t h e column oven temperature was 80 "C.
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978 :A,
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Figure 4. TIC chromatogram, mass fragmentogram, and mass spectrum for the mixture of benzene (0.1 YO),toluene (0.1%), and rn-xylene (0.1 %)/cyclohexane. (A), TIC chromatogram. (B), (C), (E), and (G)are mass fragmentograms for r n l e = 78, 84, 92, and 106, respectively. (D), (F), and (H) are mass spectra for peaks d, f, and h, respectively. Column: SE-30 (2 m). Column temp.: 80-200 OC, 5 OC/min. Lamp: Kr lamp (MgF,). Injection: 0.1 pL. Mass scanning: 60- 120
913
T h e argon lamp with a LiF window was used for photoionization. Figure (A) in Figure 6 is the TIC chromatogram for a 0.2-pL injection of a 10% solution of biacetyl in water. Only the parent ion a t mass 86 appeared (see (B)). Figure (C) in Figure 6 shows the TIC chromatogram when 0.2 pL of water containing 10% each of acetaldehyde and propionaldehyde was injected. The scanning mass range was from mass 10 to 70. Since the ionization potential of water is higher than the argon resonance lines, water could not be photoionized. In addition to large parent ion peaks, small (M - 1)' ion peaks appeared for both acetaldehyde and propionaldehyde (see (D) and (E)). The appearance of these (hl - 1)' ions agreed with the results of Driscoll and Warneck (9). Figures (F) and (G) in Figure 6 show t h e results of the nitromethane/trichloroethylene solution. T h e parent ion peak a t mass 61 was obtained. Figure 7 shows the results of formaldehyde (10%) and glyoxal (10%) in a water solution. Porapak T was used for the GC column packing material. The column length was 2 m and the temperature of the column oven was 150 "C. The TIC chromatogram for the 0.2-pL injection and mass spectra for formaldehyde ( m / e = 30) and glyoxal ( m / e = 58) are shown. T h e results for biacetyl and glyoxal seem to be very promising for the analysis of these cclmpounds by the use of GC/PIMS. For these two compounds, the parent ions formed by the electron impact with 70 eV are very weak (16). The strongest fragment ion is CH3CO' ( m i e = 43) for biacetyl and H20+( m / e = 18) for glyoxal. Therefore the GC/PIMS should be very useful for the analysis of these compounds. Diluting the solution and changing the injection volume, the sensitivities ( S / N -3) for 18 organic compounds were obtained. T h e results are listed in Table I. The sensitivity of the GC/PIMS should depend on the intensity and photon energy of the VUV lamp and also depend on the GC separation (whether the GC peak is sharp or broad). In our results, the lower sensitivity for cyclo- and n-hexane photoionized b57 the krypton lamp results from the lower ionization cross sections for alkanes just above the ionization potentials as mentioned previously. When t h e argon lamp was used, the sensitivity for cyclo- and n-hexane increased about three times. The lower sensitivity for glyoxal is thought to result from poor GC resolution, since the GC peak of the compound is much broader in comparison with the case of other compounds as shown in Figure 7 (A). In summary, the sensitivities to S I N -3 for organic compounds are from a hundred to several
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Figure 5. TIC chromatogram and mass spectrum for the mixture of benzaldehyde (1 %), rn-cresol (1 % ) , nitrobenzene ( 1 %), 2,4-xylenol (1 %), and m-nitrotoluene ( 1 %)/acetone. (A), TIC chromatogram. (E),(C), (D), (E), and (F) are mass spectra for peak b, c, d, e, and f in (A). Column: SE-30 ( 2 m). Column temp.: 80-200 OC, 5 OC/min. Lamp: Kr lamp (MgF,). Injection: 0 . 2 pL. Mass scanning range: 90-150
914
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
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Figure 7. TIC chromatogram and mass spectrum for formaldehyde ( 1 0 % ) and glyoxal (lO%)/water. (A), TIC chromatogram. (B) and (C) mass spectrum for peaks b and c, respectively. Column: Porapak-T (2 m). Column temp.: 150 OC. Lamp: Ar lamp (LiF). Injection: 0.2 pL. Mass scanning range: 10-70 Figure 6. TIC chromatogram and mass spectra for the mixture of biacetyl (10%)/water, acetaldehyde ( l o % ) , and propionaldehyde (10%)/water and nitromethane (lO%)/trichlorothylene. (A), TIC chromatogram for biacetyl (lO%)/water. (B), mass spectrum for peak b. (C), TIC chromatogram for acetaldehyde (10%) and propionaldehyde ( l O % ) / w a t e r . (D) and (E) are mass spectra for peak d and e, respectively. (F), TIC chromatogram for nitromethane (1 0 %)/trichloroethylene. (G), mass spectrum for peak g. Column: SE-30 ( 2 m). Column temp.: 80 OC. Lamp: Ar lamp (LiF). Injection: 0.2 pL. Mass scanning range: 40-100 for (A), (B), (F), and (G); 10-70 for (C), (D),and (E)
hundred ng for this equipment. The number of ions (particle/s) formed by photoionization,
F]
Table I. Sensitivities of Organic Compounds t o S/N
Compounds Toluene rn-Xylene Benzene Benzaldehyde rn-Cresol 2,4-Xylenol Nitrobenzene rn-Nitrotoluene Nitrocresol Cyclohexane n-Hexane Acetone Cyclohexane n-Hexane Formaldehyde Acetaldehyde Propionalde hyde Nitromethane Glyoxal Biacetyl
I.P. from ref. 17, (eV)
9.65
ivy, and electron impact, NF,are expressed by the following equations:
where ILand IE represent the intensity of light and electrons (photon and electron/s), crL and uE are the ionization cross sections for the photoionization and the electron impact (cm*/particle), N is the number of molecules and d L and d E are the length of absorption (cm). Assuming IE/IL = (10 lo2), VE/UL = (10' lo3), and dL/dE = (1 10) for photoionization and conventional electron impact with 70 eV, the
-3 Lamp (window) energy, eV
GC column (m), Temp.
100 150 100
SE-30 ( 2 m ) 80
WMgF,) 10.64 eV (20%) 10.03 eV (80%)
- 200 'C
9.69 9.88
10.18 9.98 11.08
9.48 9.23
}
-
100
9.88
10.90
Sensitivity t o S I N 3, ng 100
10.18
10.21
-
-
-
Porapak-T ( 2 m) 160 " C SE-30 ( 2 m ) 80 "C
Porapak-T ( 2 m ) 1 5 0 "C SE-30 ( 2 m ) 80 "C
,
Ar(LiF) 11.83 eV (45%) 11.62 eV (55%)
150 250 250 250 500 600 200 200 200 500 300 300
300 800
300
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
N'/NF is The sensitivity for the parent ion by the commercial G C / M S (NEVA, TE-600) using electron impact with 70 eV is 1 10 ng ( S / N -3) for the above organic compounds when the channeltron was used as the detector. Since ions formed by photoionization concentrate a t the parent ion, the sensitivities in Table I are reasonable values for the present GC/PIMS. The measurements were done for nitrates and nitrites (C2H50NOZ,C2H50N0,and C3H70NOZ).The krypton lamp could not photoionize these compounds. The argon lamp could photoionize these compounds; however, fragmentations did occur and fragment ions were much stronger than the parent ion. Although the sensitivity of GC/PIMS is lower than GC/MS using electron impact, itseems to be a useful instrument for the analysis of compounds since ions formed by photoionization are mostly parent ions. For example, this instrument is especially useful for the analysis of the reaction products in smog chamber experiments. This work has been undertaken in our laboratory. In the smog chamber experiment, reaction products such as aldehydes, nitro compounds, glyoxal, and biacetyl have to be analyzed in the presence of a large amount of water, NO, NOz, and air. The interference by fragment ions of reactants and products due to poor GC separation is often an annoying problem in conventional G C / M S analysis. In such a case, it has been found that the GC/PIMS can analyze these products without the interference of fragment ions and the background peaks of water and COz. Photoionization apparently yields data qualitatively similar to chemical ionization and field ionization. The latter is not generally applicable to gas chromatography since the high voltages and close physical proximity of emitter to source cause arcing. Chemical ionization is a well established ionization technique with demonstrated high sensitivity. Photoionization may provide some advantages in that it should not be prone to matrix effects and can be selective for certain classes of compounds.
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915
ACKNOWLEDGMENT We thank Kyle D. Bayes for his useful suggestions on the photoionization mass spectrometer and T. Sakai, J. Takada, and K. Nakamura in NEVA Co. Ltd. and G. Inoue and T. Fujii in our institute for their technical assistance and discussions about the GC/PIMS.
LITERATURE CITED E. P. Wigner, Phys. Rev., 73, 1002 (1946). G. H. Wannier, Phys. Rev., 90, 617 (1953). S. Geltman, Phys. Rev., 102, 171 (1956): 112, 1976 (1958). I.T. N. Jones and K. D. Bayes, J . Am. Chem. SOC., 94, 6869 (1972). N. Washida and K. D. Bayes, Int. J . Chem. Kinet., 8, 777 (1976). J. R. Kanofsky and D. Gutman, Chem. Phys. Lett., 15, 236 (1972). J. R. Gilbert, I.R. Sbgle, R. E. Graham, and D. Gutman, J. Phys. Chem., 80, 14 (1976). (6) F. Slemr and P. Warneck, 5 e r . Bunsenges. Phys. Chem., 79, 152 (1975); Int. J . Chem. Kinet., 9, 267 (1977). (9) J. N. Driscoll and P. Warneck, J. AbPo//uf. ConfroiAssoc., 23, 656 (1973).
(1) (2) (3) (4) (5) (6) (7)
(IO) W. H. McFadden, "Technique of Combined Gas Chromatography/Mass Spectrometer: Application in Organic Analysis", Wiley-Interscience, New York, N.Y., 1973. ( I 1) J. T. Watson, in "Ancillary Techniques of Gas Chromatography", L. S. Ettre and W. ti McFadden, Ed., Wiley-Interscience, New York, N.Y., 1969. (12) J. N. Driscoil. J . Chromatogr., 134, 49 (1977). (13) R. Gordon, R. E. Rebbert, and P. Ausloos, "Rare Gas Resonance Lamps", Natl. Bur. Std. ( U . S . ) , Tech. Note, 496 (1969). (14) B. Brehm, 2. Naturforsch. A , 21, 196 (1966). (15) W. A. Chupka and J. Berkowitz, J . Chem. Phys., 47, 2921 (1967). (16) E. Stenhagen, S.Abrahamsson, and F. W. McLafferty, "Registry of Mass Spectral Data", Vol. 1, Wiley-Interscience, New York. N.Y., 1974. (17) J. L. Franklin, J. G.Dillard, H. M. Rosenstock, J. T. Herron, K. Draxl, and F. H. Field, "Ionization Potentials, Appearance Potentials, and Heats of Formatlon of Gases Positive Ions", NSRDS-NBS26 (1969).
RECEIVED for review November 7, 1977. Accepted March 6, 1978.
