Anal. Chem. 1980, 5 2 , 891-896 (10) Alibert, G.; Puech. J. L. J . Chromatogr. 1977, 124, 369. (11) Mourey, T. H.; Carpenter, A. P., Jr.; Siggia, S.;Lane, A. Anal. Chem. 1976, 4 8 , 1592. (12) Carpenter, A. P., Jr.; Carter, S.; Siggia, S. Anal. Chem. 1976, 48,225. (13) Klimisch, H. J. J . Chromatogr. 1973, 83, 11. (14) Hunt, D. C.; Wild, P. J.; Crosby, N. T. J . Chromafogr. 1977, 130, 320. (15) Okubo, T.; Ise, N. J . fhys. Chem. 1969, 73, 1488. (16) Nelson, W. T. (to Phillips Petroleum Co.) U S . Patent 2 737 538, March 6, 1956. (17) Goldstein, G. J . Chromafogr. 1976, 129, 61. (18) Olsson, L.; Samuelson, 0. J . Chromatogr. 1975, 106, 139. (19) Thorns, R.; Zander, M. Fresenius 2. Anal. Chem. 1976, 282, 443. (20) Grant, D. W.; Meiris, R. B. J . Chromatogr. 1977, 142, 339. (21) Sleight, R. 6.J . Chromafogr. 1973, 83, 31. (22) Kratulovic, A. M.; Rosie. D. M.; Brown, P. R. Anal. Chem. 1976, 4 8 , 1383. (23) Wise, S.A.; Cheder, S. N.; Hertz, H. S.; Hilpert. L. R.; May, W. E. Anal. Chem. 1977, 49, 2306. (24) Jensen, H. B.; "Analytical Chemistry Pertaining to Oil Shale and Shale Oil", Siggia, S.,Wen, P. C., Eds., Report of NSF Conference Workshop, Washington D.C.; National Science Foundation: Washington, D.C., June 1974; p 24.
89 1
(25) Hurtubise, R. J.; Schabron, J. F.; Feaster, J. D.; Therkildsen, D. H.; Poulson, R. E. Anal. Chlm. Acta 1977, 89, 377. (26) Mamantov, G.; Wehry, E. L.; Kemmerer, R. R.; Stroupe, R. C.; Hinton, E. R.; Goldstein, G. Adv. Chem. Ser. 1978, 170, 99. (27) Mourey, T. H.; Siggia, S. Anal. Chem. 1979, 51, 763. (28) Shostakavskii, M. F.; Sidei'kovskaya, F. P.; Kslodkin, F. L. Vysokomol. Soedin. 1960, 2, 1794; Chem. Abstr. 1961, 55, 26516b. (29) Oster, G.; Immergut. E. h. J . Am. Chem. SOC.1954, 76, 1303. (30) Ned, J.; Sebille, 6.Compt. Rend., 1961, 252, 405. (31) DiSanzo. F. P. Ph.D. Dissertation, University of Massachusetts, Amherst, Mass., 1979. (32) Carpenter, A. P.; Jr., Ph.D. Dissertation, University of Masshachusetts, Amherst, Mass., 1978.
RECEIVED for review September 4,1979. Accepted February 11,1980. This work was supported in part by National Science Foundation Grants CHE74-15244 and CHE76-07378, and by an American Chemical Society Summer Fellowship (1978), sponsored by the Division of Analytical Chemistry (T.H.M.).
Saturation Region of Helium Ionization Detector for Gas-Solid and Gas-Liquid Chromatography Fikry F. Andrawes" Lockheed Engineering and Management Services Co.,
Inc., 1830 NASA
Road 1, Houston, Texas 77058
Roswltha S. Braze11 Chemistry Department, University of Houston, Houston, Texas 77004
Everett K. Gibson SN7/Geochemistry Branch, NASA -Johnson Space Center, Houston, Texas 77058
I n the saturation region of the helium detector field intensity, the detector response Is Independent of the electrical field. I n this region (at applied potential between 200 to 2000 volts per centimeter of electrode surface) the detector Is operated at a low background current, and a low noise level, but It still exhibits a stable and sensitive response. The detector In this region can be operated with high purity grade helium without any further elaborate purification processes to yield a positive response to all compounds and gases tested. The operation of the detector In this mode has been adapted to both gassolid and gas-liquid chromatography, with temperature programming. A sample can be introduced to the column vla a gas sampling injection valve or via a syringe by direct injection Into a modified Injection port. The detector response Is linear over a range of 10' units with a detection limit In the picogram range for organic compounds.
T h e mechanism behind the helium ionization detector is nonselective, meaning that the detector is capable of responding to all species. The metastable helium generated in the detector via a radioactive source has an ionization potential of 19.8 eV which is higher than any other atom or molecule, with the exception of neon which has an ionization potential of 21.5 eV. The sensitivity of the helium detector was reported earlier by its inventor J. E. Lovelock ( I ) as well as by others (2-5). In spite of the high sensitivity and the universal detection mechanism of the helium detector, it remains an un0003-2700/80/0352-0891$01 .OO/O
popular detector, and in most cases its use has been limited to the analysis of high purity gases, with a few other applications (6-10). The reluctance to use the helium detector is based on several factors including (a) the belief that extreme care has to be taken in using ultrapure helium as carrier gas (11,12),and (b) the belief that the extreme sensitivity of this detector requires an excessively clean chromatographic system with the use of adsorbtion columns that produce minimal or no bleed (11, 12). Other factors which have added to the reluctance to use this detector are: (c) the failure of the current detector mechanism to explain the negative and the bipolar detector responses to selected gases (1,3-15), (d) conflicting reports in the literature concerning the polarity of the detector signal with the purity of the helium carrier gas, and (e) lack of adequate experimental data with varying experimental conditions (16). Most of the data previously reported on the helium detector was collected in the multiplication region of the detector's volt-ampere curve. In this region the detector response increases exponentially with applied potential. 'The microvolume helium detector (the only type commercially available) in this region is normally operated a t 4000 to 7000 volts per centimeter of electrode surface. At such high applied potentials, the detector response is high, but the background current and the noise level are also high. The system in this region is highly unstable, and a disconnection of any part of the system requires a day or more to restabilize the detector (17). In an attempt to minimize any atmospheric diffusion that increases the background current, some authors have gone 0 1980 American Chemical Society
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O R I G I N I L Z T I O N PORT
Figure 2.
The volt-ampere curve of the helium detector
HELIUM
Figure 1. Modified injection port
so far as to operate the entire chromatograph in a helium atmosphere (18). With such high sensitivity, a sample could be introduced t o the column only via a tight gas sampling injection valve. However, these tight valves were found to leak some air into the carrier gas stream in trace amounts, but in sufficient quantities t o invert the detector polarity t o some gases (19). With such difficulties, the function of the helium detector has been very limited and its use has been primarily restricted t o the analysis of pure gases. On the other hand, in the saturation region of the detector field intensity, the operation of the detector is independent of the applied voltage and is characterked by low background current and low noise level. In the work described here, we investigated the use of the helium detector in t.he saturation region hoping t o extend the applications of this sensitive detector.
EXPERIMENTAL A Varian 1700 gas chromatograph equipped with a helium ionization detector was used. The detector itself has a dead volume of 160 GL and the electrodes are arranged in a parallel plate geometry. Voltage is applied to the upper electrode while the bottom contains a 250-mCi tritium foil ( 5 ) . The bucking current circuit of the instrument was modified and calibrated to allow absolute measurement of the detector output current. To enable direct syringe injections of liquid samples, a Hewlett-Packard 5830 injection port was adapted to the Varian chromatograph as shown in Figure 1. The Hewlett-Packard injection port was soldered to the original injection port on the instrument and helium was allowed to flow between the two septa. This arrangement minimizes atmospheric leaks and the introduction of air during sample introduction. The heating block of the injection port was maintained a t 200 "C. When wall coated open tubular (WCOT) columns were used, make-up gas was added to the detector cell using a "T'connection. The capillary end of the coluim was extended through the "T" connection to the base of the detector to minimize dead volume, and the make-up gas was of the same purity as that going to the analytical column. An 8-port zero dead volume gas sampling injection valve with 1OO-wL sample loop was also adapted to the chromatograph (Valco Instruments Co., Houston, Texas). Two columns were used in the course of this work: (1) an adsorption column of stainless steel tubing (2 m X 1.1mm Ld. x 1.6 mm 0.d.) packed with 60/80 mesh 13X molecular sieve (Alltech Associates, Arlington Heights, Ill.), and (2) a WCOT stainless steel column (100 m x 0.2 mm id.) coated with Witconol LA-23 (Witco, Houston, Texas). The packed column was conditioned overnight a t 150 "C with a helium flow of 14 mL/min while the capillary was condit,ioned at 170 O C wit,h 3 mL/min helium flow. In previous work, we have used the research grade helium for carrier gas, as have most other users of the helium detector (13, 15, 17, 1 9 ) . With this grade of helium, (which has a minimum purity of 99.9995%) in a leak free chromatographic system, the detector provides a negative response to Ne, H2, Ar, Oz. and Nz (13-15, 17, 19). T o obtain a positive response to all gases, a few parts per million of gaseous additive need to be added to the helium carrier gas (13-15, 17, 1 9 ) . Knowing that one has t o
decrease the purity of the helium carrier gas to obtain a positive response to all gases, we did not use the expensive research grade helium. Instead we used the high purity grade helium (minimum purity of 99.995%) in all the work described here. This is the grade most often used in general chromatography. The high purity helium used as a carrier was dried with a Supelco gas purifier (Supelco Inc., Bellefonte, Pa.). Three standard gas sample mixtures were used: (1) 11 ppm H,, 24 ppm Argon. 9 ppm 02,29 ppni N,, 11 ppm CH,, and 23 ppm CO in helium, (2) 104 ppm ethylene, 108 pprn propylene, 106 ppm 1-butene, 103 ppm pentene, and 104 ppm I-hexene in helium, and (3) 970 ppm ethylene, 1030 ppm propylene, 1050 ppm 1-butene, 1390 ppm pentene, and 1190 ppm 1-hexene in helium. The first (inorganic) mixture was analyzed on the mclecular sieve column, while the second and third mixtures (organic) were analyzed on the capillary column and were used to evaluate the effect of the flow rste and detector temperature on detector response. The three gaseous mixtures were introduced t o the analytical column via the gas sampling valve. A glass exponential dilution flask (Varian Aerograph, Palo Alto, Calif.) was used as previously described ( 5 ) to generate lower concentrations of the organic mixture in order to calibrate the detector response to these gases. For the calibrdon of' inorganic gases, high purity grade gases were used in the flask. The helium going to the flask was the same purit,y as that going to the analytical column. A hydrocarbon mixture of C5to CI4was purchased from Alltech Associates, and was used to illustrate the temperature programming experiment. Reagent grade benzene (containing 0.05% water) and xylene were obtained from J. T. Baker Chemical Co., Phillipsburg, N.J. Chromatograms were recorded at I-mV full scale on an Omniscribe strip chart recorder (Houston Instrument) or a Spectra Physics SP-4050 (Santa Clara, Calif.j print,er plotter. A Spectra Physics SP-4000 data system was used to integrate peak areas.
RESULTS AND DISCUSSION Inside the helium detector, the metastable helium will ionize all species present. T h e electrons formed will be collected at the anode and the cations will be collected on the cathode. T h e ionization current depends on the applied potential. Figure 2 shows the ampere-voltage curve for the helium detector used in this work. T h e current was measured with the helium carrier gas flowing through the molecular sieve column to the detector. Tb.e volt-ampere curve for t,he helium detector has been previously reported ( 5 , 16) and it consists of three parts. The initial linear increase in the detector response between 0 to 20 applied voltage is called the collection region. In this region, the charged particles are gradually collected as a function of increasing applied voltage. Between 20 and 200 V, the detector response is independent of the applied voltage. This region is called the saturation region, in which there is an equilibrium between collected and produced charged pzrticles. Above 200 V, the detector response increases exponentially with an increase in applied voltage, and accelerated secondary electrons are involved in the ionization process. It is in this region t h a t most helium detectors are operated. T h e multiplication region is characterized by sensitivity, high background current, high noise level, and high instability ( 1 6 ) . In the work described here, we attempted to evaluate the detector response in the saturation region of the detector field intensity (between 20 and 200 V). The initial
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Figure 5. Calibration of detector response for H, Ar, 02, N, CH,, and CO. Sample size, 100 kL. Condition as in Figure 3
Flgure 3. Detector response to 11 ppm H,, 24 ppm Ar, 9 ppm 0,, 29 ppm N,, 1 1 ppm CH,. and 23 ppm CO. Column, 2 m X 1.1 mm i.d. packed with molecular sieve 13X, 60/80mesh. Carrier flow, 14
cm3/min;column temperature, 40 and sample size, 100 pL
OC;
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FLOW THROUGH DETECTOR CC/MIN
rate on t h e detector response and background current. The broken lines indicate a change to negative polarity Flgure 4. Effect of flow
results showed that the detector exhibits high sensitivity, and Figure 3 shows the magnitude of the detector response in this region to a few parts-per-million of some permanent gases. 'The helium detector has been operated previously a t flow rates higher than 45 cm3/min. By operating the detector at such high flow rates, the background is lower and the detector is more stable. We found that the detector in the saturation
region could be operated a t much lower flow rates and still exhibit high stability. Figure 4 shows the effect of varying the make-up gas flow rate between 14 and 100 (:m3/min while maintaining a constant column flow at 14 cm:'/min. Figure 4 shows that t h e detector response to CH4 and CO was optimized a t a flow of 30 cm3/min, but that the detector could be operated at higher flow rates with only slight decreases in response. The effect of varying the flow rate on the detector response to Ha,Ar + O2 and N2was different from that of CH4 and CO. Any increase in the flow rat e decreased the detector response t o these 4 gases and eventually they provide a Ar, 02,N?, and H 2 0 are the negative response. Since Ne, Hz, major impurities present in helium, increasing the flow rate means more of these impurities enter the detector in a given time period. The detector responds to the net difference between the amounts of these gases i n the carrier gas and the amounts in the sample analyzed ( 5 ) . If a point is reached where the sample analyzed has a lower concentration of any given gas than the carrier gas, this will decrease the background current and the detector will provide a negative response as seen in Figure 4. This decrease in the detector response to these four gases can be of some advantage. For example, if the detection of air will interfere with the detection of the sample analyzed, as may be the case in the analysis of organic compounds by direct syringe injection, imd if the flow rate increases, the response to the air peak will decrease drastically while the response to the organic compound would not be significantly affected. Figure 5 shows the linearity and detection limits of six gases in the saturation region. The detection limits for these gases were found to be: 3.5 ppm Hz, 0.06 ppm Ar, 0.03 ppm NS, 0.004 ppm 02,0.004 ppm CH,, and 0.02 ppm CO. The results shown in Figures 3 and 5 compare well with the results previously reported for the helium detector in the multiplication region (2-5, 13, 14, 17,21). Most important is the fact that the operation of the detector in the saturation region is characterized by low standing current, low noise level, and high stability. This may explain why the detector still provides high sensitivity at such low applied potential. We further found that we were able to program the temperature of the column without hindering the function of the detector. With the capillary column in line, it appeared that the detector could still function properly. The gas sampling injection valve was used to introduce the gaseous samples to the capillary column. Figure 6A shows the detector response to a few ppm helium sample. It shows good of five organic gases in a 100-~L response with a very stable background current. Using the 100 ppm organic gaseous mixture, we also investigated the effects of flow rate on the detector response. The flow rate through the column remained constant while the make-up gas was varied. As seen in Figure 7, the detector response was
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Figure 8. Effect of detector temperature on the response and background current. Response measured for 103 pprn 1-hexene
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Figure 6. (A) Detector response to 17 ppm of ethylene, propylene, l-butene, l-pentene, and l-hexene. Column, 100 rn X 0.25 mrn coated with Witconol. Column flow, 3.5 cm3/rnln; make-up, 40 cm3/min. Column temperature, 60 OC; detector temperature 175 " C ; sample size, 100 pL. (B) Detector response to 40 ppb; conditions same as in A optimized a t about 35 mL/min, while the background current kept decreasing as the flow increased. It is important to point out that, if the make-up gas a t the detector is of low purity due to contamination with water or air, there will be a decrease in the detector response because of the decrease in the number of metastable atoms. Figure 7 shows that the detector could be operated a t a flow rate of 20 mL/min with only a slight decrease in the detector response. The initial increase in the detector response a t increased flow rates is probably due to the reduction in the atmospheric back diffusion to the detector cell, while the decrease in the detector response as the flow
rate increases above 50 mL/min is probably due to a decrease in the collision probability between the metastable helium and the detectable species. T h e effect of the helium detector temperature on the response has not been systematically studied. Previously, the detector has been operated either a t room temperature or a t low temperatures (below 100 "C). If the detector is to be used for the analysis of organic compounds, temperature is an important factor. Most organic compounds are eluted a t high temperature (above 100 "C), and consequently the detector has to be heated above the column temperature to prevent any condensation on the detector. We investigated the effect of temperature on the detector response as indicated by the response to a sample concentration of 100 ppm of 1-hexene in a 100-pLhelium sample. Figure 8 shows that the response is optimized a t 175 "C but the detector can be operated u p to 200 "C without much loss of sensitivity. Above 200 "C, tritium is released from the foil (22),and therefore we did not exceed this temperature. Figure 6B shows the detector response to 40 ppb of each of 1-butene, 1-pentene, and 1-hexene
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
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The M-shaped peak resulting from detector overloading. Column as in Figure 6. Column temperature, 90 OC; sample size, 0.01 pL: 0.05% water Figure 10.
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.
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Detector response to direct syringe injection. Column as in Figure 6. Column temperature, 60' C; sample size, 0.1 p L of benzene in xylene Flgure 9.
at an elevated temperature. There is no significant difference in the response of the detector to these gases due to their low comparable ionization potential (9.5 eV). LJsing the exponential dilution flask, we found that the detector responds linearly and symmetrically to sample concentrations up to 0.5% for the three organic gases. Above this concentration the detector response is deformed in an M-shaped peak. This upper linear limit, is similar to that shown in Figure 5 for inorganic gases. Most organic compounds are liquids, so for practical use of the helium detector in the analysis of organic compounds, one has t,o be able to use a direct syringe injection method. T h e problems involved in introducing a liquid sample by syringe are associated with the possible introduction of air that could overload the detector response and obscure a part of the chromatogram. T o minimize air introduction to the analytical column, we modified the injection port as shown in Figure 1. When the modified injection port was tested i t appeared that the amounts of air introduced with the injection are within acceptable limits. Figure 9 shows the detector response to 8 ng of benzene in a xylene sample. A f k r the benzene sample was eluted, the column was heated to 140 "C to elute the xylene. The detection limit of benzene was found to be less than 80 pg. Figure 10 shows the detector response to 0.1 p L of benzene by direct syringe injection. It shows the M-shaped peak which results from overloading the detector causing a decrease or loss of the ionization current due to quenching o f t h e metastable atoms. It also shows the detector response to 0.05% water in the benzene sample. Chromatographers often use a temperature programming method to shorten the analysis time and tu improve efficiency. Figure 11 shows a temperature programmed analysis from 90 to 160 "C for the analysis of some hydrocarbons. The sample
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Figure 11. Temperature-programmed analysis for hydrocarbons C, to C,4. InRial temperature, 90 OC for 2 min, programmed at 10 OC/min to 160 O C . Injection port temperature, 200 'C; detector temperature, 175 OC Column as iv Figure 6
(0.01 &) was introduced by direct syringe injection. Figure 11 shows some acceptable column bleed. This column is routinely used in our laboratory and bleed is often also detected by FTD operation at high temperatures. Column bleed is a universal problem, and users of the helium ionization detector might benefit from investigation of other materials such as liquid crystal stationary phases which produce little or no bleed. This, however, is outside the scope of the work described here.
CONCLUSION With the exception of the detector response to H2, Ar, 02, and N2, the mechanism behind the helium detector in the
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saturation region is ionization by metastable helium. Secondary electrons do not have sufficient energy in this region to take part in the ionization process. The mechanism by which the detector responds to H2, Ar, Oz, and N, is not fully understood. T h e effect of carrier gas purity on the detector response in the saturation region is currently under investigation. Generally the linear dynamic range is directly proportional to the number of metastable helium atoms available for ionization. Any impurity in the helium carrier gas will consume some of the metastable helium, and hence will shorten the linear dynamic range. However, a minimum amount of an impurity is needed to obtain a positive response to H2, Ar, O z , and Nz, otherwise the detector will provide a negative response to these gases (19). Apparently, the high purity grade of helium used in this work contains enough impurity to provide a positive response to all gases except neon. The detector is capable of providing a positive response for neon as well, at different conditions as previously reported (13). The helium detector is a simple and sensitive detector and ideal for the analysis of compounds such as certain halocarbons, low molecular weight hydrocarbons, hydrogen cyanide, ammonia, water, and phosphorus, sulfur and nitrous gases. I t is also suitable for detecting volatiles released from pyrolyzed metals, organic materials, and geological samples (23). T h e helium detector could also be useful in combined gas chromatography-atomospheric pressure ionization mass spectrometry (24). Due to the high sensitivity of this detector, it could be used to determine certain pollutants directly without the need for a concentration step that is often needed when other detectors are used. On the other hand, the helium detector used in this work, suffers from some temperature limitations (maximum 200 O C ) , a problem which could be overcome by using a more thermally stable radioactive source (25). When using the helium detector, special attention should be paid to air leakage in the carrier gas. Our experience with this detector over the past three years indicates that the major problem is contamination of the carrier gas by air leakage into the system. If leakage into the system is minimal, and the
column used is well conditioned, the detector behavior should be satisfactory.
ACKNOWLEDGMENT We thank T. Byers and 0. Mullins of Lockheed Engineering and Management Services Co., Inc., for modifying the electronics of the gas chromatograph.
LITERATURE CITED (1) J. E. Lovelock, J. Chromatogr., 1, 35 (1958). (2) R . Berry, "Gas Chromatography, 1962", M. Van Swaay, Ed.. Butterworths, London, 1962, p 321. (3) P. J. Bourke, R. W. Dawson, and W. H. Denton, J. Chromatogr., 14, 387 (1964). (4) S. R . Lipsky and M. M. Shahin, Nature (London),197, 625 (1963). (5) C. H. Hartrnan and K. P. Dimick. J. Gas Chromatogr., 4, 163 (1966). (6) F. Andrawes and E. K. Gibson, Am. Mineral. 64, 453 (1979). (7) A. Karmen. L. Guiffrida, and R. Bowman, J. Chromatcgr.,9 , 13 (1962). (8) H. Hachenberg and J. Gutberlet, Brennst.-Chem., 49, 242 (1968). (9) R. Villalobos, Chem. Eng. Prcgr., 64, 55 (1968). (10) J. Lasa, E. Bros., 2 . Kosc. and B. Mendyka, Chem. Anal. (Warsaw), 18, 1033 (1973). ( 1 1) D. J. David, "Gas Chromatographic Detectors", John Wiiey 8 Sons, New York, 1965,: p 165. (12) J. Sevcik, Detectors in Gas Chromatography", Elsevier, New York, 1976, p 131. (13) F. Andrawes and E. Gibson, Anal. Chem., 50, 1146 (1978). (14) J. Lasa and E. Bros, J. Chromatogr. Sci., 12, 806 (1975). (15) R. T. Parkinson and R. E. Wilson, J. Chromatogr., 24, 412 (1966). (16) S. Lukac and J. Sevcik, Chromatographia, 5, 195 (1972). (17) E. Bros and J. Lasa, J. Chromatogr., 174, 258 (1979). (18) M. L. Bruening and L. H. Wullstein, Environ. Sci. Techno/.,8 , 72 (1974). (19) F. Andrawes and E. K. Gibson, Anal. Chem., in press. (20) S. Lukac, and J. Sevcik, Chromatographia, 5, 311 (1972). (21) F. Poy and R. Verga, The Characteristics and Performance of a New Helium Ionization Detector in a Gas Chromatographic System", USSRItaly, Symposium at Tbilisi (Georgia), May 1970. (22) G. R. Shoemake, J. E. Lovelock, and A. Zhtkis, J. Chromatogr., 12, 314 (1963). (23) F. Andrawes and E. K. Gibson, unpublished work. (24) E. C. Horning, M. G. Horning, D. I. Carroll, I.Dzidic, and R. N. Stillwell, Anal. Chem., 45, 936 (1973). (25) G. R. Shoemake, D.C. Fenimore, and A. Zlatkis. J. Gas Chromatogr., 3 , 285 (1965).
RECEIVED for review December 21,1970. Accepted February 19,1980. This work was performed, in part, under the auspices of the National Aeronautics and Svace Administration, Contract NAS 9-15800 to Lockheed Electronics Co., Inc.
