CORRESPONDENCE Hydrogen Flame Ionization Detection of lnerts Using an Ionizing Carrier Gas SIR: One serious disadvantage of the hydrogen flame ionization detector (FID) is its low response or lack of response to certain compounds, Le., carbon dioxide, sulfur dioxide, nitrogen, helium, etc ( I , 2). The purpose of this paper is to report a technique that permits detection of inerts by a FID.
Table I. FID Response to Inerts Using Perfluoromethane as Carrier Gas
EXPERIMENTAL
Apparatus and Reagents. The chromatograph used in this work was a dual column, dual hydrogen flame ionization detector Varian Aerograph Model 1840-3 gas chromatograph. The recording instrument was a 1-mV Varian Aerograph Model 30 strip chart recorder. Peak area measurements were made with a polar planimeter. The gas chromatograph was fitted with a gas sample valve which was connected to a glass vacuum system. Two 1/8-in. stainless steel silica gel columns, 21 feet long, packed with Varian Aerograph No. 82-3060 30/60 mesh silica gel were installed in the chromatograph. Hydrogen and air were supplied to the chromatograph from commercial compressed gas cylinders. Nitrogen, helium, carbon dioxide, and sulfur dioxide were commercial gases supplied in compressed gas cylinders or lecture bottles. The ionizing carrier gas used in this work was perfluoromethane (Freon 14) from E. I. du Pont de Nemours and Company, Wilmington, Del. Procedure. The chromatograph was operated in a single channel mode for the results presented here; however, similar results were obtained when the chromatograph was operated in a differential mode of operation. The chromatograph was set up and operated in a normal fashion except that the carrier gas was connected to the chromatograph by a Swagelok quick-connect fitting. The chromatograph column oven was held at 80 OC and the detector oven was maintained at 275 “C. The air and hydrogen flow rates used were 291 cc/min and 23.2 cc/min, respectively. The range setting on the electrometer was lo-” A/mV. The electrometer was not saturated at this setting, and all chromatograph peaks could be kept on the recorder scale by appropriate attenuation. Initially the chromatograph was set up and balanced with a nitrogen carrier gas flow rate of 25.3 cc/min. After a stable base line was obtained on the recorder, the carrier gas was quickly changed to perfluoromethane by the Swagelok quickconnect fitting. The change from nitrogen to perfluoromethane carrier gas caused a large positive shift in the base line on the recorder but after approximatly ten more minutes of operation, a new stable base line was obtained with the perfluoromethane carrier gas. The flow rate of the perfluoromethane carrier gas was 20.7 cc/min. Various amounts of gaseous samples were injected into the chromatograph by the gas sample valve or by a gas-tight syringe and responses were measured. The responses were “negative peaks” for nitrogen, helium, carbon dioxide, and sulfur dioxide. A sample of natural gas was injected into the chromatograph and typical hydrocarbon responses (positive peaks) were obtained for the components in the natural gas sample. Changes in the hydrogen, air, and perfluoro(1) H. M. McNair and E. J. Bonelli, “Basic Gas Chromatography,” Varian Aerograph, Walnut Creek, Calif., 1969. (2) H. Purnell, “Gas Chromatography,” John Wiley and Sons, New York, N.Y., 1962.
Gram moles of inert sample
Response in coulombs
Response value, coulombs/gram mole
Nitrogen 1.05 x 10-4 5.11 x 10-5
4.18 X 10-lo 2.52 X 10-lo
3.98 X 4.93 x 10-6
Helium 9.69 x 10-5 5.11 x 10-5
6.22 X l0-lo 3.48 x 10-lo
6.42 x 10-6 6.81 X
Carbon dioxide 8.88 X 4.47 x 10-5
7.68 X 2.52 X 10-lo
8.65 5.64
x x
10-8
Sulfur dioxide 2.05 x 10-4 1.23 X lo-‘
4.35 2.44
x x
2.12 1.98
x x
10-7 10-7
10-11 10-11
methane flow rates had large effects on the response values for the inert compounds ; however, maximum response values for the inerts were not determined. Perfluoromethane (Freon 116) was also used as a carrier gas and it gave larger response values for the inerts (higher sensitivity) than did perfluoromethane. This was expected because perfluoroethane has a higher FID response value than does perfluoromethane ( 3 , 4 ) .
RESULTS AND DISCUSSION
Negative chromatographic peaks have been observed and explained in thermal conductivity gas chromatography (5) and various carrier gases have been compared when using FID analysis of organic samples (6); however, the author could find no published results on the concept of FID detection of inerts by producing negative peaks in an ionizing carrier gas stream. Response values (calculated from negative peak areas) of the FID to inerts using perfluoromethane as a carrier gas are given in Table I. The accuracy of the data in Table I is estimated to be within 5 %. Response values for nitrogen, helium, and carbon dioxide are all about 5 X coulomb per gram mole which is the same order of magnitude as when perfluoromethane is analyzed by a FID using a coulomb per gram mole nitrogen carrier gas; i.e., 1.1 x (3, 4 ) . The smallest detectable amount of nitrogen in the CF4 carrier gas stream was approximately 1 X lov6 gram mole at the electrometer setting of A/mV. The dilution of the ionizing carrier gas stream by the inert sample appears to decrease the ion current produced in the FID and thus (3) K. D. Maduskar, M.S. Thesis, Auburn University, Auburn, Ala., 1971. (4) W. C. Askew and K. D. Maduskar, J. Chromatogr. Sei.,Nov. (1971). ( 5 ) R. A. Ginskey and F. Schmidt-Bleek, ibid.,7, 371 (1969). (6) J. A. Singleton, L. W. Aurand, and T. A. Bell, J. Gas Chromarogr., 3 (lo), 357 (1965).
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create negative peaks for the inert samples. Hydrocarbon samples, highly ionizing compounds, still give positive response peaks. As seen in Table I, the response values for the compounds tested are not linear for the sample sizes injected. Linearity, linear dynamic range, sensitivity, and maximum responses were not studied in this work but this study is planned for future work. Separating ability for mixtures and peak shape depends upon the chromatographic column
and analytical operating conditions used as in regular FID chromatography. WILLIAMC. ASKEW Chemical Engineering Department Auburn University Auburn, Ala. 36830
RECEIVED for review July 23, 1971. Accepted October 13, 1971.
Capillary Gas Chromatography with Two New Moderately High Temperature Phases SIR: Dexsil 300-GC ( I ) ( O h Corporation), a linear polym-carboranylene-siloxane has been found to be quite stable at higher temperatures than are normally used in capillary gas chromatography (2, 3). Because of my interest in gas chromatographic applications for the resolution of N-trifluoroactyl (N-TFA)-DL-amino acid (+)-2-butyl esters (4, 5 ) in capillary columns, I am reporting on two new Dexsil phases, 400-GC,* end groups uncapped and capped. The Dexsil400-GC phases ( I ) are similar to Dexsil 300-GC except that some methyl groups have been replaced with phenyl groups. The uncapped Dexsil 400-GC has free hydroxyl end groups while the end groups of the capped phase have been covered with trimethyl silyl groups. I had tested the Dexsil 300-GC for application to resolution of N-TFADL-amino acid-( +)-2-butyl esters and found to my surprise, that this phase with its SE-30 backbone gave fair resolution of several derivatives. Since only polar phases are reasonably successful in resolving these derivatives, it must be assumed that the icosahedral carborane moiety gives some polarity to the Dexsil 300-GC. It would be expected that substitution of phenyl for methyl groups would further increase polarity, hence our interest in the new phases. Capillary columns were prepared by the plug method (6). A 10% solution (w/v) of the phase was made (acetone solvent for the uncapped phase and methylene chloride for the capped). Two milliliters of the solution were transferred to the coating reservoir and pressed through the 0.02-in. X 150-ft capillary column using 10-lb nitrogen pressure. This procedure coated about 66 mg of uncapped phase in the column and 64 mg of capped phase. The columns were conditioned at room temperature until the solvent was no longer detectable. They were then conditioned at 125 "C for 30 minutes at 20-lb helium pressure flowing in the opposite direction to the coating direction. The temperature was then raised to 250 'C for 2 hr and then to 300 'C for 2 hr for the uncapped phase column. The capped phase column was kept at 300 "C for 18 hr, and then raised to 350 "C for 1 hr.
* Dexsil 400-GC was called Dexsil 400-6 by the manufacturer in the development stage. (1) Karl 0. Knollmueller, Robert N. Scott, Herbert Kwasnik, and John F. Sieckhaus, J . Polymer Sci., Part A - I , 9, 1071 (1971). (2) M. Novotny and A. Zlatkis, J . Chromatogr.,56, 353 (1971). (3) R. W. Finch, Analabs Research Notes, 10,NO. 3 , l(1970). (4) Glenn E. Pollock and L. H. Frommhagen, Anal. Biochem., 24, 18 (1968). (5) G. Pollock and A. Kawauchi, ANAL.CHEM., 40, 1356 (1968). (6) R. Kaiser, "Gas Phase Chromatography," Butterworths, Inc., Washington, 1963, p 45. 634
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
ATTENUATION 8OX 100-18O'C HELIUM G m l i m i n r KIN ELMER 900
AT 2'imi-
ATTENUATION 16OX 100-Z15pC AT B0/min HELIUM 8 2 ml/min PEAK ELMER 900
MV
i 35 30 25 20 15 IO
5 0 30 25 20 15 TIME, min
IO
5
0
Figure 1. a . Separation of N-TFA-DL-amino acid (+)-2-butyl esters
b. Separation of N-TFA-amino acid-n-butyl esters
Table I. (Column bleed at 20 X Dexsil MO-GC, Uncapped)
Temp, "C Begin at 100 150 200 225 250 27 5 300 a
Chart reading, mV
Increase, mV
0.04 0.043 0.047 0.050 0.060 0. loo 0.210
0 0.003 0.007 0.010 0.020 0.060 0.170
Dexsil4mGC, capped had about half the bleed of these values.
These columns were then used for the application survey shown in this correspondence. These two phases were tested and found to be quite stable. With single column operation, the capped form produces only a small base-line drift at temperatures up to 325 "C. It could be used to 35@-400 "C for short periods of time, but at temperatures above 325 "C, bleeding increased significantly. After 5 hours at 400 "C, the column was too degraded for further use. The uncapped phase was not as stable as the capped phase at very high temperatures. I repeatedly used
