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 ' i m i -
ATTENUATION 16OX 100-Z15pC AT B0/min HELIUM 8 2 m l / m i n 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)
a
Temp, "C Chart reading, mV Increase, mV Begin at 100 0.04 0 150 0.043 0.003 0.047 0.007 200 0.050 0.010 225 0.060 0.020 250 0. loo 0.060 275 0.210 0.170 300 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
FATTY ACID METHYL ESTERS PUALITATIVE STANDARDSDEXSIL 4009,UNCAPPED 0 0 2 i n x15011(66mg) ATTENUATION i 6 o x i o o - w c AT lOo/min
PUALITATIVE STANDAROSDEXSIL 400+, CAPPED 0 0 2 i n xISOft(64mg)
1
0 D
0
MV
00
I
35 30 25 20
15
10
5
I
I
0 35 30 25 20 TIME, min
I
I
I
I
15
10
5
0
Figure 2. Separation of saturated and unsaturated fatty acid methyl esters a.
Dexsil400-GC, capped
b. Dexsil W-GC, uncapped SATURATED HYDROCARBON PUALITATIVE STANOARDSDEXSIL 400$,CAPPEO O 0 2 i n xISOft(64mgl
35 3 0 25 20
15
n
IO
5
0 TIME,
SATURATED HYDROCARBON OUALITATIVE STANlAROS DEXSIL 400+,UNCAPPED (66 mg) ATTENUA';OV l6OX 1 2 0 - 2 5 O ~ CAT S Y m ~ n HELIUM 8 2nIlmii
in
Figure 3. Separation of saturated hydrocarbons a.
Dexsil 400-GC, capped
b. Dexsil400-GC, uncapped
this column, however, to 225-250 "C and further to 300 "C for periods up to 1 hr. Column bleeding was minimal at temperatures of 225 "C and was acceptable above 225 to 300 "C. (See Table I.) When this column was tested with a gas chromatograph-mass spectrometer system, using a Varian membrane separator heated to 250 "C, the column at 280 "C gave virtually no background.
These two phases were surveyed for their ability to resolve N-TFA-~~-(+)-2-butyl esters and n-butyl esters (7) as well as fatty acid methyl esters and hydrocarbon standards. Generally, the more polar Dexsil 400-GC, uncapped phase was superior to the capped phase in all separation comparisons, except possibly hydrocarbons. Figure l a shows a chromatogram of five DL-amino acid (+)2-butyl ester derivatives on the uncapped phase. The capped phase does not resolve these derivatives as well and is not shown. Figure l b shows the separation of N-TFA-amino acid-nbutyl esters on the uncapped phase. The capped phase is not shown, but it gives a comparable separation of the components ; however, slightly more tailing occurs with the capped phase. This fault could conceivably be minimized by optimizing the coating thickness and other gas cbromatographic parameters such as carrier gas flow and program. Comparisons of fatty acid methyl ester separations and hydrocarbon separations are shown in Figures 2a and 2b and 3a and 36, respectively. It is obvious from Figures 2a and 26 that bleed with the uncapped phase (Figure 26) is much more noticeable from 250-300 "C than the capped phase. The tailing of esters Cg through C11 on the capped phase is significantly worse than the uncapped phase. It should be noted that the saturated and unsaturated fatty acid methyl esters are separated by both columns. The hydrocarbons are separated by both columns and it should be noted that pristane is partially separated from C17while phytane is not separated from Cis. These branched chain hydrocarbons (pristane and phytane) are generally of interest to geochemists and others interested in chemical evolution or the origin of petroleum. Under the conditions tested, the uncapped phase gives less tailing and is superior in resolving the (+)-2-butyl and n-butyl amino acid ester derivatives. As indicated in Figures 2a and 2b and 3a and 36, the capped phase appears to be as good as the uncapped phase for fatty acid esters over CI1and possibly better for hydrocarbons. One could certainly more easily chromatograph longer chain fatty acid esters and hydrocarbons on the capped phase. In general, these two new phases will materially aid in the extension of gas chromatographymass spectrometry applications as well as increase the temperature range over which capillary gas chromatography can operate. ACKNOWLEDGMENT I wish to thank Dr. Hans Schroeder for supplying me with these phases and Dr. J. Lawless for testing the uncapped phase with his GC-MS system. GLENNE. POLLOCK Ames Research Center NASA Moffett Field, Calif. 94035 RECEIVED for review August 19, 1971. Accepted October 21, 1971. (7) W. M. Lambkin and C. W. Gehrke, ANAL.CHEM.,37, 383 (1965).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 3, MARCH 1972
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