(27) Lovelock, J. E., Lipsky, S. R ,J . Am. Chem. SOC.82,431 (1960). (28) McWilliam, I. G., Dewar, R. A., “Gas Chromatography,” D. H. Desty, ed., Vol. 2 , p. 142, Butterworths, London, 1958. (29) Martin, A. J. P., James, A. T., Biochem. J . 63, 138 (1956). (30) Ongkiehong, L., “Gas Chromatography,” R. P. FT’. Scott, ed., Butterworths, London, in press. (31) Otvoe, J. W.,Stevenson, D. P.,
J . Am. C h e m SOC.78,546 (1956). (32) Pickethly, R. C., ANAL.CHEM.30, 1309 (1958). (33) Pompeo, D. J., Otvos, J. W. ( t o Shell Development Co.), U. S. Patent 2,641,710 ( 1953). (34) Ryce, S. .4.,Bryce, W. A , Can. J . Chem. 35, 1293 (195’ (35) Scot;, R. P. W.,‘)“Gas Chromatography, R. P. W. Scott, ed., Britterworths, London, in press. (36) Sharp?, J., “Siiclenr Radiation De-
tectors,” pp. 130-4, blethuens, London, 1955. (37) Stern, O., quoted by Lewis, B.. Von Elbe, G., in “Combustions, Flames and Explosions of Gases,” p. 206, Academic Press, New York, 1951. (38) Townsend, J., “Electrons in Gases,” Hutchinsons, London, 1947. (39) Willis, Y.,-Vuture 183,1754 (1959). RECEIVEDfor review November 3, 1960. Accepted December 8, 1960.
Adsorption Characteristics of Some Gas-Liquid Chromatographic Supports EVERETT M. BENS Research Department, Chemistry Division,
b Different solid supports were studied to understand more completely the causes for tailing in gas-liquid chromatographic separations. The adsorption of some common solvents on several solid supports was studied by measurement of the retention volumes of the solvents on the supports at several temperatures. The retention volumes of some aliphatic and aromatic hydrocarbons, alcohols, and ketones on C-22 firebrick, glass spheres, and Tide were plotted against inverse temperature; all produced linear isotherms. Effects of small quantities of residual solvent upon the retention time and its effect upon the above plot are discussed. A graphical comparison of the various solid supports is made so that the effects of adsorption may be evaluated when separations are made using small quantities of stationary phase. Rapid evaluation of the change due to different treatments of the solid support is readily made.
T
of gas-solid chromatography has been retarded by three major disadvantages: chemical changes induced by certain adsorbants, serious tailing effects due to nonlinearity of the adsorption isotherms, and displacement effects due to the dependence of individual isotherms upon the nature, number, and concentration of the components in the sample. While the chemical changes have been particularly prevalent in gas-solid chromatography, Vilkas and Abraham (29) have recently reported isomerism of 8-pinene when a nonpolar substrate was used on either firebrick or Celite. When more polar substrates were used the active sites of the support were deactivated but reactivated after aging. HE WIDESPREAD USE
178
ANALYTICAL CHEMISTRY
U. S.
Naval Ordnance Test Station, China lake, Calif.
The importance of this characteristic has been recognized by the British suppliers of gas-liquid support materials (20) who devised a special test of the catalytic action of their supports. The nonlinearity of adsorption isotherms nhich causes tailing has been a problem in both gas-solid and gasliquid chromatography. Knight ( I S ) improved the symmetry of elution peaks for hydroxyl and amino compounds by saturating the carrier gas with a polar material similar to the sample, thus reducing the activity of the support. Other investigators of gas-liquid chromatographic supports have used varying amounts of stationary phase to eraluate the effects of the support upon separation and tailing. I n one such study, Johns (16) found that a decrease in particle size of the support increased the retention time. I n addition, repeated sampling tended to increase the peak height observed for polar materials, an effect IT-hich may be ascribed to decreased adsorption due to saturation of active sites. Most workers h a r e used chemical treatment of support materials to remove the active sites and, thereby, reduce the tailing effects. It has become common practice to acid-wash, or caustic-treat solid supports so that tailing of the more polar materials is reduced. -4novel treatment by Ormerod and Scott (21) consisted of the deposition of an equal weight of silver upon the support and in this manner they reduced tailing successfully. Another worker (10) has evaluated the effect of varied amounts of support and stationary phases upon relative retention values. He found the retention times of hydrocarbons to be least affected by the support material, while the more polar materials such as
water, alcohols, amines, ketones, and aldehydes 17-ere changed considerably. A better understanding of the part played by the solid support may nom be obtained from a thorough report prepared by Ottenstein (26) on the more commonly used solid support materials. He discusses the differences between the Chromosorbs, firebrick, and Celite 545 as \vel1 as reviewing the attempts of other workers to deactivate these materials, either by chemical treatment to remove the active sites, or by saturation of the active sites with a more polar inaterial in the stationary phase. A brief discussion of other solid supports such as glass beads, metal helices, Tide, and Teflon n a s included. The use of glass beads and small amounts of stationary phase has been particularly applicable for the separation of some solid organic compounds (12). A recent study (3) of the adsorptivity of Silocel firebrick was made in which the adsorption isotherm, determined by frontal development, was correlated with the elution method using uncoated firebrick. Emphasis was placed on the chemical treatment of the active adsorption sites, believed to be the hydroxyl groups of the siliceous material, n ith hexamethyldisilizane to modify the support and thus prevent tailing. Suppression of any remaining activity with a trace of polar material (polyethylene glycol) was recommended. Since the affinity of many materials has been studied in relation to the solid support and some stationary phase, it would appear that further study of the support material alone would be beneficial. I n such a study the retention volumes of any material on the support should be the same as that of any inert gas if there were no adsorption effects. By such a study the effect of chemical
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TIDE (PETROLEUM ETHER EXTRACTED-DECORA) 40-60MESH I 3.6 GRAMS/FT.
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or physical changes in the support materials can then be evaluated, new support materials may be compared, proper tailing suppressors may be selected, and possible chemical interaction of the support and proposed sample may be estimated. Since much information is available on zeolites and Linde Molecular Sieves ( 4 , 5 , 1 1 , 1 4 , 25), silicagel ( 2 , 8 , 9 , 1 1 , 1 9 , $4, 26, 27, 28), carbon blacks ( I , 11, 16), alumina (6, 11, I Y ) , and bentonites ( I S , as), interest has been focused upon a commercial preparation of C-22 firebrick, glass spheres, and Tide, extracted in the manner of Decora ( 7 ) . Data on these materials v-ere treated much in the same manner as Phillips (23) treated gas-liquid chromatographic data, in which the log of the corrected retention (V'R) mas plotted against reciprocal temperature, for each solid support material. From these data. heats of adsorption could be calculated in the manner of Greene and Pust (11). In addition, the logarithm of corrected retcntion volumes or retention times for rach of two materials was plotted so that comparison of the adsorption of different chemical species upon the supports was facilitated. EXPERIMENTAL
A Perkin-Elmer Model 154C Vapor Fractometer having a thermistor-type detector was used with a k e d s &
Figure 2. Log retention volumes vs. inverse temperature for ketones, acetates, and other solvents on Tide support
Xorthrup Speedomax G recorder. The recorder has a 6 m v . span, 1-second response, and a chart speed of l / 2 inch per minute. The instrument was altered so that the carrier gas was heated t o 71" C. when the oven temperature was 120" C., and 52" C. if the oven was 60" C. by heating the injection block with the oven heater alone. Later work was carried out with the 60-watt heater of the injection block connected to the standard 20watt heater power supply and the power control set a t 80 so that the carrier gas temDerature was 145" C. in all determinations. In all cases the helium flow rate a t the column exit was held a t 43 cc. Der min. One-fourth-inch diameter coppei tubing, 6 feet in length, was used for all columns with the exception of the Tide columns where both stainless steel and copper tubing were used The tubing was cleaned with acetone, dried, and filled with either 13.5 grams of GC-22 Super Support (42- to 60-mesh), from Coast Engineering Laboratories, Herniosa Beach, Calif., 0.2-mm. glass spheres from the Minnesota Mining Co., or 13.6 grams of Tide (40- to 60-mesh) n-hich had been petroleum ether-e'xtracted as described in ( 7 ) . An additional support was prepared by a Soxhlet extraction of Tide with absolute ethyl alcohol for a period of 4 hours followed by drying in a vacuum oven a t 50" C. The supports were compacted by \
