Separation of Gases by Gas Adsorption Chromatography S. A. GREENE, M. L. MOBERG, and E. M. WILSON Aerojet-General Corp., Azusa, Calif.
Gas adsorption chromatography was investigated as a method of separating some low-boiling gases and hydrocarbons. Adsorbent columns containing activated charcoal and alumina were continuously heated during elution. This method is advantageous for analyzing gas mixtures with wide boiling ranges, as it sharpens peaks, improves peak symmetry, and reduces time for analysis.
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HE various techniques of gas chromatography have resulted in very useful methods for the separation and quantitative analysis of gaseous mixtures. In particular, Patton and coworkers ( 3 ) have shown good separations of some gases; the mixtures were separated on thermostated columns packed with charcoal, alumina, or silica gel, by elution with an inert carrier gas. They state that continually increasing column temperature during elution permits a more efficient separation of materials with widely different affinities for adsorbents, but were unable to take advantage of this technique because of the experimental difficulties attending reproduction of heating and carrier gas flow rate. For gas-liquid partition columns, Lichtenfels and coworkers (1 ) found that increasing column temperature during elution is advantageous for analysis of gas mixtures containing compounds with wide boiling ranges, but the reproduction of temperature rise and carrier gas flow rate is too poor for quantitative analysis. This paper describes the separation and quantitative analysis of some low-boiling gases and hydrocarbons by elution from columns packed with activated charcoal or alumina, while the column temperature is continually increased.
orifices (micrometer needle valves) located downstream of the thermal conductivity cell. The apparatus is schematically shown in Figure 1. Once a sample was being analyzed and the column heated, visual inspection of the manometer (which could be read to =tl mm. of water) indicated decreasing ressure drop across the orifice and thus decreasing flow througf: the sample channel. The correct pressure drop (and flow) was continually restored by suitable adjustments of the pressure regulator. The columns were fabricated from copper tubing 0.25 inch in outside diameter, and after being filled with the adsorbent were wound into a coil 3 inches in inside diameter. Columns were immersed in a stirred oil bath (initially a t ambient temperatures), and a t the beginning of the run immersion heaters were plugged into a 110-volt source. Resultant heating rates were extremely reproducible. When a chart speed of 1 inch per minute was used, the position of peaks for the same gases during different runs did not vary in position on the chart paper by more than k 0 . 5 inch. A detector (Gow-Mac thermal conductivity cell) was used with a 0 to 5-, lo-, and 25-mv. recording potentiometer. The helium carrier gas flow rate was 100 ml. per minute. Gas samples (approximately 10 ml.) were intro-
EXPERIMENTAL WORK
Continuously increasing the column temperature during elution results in a decreased flow to the flow-sensitive channels of a detector thermal conductivity cell. To maintain constant flow, the pressure upstream of the column should be continually increased. If reference and sample channel are connected in series, increased pressure in the reference channel will result in an annoying base-line drift of the recording potentiometer, used in conjunction with the thermal conductivity cell ( 3 ) .
TIME (YIN.)
Figure 2.
This difficulty was obviated by splitting the carrier gas, and controlling gas flow to both reference and sample channels by means of pressure regulators (Conoflow Corp., Philadelphia, Pa.). Gas flows were manometrically measured by the pressure drops (approximately 8 inches of water) across tn o variable
Separation of gas mixture on a !%footcharcoal column, heated to 170' C.
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Figure 3. Schematic diagram of apparatus
1369
Separation of hydrocarbon gases on a 20-foot alumina column, heated to 150' C.
ANALYTICAL CHEMISTRY
1370 duced through a metal sample system similar to that of Patton and coworkers (3).
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Figure 2 shows the separation of a mixture of hydrogen] carbon monoxide, air, methane, carbon dioxide, ethylene] and acetylene on a 9-foot column packed with 40- to 60-mesh activated charcoal. The sensitivity to hydrogen could have been increased by elution with nitrogen rather than helium, but this would have resulted in a large loss in carbon monoxide and air sensitivity. Column temperature a t the end of the run was 170' C. Figure 3 shows the results obtained with some hydrocarbons below Cson a 20-foot column packed with 60- to 80-mesh activated alumina, and heated to 150' C. The sample size was 10 ml., and all components were completely separated when a sample size of 0.3 ml. was used. Figure 4 illustrates the quantitative determination of four hydrocarbons by plotting the planimetered areas of peaks against the pressures in a constantcvolume sample tube which contained the mixtures. Precision of the determinations wm within 5 2 % . The adjustment of pressure during heating and elution does not seem to interfere with quantitative measurements. Aside from column length, the most critical factor in the separation of hydrocarbon mixtures is the rate of column heating. The results of Figure 2 were obtained using a single 250-watt heater; two 250-watt heaters resulted in a single peak for isobutylene and 1-butene and a double peak for acetylene, isobutane, and nbutane. Where increased heating rates can be tolerated, the symmetry of the peaks will be improved. ACKNOWLEDGMENT
The writers express their appreciation to the Air Force for financial assistance.
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RESULTS AND DISCUSSION
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Figure 4. Plot of planimetered areal of peaks against pressures
LITERATURE CITED
(1) Lichtenfela, D.H., Fleck, S. A., Burow, F. H., ANAL.CHEM.27, 1610-13 (1955). (2) Ibid.,Figure 7. (3) Patton, H.W., Lewis, J. S., Kaye, W. I., Ibid., 27, 170-4 (1955)
RECEIVED for review March 19, 1956. Accepted M a y 15,1956.
Qualitative Gas Chromatographic Analysis Using Two Columns of Different Characteristics J. S. LEWIS, H. W. PATTON, and W. 1. KAYE1 Research Laboratories, Tennessee fastman Co, Division of fastman Kodak Co., Kingsport, Tenn.
Information as to the time required for a member of a given homologous series to pass through a gas chromatographic column is often sufficient for identification of the compound. However, if members from other series are present, the time of emergence from a single column does not positively identify a compound. This paper reports the results of experiments using two columns for the systematic identification of volatile materials. Elution times of known compounds from two gas-liquid partition columns having different characteristics were plotted against each other on logarithmic paper. Each compound studied had a definite location on this plot, which was somewhat comparable to a two-dimensional paper chromatogram. This method was found to be simple and effective for the identification of alkanes, cycloalkanes, esters, aldehydes, ketones, and alcohols. I t is probable that the method could be easily extended to include other classes of compounds.
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WIDESPREAD interest has recently developed in the use of gas chromatography for the separation of gases and
volatile liquids. The gas-liquid partition method in particular has been shown to be exceptionally versatile and effective for the resolution of mixtures that are difficult to separate by other means. This method was first applied to the analysis of volatile fatty acids by James and Martin (16-17). It has since been used by a number of workers to separate a variety of compounds (1-26). In general, a separated component is identified by the time required for it to pass through the chromatographic column. However, this is sometimes insufficient information for characterization] particularly of fractions from very complex mixtures which may contain a variety of molecular types. Although members of a homologous series are generally separated with ease, the characteristic times for members of different series may not be sufliciently different to avoid confusion. 1
Present addreea, Beckman Instruments, Inc.. Fullerton, Calif.
