James M. Miller Drew University Madison, N e w Jersey
Dual Column Gas Chromatography A teaching to01
Soon after the technique of programed temperature gas chromatography became popular, dual parallel column instruments were used to cancel baseline drift due to the bleeding of the liquid phase. In such applications, both columns in the chromatograph are the same. The use of two differentcolumns has been suggested but not fully exploited. This technique lends itself nicely to teaching gas chromatography in analytical chemistry. There are three advantages in using dual cblumns. The principles of gas chromatography can be more easily taught since several parameters are being held constant; namely, the oven and injection temperatures, the flow, the sample size, and the detector and read-out systems. It is not necessary to turn off the instrument and change columns since two columns are
always available; thus this technique saves quite a bit of laboratory time. These instruments are only slightly, if any, more expensive than single column instruments. Hence, for these applications a dual column instrument is almost as good as having two single column instruments. Before dual column chromatographs became available, Amy1 described a modification of a commercial single column instrument so that three different columns could be used. He also described a simple experiment to illustrate the selectivity of liquid phases. A homemade dual column chromatograph has been constructed for less than $300. The flow diagram is similar to that of the commercial instruments and the component parts are similar to those which have been described.% The only component which is not readily
Presented at the third Annual Metropolitan Regional Meeting of the North Jersey and New York Sections of the American Chemical Society, Jt~nuery27, 1964.
'AMY,J. W., Perkin-ElmerInstrumat News., 1 3 , l (1962). MILLER,J. M., "Experimental Gaa Chxnatography," Gow-Mac Instrument Co., Madison, N. J., 1963, pp. 11-24.
Volume 41, Number 8, August 1964
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available is the dual injection port which can he heated above the column temperature. An injection block was made by drilling out a piece of brass and attaching the necessary pipe fittings, tubes, and cartridge heater.3 Alternatively, pipe tees wound with a heating tape could be used. Typical Experiments
Another column variable which can be investigated is the extent of liquid loading. Table 4 gives data for two Carbowax columns, one containing 15% and the other 0.5% liquid phase. The shorter retention times and the failure to separate two of the components on the column containing 0.5% liquid phase aptly illustrate this colhnnvariable. Table 3.
The most obvious experiment is one which illustrates the differences in selectivity between different liquid phases. Table 1 lists the conditions used for one such experiment. Two columns are used which differ markedly in their polarity and hence in their selectivity. Table 1. Sample: Column C: Column N: Flow: Oven temperature: Injection temperature: Detector:
instrumental Conditions 1 r l Synthetic mixture in.; 15% Carhowax 400 on 4 ft X 60/80 mesh Chromasorb W 4 ft X '/, in.: 157" Nuiujol on 60/80 mesh Chronksorb-W 89 ml/min 46'C 125T Cow-Mac hot wire thermal conductivity cell
Table 2.
Comoound
80.1 82.3 97.2 99.3 110.6
Benzene iso-Propanol n-Propanol iso-Octane Toluene
Adjusted Retention Vx' (ml).--Volume, Column C Column N 206 363 794 21.4 420
313 46.3 82.0 491 960
a Vn' is "adjusted" retention volumn, i.e., measured retention time from the air peak.
Table 2 shows the results of a separation of a synthetic mixture composed of two paraffins, two alcohols, and two aromatics. Such a mixture would be difficult to separate by distillation. The most distinguishing characteristic of these chromatograms is the order of elution: the Carhowax column retains the alcohols and allows the paraffins to be eluted very quickly; on the other hand, the Nujol column retains the paraffins and allows both alcohols to be eluted quickly. I n both cases, the order of elution is different from the order of increasing boiling point. Note that the total time for each chromatogram is about twelve minutes. Even though this experiment is intended to show liquid phase selectivities, one can also use the data to calculate the number of theoretical plates for each column, separation factors between two components, and other quantities of interest,. These columns can also he used to illustrate a procedure for qualitative analysis of mixtures which contain members of homologous ~ e r i e s . ~Table 3 gives the data for a series of n-alcohols and acetates. Straight lines of differing slopes are obtained by plotting retention volume on one column versus retention volume on the other. T h e cooperation of the Cow-Mac Instrument Company is gratefully acknowledged. (KEULEMANS,A. I. M., "Gas Chromatography," 2nd ed., Reinhold Publishing Corp., New York, 1959, p. 31.
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Table 4.
Journal of Chemical Education
Effect of Liauid Loadina (T = 49°C) Vn' (ml)
Compound
-
15% C
0.57" C
n-Hexme iso-Octane Benaene iso-propanol Toluene n-Prooanol
Retention Data
Boiling ~ 0 i n ("C) t
414
Homologous Series I T = V 0 C )
" Unseparated on 0.5% column.
The effect of the solid support can also be studied. iVIanufacturers' literatures illustrates the advantages of a fluorocarbon over the traditional diatomaceous earth supports for polar materials and water. It should be noted, however, that a decrease in tailing is also accompanied in most cases by a loss in efficiency. Another procedure commonly used to obviate solid support effects is to treat the solid support. These treatments include acid washing, chemical treatments with methyl silicones, with hexamethyldisilazane, and similar chemicals. Examples of the effects of such treatment have been publi~hed.~Columns of two of these materials could be used to illustrate solid support effects. There are other less important parameters which can be investigated. These include column diameter, colun~nlength, mesh of solid support, mesh range of solid support, etc. It is hoped that this article will stimulate others to try experiments which illustrate these and other column variables. Tables of retention data can he consulted in designing experiments like the first one described. It is also anticipated that the use of two different colun~nsshould be helpful in qualitative organic analysis. Proper choice of chromatographic columns and student unknowns should allow separation of the components of most unknowns on a t least one of the columns followed by collection and spectroscopic identification. See, for example, drtta sheets an "Haloport F" available from F&M Scientific Corp., Avondale, Pa.; "Gas-Pack F" available from Chemical Research Services, Inc., Addison, Ill.; and "Fluro~ak," Armgraph Research Notes, Spring 1960, Wilkens Instrument Co., Walnut Creek, Calif. 'Bulletin FF-114, Johns-Manville Corporation, New York, 1963.
