John McLean and Peter 1. Pauson Royal College of Science and Technology G I ~ S ~ Scotland OW,
A Gas Chromatography Demonstration Apparatus
A n inexpensive apparatus for the separation of volatile organic compounds by gas chromatography has been described,' in which hydrogen was used as the carrier gas. The gas was burned as it emerged from the apparatus and the detection of components was accomplished by studying visual changes in the nature and height of the flame. For the purpose of class demonstration of gas chromatography it is necessary to use hydrogen under a pressure of 2-3 Ib/sq in. in order to get a flame which is long enough to
Figure 1.
partitioner. The coal gas was ignited a t the emergent orifice to give a luminous flame 2- -3-in. high. Oxygen at a pressure of 3 lb/sq in. or air a t a slightly higher pressure was then passed through the apparatus; and if the coal gas flame remained luminous, the coal gas supply was adjusted accordingly. The sample (0.02 ml) which consisted of a ternary mixture of ethyl chloride, methylene chloride, and chloroforn~in the volume ratio of 1 : 1 :1, was introduced via the serum cap by means of a hypodermic syringe. When one of these components appeared in the emergent gas, it imparted a green color to the flame as a result of the volatility of the copper halide formed on the copper wire detector. When the apparatus was operated at ambient temperature (approximately 14'C) ethyl chloride appeared in the emergent stream after 45 sec, the flame coloration reachmg a maximum after an additional 12 sec and finally disappearing after a further 12 sec. Methylene chloride appeared after the lapse of 100 sec from the time of introduction of the sample and burned for a total period of 50 sec while chloroform appeared after 250 sec and burned for 50 see. Besides offering a safe and easily visible demonstration of the separation of organic liquids by gas chromatography, the apparatus lends itself to the drawing of curves in which time is plotted against the intensity of the flame luminosity. By conducting the experiment in a semidark room and by focusing the copper halide flame on a Weston Master I11 universal exposure
Diagramof the apporatur.
be visible. This introduces a hazard as the serum cap may he blown off and the pressure hydrogen become ignited by the burning emergent gas. These dificulities with respect to flame height and fire hazard have been overcome by modifying the technique so that hydrogen is replaced by oxygen or air as carrier gas and the emergent gas is led into a coal gas flame which may have a height of 2 to 3 in. A convenient mixture for demonstration purposes is one containing chlorinated hydrocarbons which can be detected readily by means of a copper wire suspended in the coal gas flame. The apparatus consists essentially of a Pyrex U-tube (30 in. in length; 6.5 mm id; 8.5 mm od). The carrier gas and coal gas inlets are arranged as shown in Figure 1.
The U-tube was packed with Celite (80-120 mesh) which had previously been treated with silicone oil as COWAN, P. J., 246 (1959).
AND
SUGIRARA, J. M., J. CAEM.EDUC.,36,
Figure 2.
Intensity of flame luminosity versus time for a ternary mixture
meter by means of a lens, the graph shown in Figure 2 was obtained for the ternary mixture above using compressed air a t a pressure of 6 lb/sq in. as carrier gas. Impressed by the success of the above separation we directed our attention to experiments of a serniquantitative nature. I n these, the Celite column was Volume 40, Number 10, October 1963 / 539
inserted in the air stream leading to a flame photometer as in Figure 3 and readings were taken on the galvanometer a t 10 sec intervals. Using mixtures of chloroform and methylene chloride in varying proportions and a copper wire detector, the galvanometer deflections were plotted against time; and it was shown that the ratio of the areas under the curves is the same as the ratio of the total chlorine content in the individual components of the mixture-see Table 1.
flame photometer could be nsed for detecting a wide range of organic compounds embracing hydrocarbons, halogenated hydrocarbons, ethers, aldehydes, ketones, amines, and esters, without the use of a special copper detector for halogenated compounds. Provided the vapor which is being detected causes an alteration in the luminosity of the flame, the photometer will serve as an efficient detector; and the only compounds tested which could not he detected satisfactorily were methanol and ethanol. Table 2.
Time and Duration of Flames for Organic Compounds in Demonstration Apparatus ~.
Appearmee of flame Camoound
Flolne Figure 3.
Photometer
Insertion of Celite column in air stream.
Table 1 .
Mixture
Carrier Gar-Compressed
Molar ratio CHClr: CH2C12
Area ratio
Air a t 7 lb/sq in.
Chlorine ratio
That the areas bore a direct relationship to the total amount of chlorine present in each component was to be expected as the copper wire detector system is in fact trapping chlorine only. In this connection it was observed that for mixture D, which contained the highest proportion of chloroform, it was necessary to replace the single strand of copper wire nsed as detector by a copper coil in order to obtain area results which showed good agreement with the chlorine ratio. Subsequent experiments revealed that the combination of the gas chromatographic column with the
540 / Journal o f Chemical Educofion
(see)
Duration of flame (see)
Acetaldehyde Ethyl chloride Acetone Diethyl ether n-Pentane Methyl iodide Methylene chloride Methyl acetate Ethyl amine Chloroform n-Hexane Benzene Cyelohexane Cvelohexene
Table 2 shows the results obtained with various classes of volatile organic compounds using compressed air a t 7 lb/sq in. as carrier gas. The time taken for the vapor to appear is noted and also the time of duration of the flame so that from the data provided in Table 2, one can predict which mixtures of the compounds shown could he separated by the procedure described. For example, there would be no difficulty in separating the components of a mixture containing acetone and chloroform, or *pentane and n-hexaue, or n-hexane and benzene. It was also found, that under a standard set of conditions, via., constant pressure of carrier gas and constant room temperature, the retention times for various compounds could be reproduced to within a few seconds representing a divergence of less than 5%. For higher boiling compounds, the U-tube may be immersed in a constant temperature oil bath, but this does not add to the teaching value of the apparatus.
