R. S. Tse University of Hong Kong Hong Kong
An Undergraduate Experiment in Gas Chromatography and C,H-Microanalysis
This paper describes an undergraduate experiment which illustrates both the basic instrumentation of gas chromatography and, in conjunction with an on-line combustion tube, its use in the microanalysis for C,H in organic compounds. A recent paper' in THIS JOURNAL has described a similar experiment in which the generated COz is determined chromatographically and the HzO by taking the difference. I n that experiment, a small error in the injected quantity could cause a large error in the difference, whereas in the present experiment, both the CO, and the H 2 0 are chromatographically determined and the C : H ratio is unaffected by errors in the amount injected. The construction of the apparatus is shown schematically in the figure. The temperature of the oven is capable of control to *l0C up to 25OoC. The column stainless steel tubing packed with is a 4 ft X I/&. 80-100 mesh Porapak Q.z The thermal conductivity detector is a Gow-Mac3 model 9454 with four tungsten filaments. The combustion tube is a 12 X 3/16-in. stainless steel tubing initially packed with fine copper wires. Copper oxide is generated on the surface of these wires by passing a stream of oxygen through the tube while it is kept at operating temperatures. The furnace is actually a 1-kw porcelain heater element commonly used in low-cost domestic room heaters, and is wrapped with asbestos tape and connected to the output of a variable transformer. Serum caps A and B allow for alternative injection points so that the apparatus can be used either as a simple gas chromatograph or as a combustion-gas chromatography instrument. The following are typical operating conditions Carrier gas: srgon Carrier gas pressure: 8 psig Flowrate: 27 cc min-' Column: 4-ft X '/,-in. stainless steel with Poravak Q Oven temperature: 1 3 5 T Furnace temperature: 700% Detector filament current: 160 mA Sample size: 1
The students were instructed to perform two experiments: Chromatography-through serum cap B, inject a sample containing a mixture of water, methanol, and ethanol, obtain a chromatogram and identify the peaks; and Analysis for C,H-through serum cap A, inject neat samples of benzene, mhexane, and a-xylene and determine the C:H ratio in n-hexane and a-xylene from the C02 and H20 areas, using the corresponding areas from benzene as a reference. Under the above operating conditions, the peaks are very well separated and tailing is noticeable only in the case of ethanol. The elution times for carbon dioxide, water, methanol, and ethanol are approximately 2, 5, 8.5, and 16.5 min, respectively. So far 36 students in groups of two each have completed this experiment and their precision is 550
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Journol of Chemical Education
A ond B are dternotive injection points for combustion-gas chromatography and for simple gor chromotogmphy, respectively. Schematic diogram of the opparolus.
Precision of C:H Ratio Obtained by 18 Groups of Students
o r Compound Average Benzene n-Hexane c-Xylene b
4 2 . 1
Range 0.8:7:0% 0.0-8.2%
-DeviationbAverage Range 3.2% 3.6% 3.3%
0.69.94% 0.0-9.07, 0.1-8.7%
Compared with theoreticalvalue. Compared with experimental mean value.
summarized in the table. These students have had no previous experience with gas chromatography and most of them have not used a microsyringe before. Each group spends about two hours doing the experiment and about another hour measuring peak areas with a planimeter and calculating the final results. These students have found the experiment interesting and instructive. Their greatest diiculties have been in injecting 1pl samples reproducibly and in the precise use of the planimeter. For experienced operators, the errors in the microanalysis can probably be reduced tenfold. This experiment is suitable for undergraduates a t second year level and higher, and is particularly relevant to an instrumental methods course. The physical arrangement of the apparatus allows the students to study the construction of a gas chromatograph and to appreciate the application of chemical and physical principles. Further development of this experiment would include the determination of oxygen contents by difference in a compound containing C, H, and 0, and the injection of solid samples using an injector such as the Hamilton4 model SS60. 'BARD,J. R., J. CHEM. EDUC., 46,441 (1969). a Waters Associates Inc., 61 Fountain Street, Framingham, Mass. 01701. Gow-Mac Instrument Co., 100 Kings Road, Madison, N. J. n794c
'Hamilton Co., P.O. Box 307, Whittier, Calif. 90608.
