Simple Classroom Demonstration of Gas Chromatography The gas chromatograph (GC) is one of the most useful tools available to chemists. It is widely used for separating and analyzing small quantities of sample, even quite complex mixtures. In discussing the GC with a class, we have often taken a small basic GC unit into the classroom, or used a set of slides, to acquaint students with how the instrument looks and works. But like many other lahoratory instruments, a GC is a box covered with knobs and dials and connected to a recording pen that sketcher out patterns on a sheet of graph paper. A simple, inexpensive model showing how the GC works can be constructed easily from common laboratow materials. It can serve as a prototype fnr explaining the principle of GC separation to a class. The GCcan be broken dawn into 3 major part?: the injection site, the separationzone, and the detector. (1) The Detector. The Beilstein test (a halogen containing compound on a copper wire produces a bluish green flame test), using a modified Bunsen burner, provides the basis for the detector.The burner hasacopper wire (about 18gauge)wrapped around its barrel, with the end of the wire extending a h w e the reducing region into the top portion of the flame's cone (2). Thr Separation Zone. This consists of a glass tube packed with a granular detergent (such as T i d e 9 Theglass body of a 25-ml buret filled withdetergent and plugged a t each end with small pieces of glass wool works well for this purpose. One end of the horizontal detergent-packed buret is attached by a foot or so of rubber tubing to a natural gas jet. The other end is directly attached with about 1in. of rubber tubing ta the copper-wired burner. (3) Thc Injection Site.Thisean be a t any point near the buret between the natural gas jet and the detergent-packed buret. The samples, which are volatile, halogen-containing compounds, are injected directly into the natural gas stream through the rubber tubing with a GC syringe. To operate the "instrument," the gas is turned on, the burner is lit, and the copper wire is adjusted to give a dean burning flame. Then a small sample (1-5 pl) of a volatile haloorganic compound such as CHzClr, C H C h CCL, CH2CICHcCI,CeHsCI, etc. is injected through the rubber tubing just hefore the packed glass column. The elapsed time, usually in the fiO to 120sec range, between the time of injection of the sample and when the flame hecomes green is noted. In 60-120 sec, depending on the size of sample injected, the flame will return to it? natural blue color. After approximately 2 mi", another sample can heinjected and the time required for theappearanceof the green flame can be compared with the first sample. Duplicate injections of the same compound will give elapsed or elution times that agree to within f-10 sec. If the halogen-a>ntainingliquidis a mixtureof2orm~~recomp~~unds, it is likely that more than one peak will beobfierved when the flame shows a green coloration. For example, the simple apparatus described here is capable of resolving the cis-trans isomers of 1,2-diehloroethylene. This particular separation not only demonstrates how s GC works but also illustrates how polarity difference in molecules can be related to their rate of movement throughaseparation column. Another easily separated system is methylene diehloride and chloroform. This simole . orototvoe . .. of a GC is auite sensitive for the detection of haloeen-containine..omanic .. comoounds as is illustrated by the fact that ~n)ectionofa water sample wh~chhas been shaken with clhndcmn ~ i v e i i ap w I t v r tc\t fur 84 halogen compound. This allows one to rslk ahout the presence of haloorganm in our watrr s y s l w n ~alld I h w dertrI i i n hy ( X : . Ifdeswed, the inzrrument can be made portahle hy weof a prupane twch as I h,.uwnc 111 rnrrlcr ~ : a < . T h ehtyher pressures available from the propane cylinders will shorten the retention times. Max A. Taylor Kenneth E. Kolb Clark E. Bricker Univwsity of Kansas Bradley University Peoria. IL 61625 Lawrence. KA 66045
Volume 58. Number 1, January 198 1 I 41
