p~
Organic Laboratory Experiments A Simplified Method for Collecting Preparative Fractions from Gas Chromatographs Sherrel Smnh Texas A&M University, College Station. TX 77843
Preparative gas chromatography has become a popular method for ueoaration of small auantities of liquids in orpanOften these separations are carried ic laboratory out on Gow-Mac chromatographs and use the glass collecstandard equipment in the tion tubes that are included elassware kits for microscale organic laboratory techniques. fn order to use these collection tubes, the exit ports of the gas chromatograph (GC) must be threaded so that a special adapter may be screwed on to the exit port. This method of collection has several drawbacks. The collection tubes are breakable and relatively expensive (-$12.50 each), the conversion of the exit ports is expensive if done by Gow-Mac, and several of the adapters need to be purchased. The following article details how short pieces of Teflon tubing may be used for collection tubes. There are several advantages to this simple method of collection. The collection tubes are unbreakable and inexpensive. The exit ports of the gas chromatograph need no conversions. No adapters to mate the collection tubes to the exit ports need be purchased. The experiments reported here were carried out with a Gow-Mac model 150 chromatograph. This method of collection was also successfully performed with Gow-Mac 550 and Perkin-Elmer 8410 chromatographs. I t should be applicable to any GC that has %-in. 0.d. exit ports. Also an exit port heater is described that facilitates collection of samples from GC's in which the exit tube is not heated. The gas chromatography was performed with a Cow-Mac model 150. Astainlesssteelcolumn, 'I*in. X 8 ft, packed with 8% Superox 20 on Supelcoport 80-100mesh wasused for the separations. Injections were made with a Unimetrics 4025TP syringe. Four-inch lengths of spaghetti Teflon tubing of the size used t o connect Waters HPLC pumps to solvent filters was used for the collection tubes. This tubing is 0.149-in. 0.d. X 0.119-in. ID. This tubing may readily be slipped over %-in. exit ports but provides a tight fit. Approximately 50 pL of condensed liquid can be successfully collected in one of these tubes. Collected fractions were transferred to 0.1-mL vials (Ace Glass cat. #9590-02). The tubing containing the collected fraction was attached to the vial with a threaded cap and 0 ring. The assembly was then centrifuged for a few seconds to transfer the collected liquid to the vial.
'
Mayo. D. W.: Pike, R. M.; Butcher, S. S. Microscale Organic Laboratory, Wiley: New York. 1989. Pavia. D. L.; Lampman, G. M.; Kriz. G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques, A Microscale Approach: Saunders: Philadelphia, 1989. Mayo, D. W.; Pike. R. M.; Butcher. S. S. Mlcroscale Organic Laboratom Wiiey: New Yoh, 1989; p 48.
Figure 1. The center bore of the heater Is %-in.. enlarged to %,In. in the ares wherethe Teflon ring and collectiontube must beslippedonmeexit port of the
GC.
Flgure 2. A view of me assembled heater in place on the exil pon of me GC
A 1:l mixtureof heptanal and cyclohexanol was separated at an oven temDerature of 140 '(1 and at a flow rare of 28 cm3/s of helium. This is the mixture chosen by Maya et aL3 for their introductory experiment on preparative gas chromatography. Ten 25-pL, injections were performed and fractions collected as described above. The average recovery was 84.7% for heptanal and 97.5% for cyclohexanol. Other compounds used to test recovery efficiency of this method were acetone (44.0% recovery), 2-heptanone (97.6% recovery), and d-limonene (95.4% recovery). The %-in. exit port tubing of Gow-Mac models 150 and 550 is not heated. The temperature a t the end of the exit tubing of the model 150 was measured with a bead thermocouple and a digital thermometer. With the oven operating Volume 67
Number 7 July 1990
615
'
at 140 O C , the exit temperature was ahout 40 OC. At this low temperature many compounds will condense in the GC exit tubing. This leads t o poor recoveries and possible contamination of separated fractions. This problem is easily overcome by equipping the exit ports with beaters. Figure 1 shows a heater machined from a %-in. 0.d. aluminum rod. The heating element was 26-gauge nichrome wire with glass fiber insulation. One layer of the wire was tightly wrapped around the heater in the small depression. An additional winding of glassfiber was used as insulation from air currents. This heater was slipped over the exit port of the
616
Journal of Chemical Education
chromatograph. The heater is retained in place by a '/,;-in. piece of the Teflon tubing that is slipped on the exit port tuhingand pushed upagainst the hack wall of the ,r-in. well in the heater. The completed assemhlv is shown in Firmre 2. One of these heaters &as placed on kach exit port o f the chromatograph. When two of these heaters were operated in series at 14 V, the exit port temperature was appr&imately 132 O C with an oven temperature of 140 O C . With an oven temperature of 180 O C , the exit port temperature was 154 O C . With this setting (14 V), compounds were successfully collected with boiling points from 56 O C to 300 O C .
