R. E. Herbenerl Hillsdale College Hillsdale, Michigan
A Demonstration Device for Gas Chromatography
This paper describes the construction and use of a simple gas chromatograph. The instrument has removable covers for the purpose of class demonstration. The use of a "Tide" column to separate mixtures of volatile organic liquids has been described in a demonstration using hydrogen flame photometry as the d e t e c t ~ r . ~The same type of column is used here, helium is the carrier gas, and an uncomplicated thermal conductivity detecting- device is employed. The hinged heating jacket was constructed from asbestos steam pipe covering doweled together end-toend to build up a desired length. Number 18 Nichrome wire serves as the heating element and plywood discs support the column. The overall dimensions of the jacket used here are 4 in. od and 93 in. length. The packed column as well as all other glass tubing is G mm Pyrex. All joints are connected with Tygon tubing. The column packing was Tide, 18-20 and 20-40 mesh. A convenient source of agitation during column packing is a vibrator-type sander. Care should be taken to avoid the excess vibration which breaks off minute particles of the material to clog the column.
Figure 1. Cut-away top view: A, Carrier gar in; 8, Arberfor jacket; C, Arbertos board box containing detectorr D and G; E, Sample in; F, Pocked column; H, Exit to flow meorurement.
A lecture bottle of helium gas will provide for several hours of operation. In attempting t o devise a gas flow splitter, some diiculty was encountered in obtaining equal flow rates in both branches of the system. To eliminate this, a third tube was added to the system (Fig. 1). The helium flows from the tank through the heating jacket to the first detector block (RI), thence back through the jacket so that its elevated temperature is maintained before it picks up the sample and goes back t o the second detector After leaving the detector, it passes through a three-way stopcock to a bent glass tube immersed in mineral oil where the rate of bubbling gives a rough indication of flow rate during operation. The other branch of the three-way stopcock leads to a simple flow-metering device consisting of a buret and leveling bulb containing water. With the aid of a stopwatch it is possible to obtain duplication of flow rates with some degree of precision. The ground glass plug was removed from a three-way T-form stopcock, and the ends of this opening were
(a).
Andrew Zeiler was the student who participated in the experimental work. COWAN, P. J., AND SUGIHARA, J. M., J. CHEM.EDUC., 36,246 (1958).
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Journal of Chemical Education
sealed with short rubber stoppers. I n this way it could be held firmly in place with a single buret clamp on a ring stand. A small rubber septum from a serum bottle fits snugly enough into the branching tube with sufficient friction to withstand the internal pressure of the gas system. After joining t o the column and to the helium return, this stopcock was wrapped with heating wire and wired in series with the column heater. A number 705 Microliter syringe fitted with a number 26- X '/An. needle was used to inject samples.
Figure 2. Deleding System. R, and Rn, Hot wire rerirtorr (model oirplone glow plugJ; S, Switch; R, Slide wire rheo3t.t I 2 0 Cll; 8, dc source (0-3"). R, ir in rtreom of helium alone. Re is in stream of helium pivr sample.
The thermal conductivity sensors employed in this system are two glow-plugs of the type intended for use in model airplane motors. These are threaded into the sides of hollow brass cylinders 1 in. diameter by 3 in. long. They act as Rl and Fh in the Wheatstone bridge shown in Fig. 2. The bridge is initially balanced with a flow of helium only through both detectors. With the passage of the components of the sample, the variation in heat conductivity of the gas passing over the heated element of the glow-plug causes a change in its resistance and a resulting unbalance in the bridge. This is noted by the time and magnitude in deflection of the galvanometer. Calibration with known samples is all that is necessary to standardize the instrument. It was found that the detecting system was sensitive to temperature changes from air currents. For this reasou it was built into an insulating box, 6 in. on each edge, with a separating partition. Each block is surrounded by a loose packing of glass wool. The top of this box can be readily removed for demonstration purposes. The demonstrations involve the obtaining of calibration data using samules of single - components and separation of mixtures of these same components. When the column temperature was 104'C, and the flow rate 82 ml/min, a 1: 1:1 mixture of ether, acetone, and chloroform separated into well defined "peaks." These appeared a t 24, 29, and 38 sec respectively, times in accord with the behavior of the individual pure components. It is also possible to demonstrate clearly that size of sample has a pronounced effect on the magnitude of the galvanometer deflection, but little effect on the retention time. ~
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