F. Sicilio', H. Bull, Ill, R. C. Palmer, and J. A. Knight Georgia Institute of Technology Atlanta
An hexpensive Versatile Multi-Column Gas Chromatograph for Students
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gas chromatography nnit has been constructed in this laboratory from materials which are readily available with the exception of some of the electronic components and a matched pair of thermistors. A senior undergraduate student assembled this nnit within one quarter as a special problem. The approximate cost of materials is $350, excluding recorder; this price can be reduced substantially by using materials normally on hand in a laboratory. Other home-made gas chromatographs have been described (1-4); the unit described herein incorporates features of added versatility and niggedness. Experiments and specific applications in gas chromatography have been described (1, 5). 'Present address: A & M College of Texas, College Station, Texns.
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The unit and its accessories, with the exception of recorder and carrier gas supply, are mounted on a plywood platform provided with casters for portability (Fig. 1). As an alternative, the assembly could be compacted for use on a desk top. In this unit, two parallel columns are employed and are housed in separate ovens. Each column with its sample injectionflash evaporator unit is connected to one side of the thermal conductivity cell. Either column can serve as the preheater for the reference stream or as the chromatographic column. In essence, there are two parallel flows through identical systems, each of which consists of three major sections: sample injection-flash evaporator system, the chromatographic column and heater, and the sensing system. This arrangement enables the two columns to be packed with different materials, which permits differeut types of analyses to be accomplished without necessitating a change of columns or temperatwe. I t is believed that the expense of the additional column is warranted because of the added versatility; however, the second column could be deleted in the interest of economy. A flow diagram for normal operation is shown in Figure 2. An extension of the two-column scheme is the preparation of a bank of column-pairs, each column-pair having a thermal conductivity cell. Each column-pair and cell could be added to the unit a t small expense, and this creates essentially a new chromatographic unit with each column-pair. The same circuitry and recorder could be used then for any column-pair by use of a multiplepole gang switch. This would make available to an analyst many different types of columns a t any desired temperature, and all column-pairs could be equilibrated simultaneously. Other points of versatility are that columns of any practical length can easily be placed in the unit and the entire unit can be readily and conveniently disassembled. Also, the heating elements for the column ovens, thermistor block, and flash evaporators are controlled by individual variable auto transformers; this allows the use of many combinations of temperatures for instructional variety.
R Figure 1. View of complete gar chromatographic unit with the exception of recorder. (A1 Sample injection-florh evaporator unitr, (81 Column ovens, IC) Thermistor oven. ID) Control panel, IE)
Soop-film Row meter, IF1 Flow regulators, and IG) Vorioble ovto tronrfarmerr.
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COLUMN A
CHROLULTOGRAPHIC COLUMNS
Figure 2 Irightl. Flow diagram for twocolumn unit. The sample can be inieded into either sheam. The other streom acts
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COLUMN B
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Figure 3. Brorr flash evoporotor unit, ride reclion. (A1 Well for thermometer, (01 Well for rubber septum, and (C) Well for cartridge heater.
The sample injection-flash evaporator unit is a brass block with a chamber and a side port through which the sample is introduced into the stream of carrier gas. The unit is heated by a cartridge heater and is insulated with asbestos paste and Flexboard. The unit is schematically shown in Figure 3. A sample can be introduced, with a syringe and hypodermic needle, through the perforated screw and rubber septum (Perkin-Elmer 154-1043). The sample injection unit also vaporizes liquid samples upon introduction to the chamber. The temperature of the chamber is measured by a thermometer placed in a well in the side of the brass block. The sample injection-flash evaporator unit is attached directly to the bottom of the column oven so as to eliminate cooling of the system a t this point. The Swagelok fitting on the top of the injection chamber protrudes through the bottom of the column oven and is connected directly to one end of the column. The coiled column is heated by an oven consisting of a 12-in. length of 4-in.-diameter steel pipe, nicbrome heating element, and insulation. The steel pipe is first wrapped with asbestos paper to insulate the nichrome heating element. The outer asbestos insulation is wrapped in cheesecloth and is painted with a suitable coating (Sauereisen cement is satisfactory). The top and bottom of the oven are square pieces of Flexboard which act both as the structural body and as insulation. The temperature of the oven is controlled by manual setting of a variable auto transformer and is monitored with a thermometer, either partial immersion glass or bimetallic dial type. The temperature of the column itself is assumed to be approximately that of the oven, a t reasonably low flow rates. Maximum usable temperatures, to the exclusion of column packing material, are in the order of 450% The circuit diagram of the bridge is given in Figure 4. A matched pair of thermistors is used in combination with two 400-ohm resistors to form the basic bridge circuitry. Coarse and fine adjustments (10turn precision potentiometers) are added to allow compensation for differences in flow rate, temperature, e t ~ . The output signal is developed a t the junction of the "sensing" thermistor and the resistor that forms that leg of the bridge. The junction of the other leg serves as a reference potential. Since the output signal of large samples is too great for a millivolt recorder an attenuator of precision resistors (Perkin-Elmer 1540228) is used to decrease the output to the recorder by factors of two. The polarity of the signal reverses when changing from one thermistor to the other; therefore, a reversing switch is needed to provide upscale deflection when samples are injected into alternate col-
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Figure 4.
Circuit for bridge.
umns. Power is supplied to the bridge by a 9-v battery (six 11/*v cells in series). A 500-ohm variable resistor is used in series with the battery to allow adjustment of the applied potential of 8 v, which is monitored by a 0-10 v voltmeter. The bridge housing is easily removed for replacement of batteries. A null balance or potentiometric voltage recorder (5 millivolt or less), such as the Sargent SR or Leeds and Xorthrup Speedomax G, is 4rutilized in conjunction Figuri 5. Steel thermirtor block, top view. with the bridge. The two thermistors (matched, Fisher Scientific, SPM 11-130-1-125) are housed in a steel block (Fig. 5). Each of the two streams of gases from the columns pass through the block by means of two parallel horizontal holes drilled lengthwise in the lower portion of the block. Vertical holes are also drilled through the top of the block into the center of each of the horizontal holes in the lower portion of the block. A shallow seat is also provided in the top of the two vertical holes. The thermistors, with their supports and electrical connections, are placed in these vertical holes. The thermistor support is a cylindrical piece of metal with the lower portion having the diameter of the vertical holes in the block. I t is able to protrude slightly into the lower hole; the upper portion of the support has the same diameter as the seat. This upper portion of the thermistor support is made to bear on an "0" ring that is placed in the seat of the vertical hole to insure no leakage. The entrance and exit tubing of the thermistor block are made similar so that either end can be connected to the top of the columns. Thermal conducVolume 38, Number 10, Ocfober 196 1
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tivitydetectors are available for$60-$loofrom firms such as Gow-Mac Instrument Company, Madison, N. J., or Victory Engineering Corporation, Union, N. J. The oven for the thermistor block also employs the body-insulation of Flexboard as used in the column ovens. The oven is lined on four sides with nichrome mire which serves as the heating element. The outside terminals for the heating element of the oven are covered by a metallic guard to prevent electrical shock. A soap-film flow meter is employed to determine the flow of the streams of gases through the two sides. The flow meter consists of a buret connected to one end of the run of a glass tee. A squeeze bulb, partially filled with soapy water, is attached to the other end of the run. The stream of gases, whose flow rate is to be determined, enters through the branch of the tee. The
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rise of a bubble in the buret is timed and the flow rate is calculated. With proper connections, the flow rate of either column can be measured. Regulation of flow is accomplished by use of pressure regulators (0-35 psi, Fisher 67581). The authors acknowledge the assistance of Mr. D.
K. Davis. Literature Cited (1) BRENNAN, D., AND KEMBALL, C., J. CHEM.EUUC.,33, 490
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(2) SULLIVAN, L. J., LOTZ,J. R., A X D WILLINGHAM, C. B., Anal. Chem., 28,495 (1956). (3) DIMBAT, M., PORTER, P. E., AND STROSS, F. H., Anal. Chem., 28,290 (1956). (4) G o n r . ~R. ~ , S., Anal. Chem., 29, 1723 (1957). (5) GREENE, S. A,, J. CHEM.EDUC.~ 34, 194 (1957).
