S. Lowell
and H. Malamud Radiation Research Cor~orotion Shames Drive Westbury, 1. I., N. Y.
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Thermal Conductivity Gas Analyzer
G a s chromatography has, in recent years, become a technique of increasing analytical importance. Gas chromatographs with high degrees of sophistication and at costs up to several thousands of dollars are commercially available. Some inexpensive gas chromatographs are being manufactured for student use and for research purposes where less sophistication is required. Even these less expensive chromatographs are often priced at a few hundred dollars, out of the reach of many who could put them to good use. Therefore, it seems desirable to make available to anyone interested the design of a gas chromatograph with the characteristics of simplicity of construction, minimum cost, good sensitivity, and ruggedness, and with pmvision for inexpensive, simple filament replacement. Instrument Construction
Filaments
Advantage is taken of the fact that most small light bulbs have tungsten filaments not very diierent from those used in commercial katharometers (thermal conductivity cells). The authors employed four No. 47 radio dial lamps, each with a cold resistance of 4.2 ohms. These four lamps were selected from a dozen bought in a local electronics parts supply store. The exact filament resistance values are not as important as finding a set of four matched for use in a balanced bridge. A Wheatstone bridge may be used for selection of the matched set. The bridge supply voltage must be low so that the current through the filaments undergoing measurement does not exceed &5 m a If higher currents are used the filament resistance will increase due to heating and inaccuracies will result.
660 / Journal of Chemical Education
Katharometer Construction
The bayonet lugs were filed from the lamp bases and then lead wires several inches long were soldered to the socket shells and the socket end contacts. The lamp bases were then mounted in No. 0 rubber stoppers having holes drilled to size. To make a clean hole the stoppers were immersed for a few minutes in liquid nitrogen and then drilled with a standard 5/16-in.twist drill. The stoppers were then cut to onehalf their length, the narrower half being used. The lamps were mounted into the stoppers with the bulb and filament projecting from the stopper's narrow end. They were cemented in place, for a gas-tight fit, using Eastman 910 cement. The glass was then broken away carefully, so as not to damage the filaments. The rubber stoppers were mounted in the ends of 6/s-in. ad copper tubing 11/2-in. in length to which copper tubing gas lines had already been hard
- >l-
Eortrnan
910 cement
Hord solder
Number 47 lorno
C % 8 "j Figure 1. Datoilr of kothorometer condruction. moved, are used ar Rlamantr.
Bulbs, with gloss re-
soldered. Aluminum foil was first cemented, using Eastman 910 cement, to the exposed surface of the rubber stoppers to reduce the area of contact with organic vapors. Care must be taken that the aluminum creates no short circuit between the lamp base and the 6/s-in. tubing katharometer body. A Teflon plug machined to shape would he an improvement over the stopper described here. Figure 1 shows the katharometer construction. Gas Flow Circuit
The katharometer was installed in the gas flow circuit as shown in Figure 2. The gas flow circuit Needle
Comer
ATTlNVATlON FACTOR
Saop bubble flow meter 0
-
DECIBELS
T 60 50 $0 U) 20
10
0
Figure 4. Bridge circuit diagrom. Anenvator resistors are "mode-up" of series-parollel combinations of standard resirtors, where precisian resistors are unavoiloble. Dete~tor
Figure 2.
Gos Row circuit.
column
Detector
Sofl copper tubing construction.
could be constructed external to the chassis for immersion in a constant temperature bath (or oven) hut then one must be certain to make the electrical wiring to the liatharometer water-tight (or heat resistant) to prevent short 'ircuiting. Figure 3, ( a ) and ( b ) , shows that only the rolumn is external to the chassis in the inst,rument described here, to provide for column tcmperat,ure control. All connections in the gas flow cireuit were made by soft soldering the joints, exrept the connections to the katharometers which were silver soldered. Swagelok fittings ('/,-in.) were used to connect the column and the needle valve to the gas flow circuit. Electrical Circuit
For student use the electrical circuit can be constructed on an open bread-board arrangement or it
can be conveniently mounted in an aluminum chassis. Figure 3, (a) and (b),shows a 'onvenient (minimum) size chassis of dimensions 12 X 8 X 3 in. A larger chassis might he helpful in promoting ease of construct,ion. Figure 4 is the schematic diagram of the circuit employed. The att,enuator was constructed logarithmically, to add a factor of 3.16 (10 decibels) for any step and a factor of 10 for every two steps. As many resistors as possible were purrhased as 1% precision units; several had to be assembled from series-parallel arrangements of two to four standard resistors, using a Wheatstone bridge for selection. A simpler and more conventional attenuation scheme could use attenuation factors of 1:10: 100: 1000. Summary and Results
Table 1 indicates all the components needed. The total cost, if all parts must be purchased, should not exceed $40. All the electrical equipment is standard and can be purchased from local or mail-order electronics supply stores. The copper tubing is also Table 1.
Part
Descri~tion
Switch Switch Pilot lamps Meter Rheostat Resistor Resistor Potentiometer
Table 2.
Compound
Renrene
10)
(left).
Top view of rhromatogroph. The externol column ( b ) (Rightl. Bottom view of chromatograph, rhowing convenient, though not
