A Simple-to-Build Thermal-Conductivity GC Detector Mark E. Jones Dow Chemical Company, Central Research and Development-Catalysis As a participant in the 1992 Dow Chemical Company Foundatioflational Science Teachers Association Workshop, I developed a n inexpensive, easy-to-build thermalconductivity detector for gas chromatography. When coupled with a n inexpensive packed column a n d c h a r t recorder, this device is suitable both for illustrating principles of chromatography and for qualitative identification of reaction products. Several examples of simple chromatographic detectors are presented in the literature (13).The thermal-condnctivity detector that I designed is easier to fabricate, and all parts are readily available. The prototype was built using parts obtained a t Radio Shack. The principal exploited is that the voltage drop across a standard silicon diode decreases i n a n approximately linear way with increasing temperature. The actual voltage drop for a silicon diode is
where 71 = 2: m = 1.5; K = 0.016; and V, = 1.21 V (4). The units [nf trmprrarurv and current an! Kclvin and ampcres. T h ~ rcduccs s lo a change ol'approxirnatel? -1.3 mV C . When a current of GeateI-than about 10 mA i s used, small signal diodes can be heated by the current flow, by so-called self-heating. Such a diode will reach some steadystate temperature determined by the thermal conductivity of the gas in contact with the diode. Helium has a coefficient of thermal conductivity t h a t is greater than most other gases. Aself-heating diode suspended i n a flow of belium will be cooler than an identical diode i n a flow of almost any other gas. Replacing the helium momentarily will increase the temperature of the diode, and the increased temperature can be detected by the change in the voltage drop across the diode. The use of the diode a s both heating element and detector is not the optimum arrangement for a precision analytical instrument. I t works well enough, however, for use in an instrument intended to illustrate the fundamentals
Laboratory, Midland, MI 48674
of gas chromatography. The components required for construction of the detector are commonly available and can be purchased for less than $10. Careful: When using a power supply, you must ground the low-voltage side to prevent damage t o the devices connected to the detector, such as a chart recorder.
Figure 1shows a schematic diagram of the entire detector circuit. An LM317 voltage regulator is configured as a constant source and delivers about 150 mA to a small signal diode (1N914). The circuit can be powered by three D-cell batteries, but the useful battery life is short. As shown in Figure 1, two voltages are supplied: V,,, and Vd. I have chosen to use a n inexpensive 9-V dc wall-transformer power supply for V,,,;. Two D-cell batteries supply a stable voltage source for the reference voltage. The signal is the difference between the measured voltage drop across t h e diode a n d t h e reference voltage. Currents used to maximize signal-to-noise are between 100 and 150 mA. The current output of the LM317 is determined by the value of the resistance of R 1 (R1) a s follows.
Figure 1. Schematicdiagram of the circuit for a single-diodethermalconductivity GC detector. R1 = 8 R for I,,, of 150 mA. For Vpower= 9 V dc, R2 = 30 0. With I,,,,,, = I50 mA, the voltage drop across the diode is approximately 0.9 V.
Volume 71
Number 11 November 1994
995
Area 17070 62159
Areas
21.541 78.451
Figure 2. Cross-sectional view of the diode detector. Special flux is required for using standard electronic solder with stainless steel.
0
R2 i s installed t o reduce t h e power dissipation of the LM317. The value of the resistance of R2 (R2) i s determined by
For a 150-mA current and a 9-V dc input, R i = 8 R and RZ = 30 R. If the circuit is to be supplied by three D-cell batteries, then the resistor R2 is not required. Figure 2 shows a cross section of a simple single-diode detector. The IN914 diode is soldered into a piece of 118-in. x 0.085-in. stainless steel tubing. Ideally, the diode should be suspended i n the tubing with minimum clearance on all sides. This configuration provides the best sensitivity by allowine heat to be removed onlv bv the flowing eas stream while mynimizing dead volume.-c&e must be FaLen during assembly to insure that the diode does not touch the metal tube. Due to the diode's sensitivity to air currents, the entire assemblv should be shielded from air movements during use. hed diode detector can also be mounted i n a compression T-fitting using polymer ferrules to produce a detector that does not require soldering.
996
Journal of Chemical Education
I
2
3
Time~~(min), Figure 3. Sample chromatogram showing the separation of oxygen and nitrogen in air using a 118-in. x 6-fl.801100-mesh molecularsieve 13X column. A sample chromatogram obtained using the diode detector with the circuit shown in Figure 1is shownin Figure 3. This chromatogram shows the separation of oxygenand nitrogen in air using a molecular sieve 13X column a t amhim; temperature.^ 18-jn. x 6-11, 801100-mesh commerciallr available column was used. An iniwtion uf 1 mL i,f air was made into a 118-in. swagelok-(a trademark of Crawford Fitting) T-fitting using a 114-in. septum. The chromatogram was recorded and integrated by a Hewletti Packard 3390A integrator. Literature Cited 1. Yarnitzky, C. N. J. Chem.Educ. 1990,67,712-713. 2. Ford. A,: Meloan. C. E. J Cham Edue. 1978.50.85-66. 3. Lowel, 3.;Malamud, H. il Chpm Ed
