Thermal conductivity detectors (TCD):
The diagram shows the cross sectional view of one of the temperature sensitive elements in a thermal conductivity detector. This detector is based upon the changes in the thermal conductivity of the gas stream. It is also called as katharometer. The sensing element in this device is an electrically heated element whose temperature at constant electrical power depends upon the thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold, tungsten, wire or semiconducting thermistor. The resistance of the wire measure of thermal conductivity of the gas.
The next diagram shows the arrangement of detector elements in a typical detector unit.
In the above system, two pairs of elements are employed. One pair is located in the flow of effluent from the column and the other pair is located in the gas stream ahead of the sample injection chamber (reference). The resistances of the pairs are compared by incorporating them into two arms of the Wheatstone bridge.
The thermal conductivities of He and H2 are roughly 6 times greater than those of most of the organic compounds. Thus in presence of even small amounts of organic compounds, a relatively high decrease in thermal conductivity of the column effluent takes place and thus the detector undergoes a rise in temperature.
Advantages of the thermal conductivity detector:
2) Large linear dynamic range
3) General response to organic and inorganic samples
4) Non destructive character
Limitations of the thermal conductivity detector:
1) Low sensitivity, which requires large sample size.
2) Thermal conductivity detector is costly.