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A TCD detector consists of an electrically-heated wire or thermistor. The temperature of the sensing element depends on the thermal conductivity of the gas flowing around it. Changes in thermal conductivity, such as when organic molecules displace some of the carrier gas, cause a temperature rise in the element which is sensed as a change in resistance. The TCD is not as sensitive as other dectectors but it is non-specific and non-destructive.
Two pairs of TCDs are used in gas chromatographs. One pair is placed in the column effluent to detect the separated components as they leave the column, and another pair is placed before the injector or in a separate reference column. The resistances of the two sets of pairs are then arranged in a bridge circuit.
Schematic of a bridge circuit for TCD detection
The bridge circuit allows amplification of resistance changes due to analytes passing over the sample thermoconductors and does not amplify changes in resistance that both sets of detectors produce due to flow rate fluctuations, etc.
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