Gas Chromatography with Thermal Conductivity Detector

The Thermal Conductivity Detector (TCD) is commonly used in Gas Chromatography because it detects almost all organic and inorganic compounds unlike FID. It is described as a universal detector.

The TCD consists of an electrically heated filament in a temperature-controlled cell and four bridged filaments supplied by a constant current. There is a stable flow of heat from the filament to the body of the detector when the carrier gas passes alone.
Two of the four filaments are swept by the carrier gas and the other two by a flow containing the sample. When adding a sample to this gas, a variation in the thermal conductivity of the sweep flow is observed. This causes a change in temperature and in the resistance of the filament.

This shift in resistance corresponds to the detector signal and is often detected by a Wheatstone bridge circuit.