Thermal conductivity detector

[1] This detector senses changes in the thermal conductivity of the column eluent and compares it to a reference flow of carrier gas.

When an analyte elutes and the thermal conductivity of the column effluent is reduced, the filament heats up and changes resistance.

The TCD is a good general purpose detector for initial investigations with an unknown sample compared to the FID that will react only to combustible compounds (Ex: hydrocarbons).

The TCD is also used in the analysis of permanent gases (argon, oxygen, nitrogen, carbon dioxide) because it responds to all these substances unlike the FID which cannot detect compounds which do not contain carbon-hydrogen bonds.

Considering detection limit, both TCD and FID reach low concentration levels (inferior to ppm or ppb).

This problem can be avoided if another reference gas is used, for example argon or nitrogen, although this will significantly reduce the detector's sensitivity towards any compounds other than hydrogen.

The results are slower to obtain compared to a mass spectrometer, but the device is inexpensive, and has good accuracy when the gases in question are known, and it is only the proportion that must be determined.