174 Advanced Operation Manual
5 Detectors
About the TCD
The TCD compares the thermal conductivities of two gas
flows—pure carrier gas (the reference gas) and carrier gas
plus sample components (the column effluent).
This detector contains a filament that is heated electrically
so that it is hotter than the detector body. The filament
temperature is held constant while alternate streams of
reference gas and column effluent pass over it. When a
sample component appears in the effluent, the power
required to keep the filament temperature constant changes.
The two gas streams are switched over the filament five
times per second (hence the ticking sound) and the power
differences are measured and recorded.
When helium (or hydrogen) is used as carrier gas, the
sample causes the thermal conductivity to fall. If nitrogen is
used, the thermal conductivity usually goes up because most
things are more conductive than nitrogen.
Because the TCD does not destroy the sample during the
detection process, this detector can be connected in series to
a flame ionization detector or other detector.
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