About The Tcd - Agilent Technologies 7820A Advanced User's Manual

About the TCD

Advanced User Guide
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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Detectors
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