it detects all molecules, the Thermal Conductivity Detector is commonly
used for fixed gas analysis (O2, N2,
CO, CO2, H2S,
NO, etc.) where the target analytes do not respond well on other, more
sensitive detectors. The TCD can detect concentrations from 100% down
to around 100 ppm on a flat baseline with sharp peaks. Where the peak
is broad or the baseline is not flat, detection limits of 300 ppm are
more realistic. For lower detection limits, the HID
detector may be more suitable for inorganics, and the FID
detector provides 1ppm detection for hydrocarbon species.
The TCD consists of
four tungsten-rhenium filaments in a Wheatstone bridge configuration.
Electric current flows through the filaments, causing them to heat up.
Carrier gas (usually helium, which has very high thermal conductivity)
flows across the filaments, removing heat at a constant rate. Two of the
filaments are exposed only to carrier gas (reference), and two are exposed
to the carrier/sample flow. When a sample molecule with lower thermal
conductivity than the carrier gas exits the column and flows across the
two sample filaments, the temperature of the filaments increases. This
temperature increase unbalances the Wheatstone bridge and generates a
peak as sample molecules transit through the detector.
A filament protection
circuit prevents filament damage by disabling the current if carrier pressure
is not detected by the GC, but cannot prevent filament damage in all circumstances.
The TCD is equipped with user-replaceable filaments in the event of a