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TCD - SRI Instruments
TCD - Thermal Conductivity Detector
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Universal detector
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Detects from 100% down to 200-500 ppm
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Easily replaceable filaments
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Thermostatted up to 275° C
Our TCD is equipped with user-replaceable filaments, so it can be quickly
returned to service in the event of a burn-out.
Because it detects all molecules, the Thermal Conductivity Detector is commonly used for fixed gas analysis (O
2
, N
2
,
CO, CO
2
, H
2
S, 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
detector may be more suitable for inorganics, and the
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 burn-out.
This chromatogram
shows 0.5mL 10,000 ppm
(1%) CO
2
separated from