TCD DETECTOR

The SRI TCD detector consists of four tungsten-rhenium filaments in a Wheatstone bridge configuration. Electric current flows through the four filaments causing them to heat up. Carrier gas ( typically helium which has very high thermal conductivity ) flows across the filaments removing heat at a constant rate. When a sample molecule with lower thermal conductivity exits the column and flows across the two sample filaments, the temperature of the filaments increases unbalancing the Wheatstone bridge and generating a peak as the sample molecules transit through the detector.

The TCD detector is useful because it detects all molecules, not just hydrocarbons, so it is commonly used for fixed gas analysis ( O2, N2, CO, CO2, H2S, NO, NO2, etc. ) where the target analytes do not respond well on other more sensitive detectors. The TCD is able to detect concentrations from 100% down to about 100 ppm, but not lower. Even 100 ppm is only possible where the chromatography permits a sharp peak on a flat baseline. Where the peak is broad or the baseline is not perfectly flat, detection limits of 300 ppm are more realistic. For lower detection limits, the HID detector may be more suitable for inorganics, and of course the FID provides 1 ppm detection for hydrocarbon species.

The standard TCD detector may be thermostatically controlled up to a maximum temperature of 160ºC, which is suitable for most applications. A high temperature TCD detector is also available which can be operated up to 300ºC. Both the standard and high temperature TCDs use identical easily replaceable filaments which allow user replacement in the event of a filament burn-out. A filament protection circuit prevents filament damage by disabling the current if carrier gas pressure is not detected at the GC, but cannot prevent filament damage under all circumstances.

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