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GCDetectors
TCD - Thermal ConductivityDetector
Because itdetectsallmolecules, theThermalConductivityDetector iscommonlyused for ixedgasanalysis
(O
2
,N
2
,CO,CO
2
,H
2
S,NO,NO
2
, etc.)where the target analytesdonot respondwell onother,moresensitive
detectors. The TCD can detect concentrations from 100% down to around 100ppm on a lat baseline
with sharp peaks. Where the peak is broad or the baseline is not perfectly lat, detection limits of 300ppm
are more realistic. For lower detection limits, the HIDmay be more suitable for inorganics, while the FID
provides 1ppmdetection for hydrocarbon species.
TheTCDconsistsof four tungsten-rhenium ilaments inaWheatstonebridgeconiguration. Electriccurrent
lows through the ilaments, causing them to heat up. Carrier gas (typically helium, which has very high
thermal conductivity) lows across the ilaments, removing heat at a constant rate. Twoof the ilaments are
exposed only to carrier gas (reference), and two are exposed to the carrier/sample low. When a sample
molecule with lower thermal conductivity than the carrier gas exits the column and lows across the two
sample ilaments, the temperature of the ilaments increases. This temperature increase unbalances the
Wheatstonebridge andgenerates apeak as samplemolecules transit through thedetector.
A ilament protection circuit prevents ilament damage by disabling the current if carrier pressure is not
detected by the GC, but cannot prevent ilament damage under all circumstances. The TCD is equipped
withuser-replaceable ilaments in the event of aburnout.
• “Universal”Detector
• Detects from 100%Down to200-500ppm
• EasilyReplaceableFilaments
• Thermostattedup to 275°C
8690-0007
TCDdetector
