Detector Systems
Thermal conductivity detector (TCD)
The thermal conductivity detector (TCD) detects the difference in thermal conductivity between column effluent flow (carrier gas + sample components) and a reference flow of carrier gas alone; it produces voltage proportional to this difference. The voltage then becomes the output signal to the connected chart recording or integrating device.
The TCD uses a single filament to examine alternately relative thermal conductivities of reference versus column effluent gas streams every
200 msec. At this frequency, the detector is insensitive to thermal drift.
Factors which influence TCD response include the following:
Temperature difference between the filament versus the surrounding detector block
Flow rate ratio between carrier effluent versus reference gas streams
Type of carrier/reference gas used
Caution The TCD filament can be permanently damaged if gas flow through the detector is interrupted while the filament is operating. Make sure the detector is off whenever changes/adjustments are made affecting gas flows through the detector.
Likewise, exposure to O2 can permanently damage the filament. Make sure the entire flow system associated with the TCD is leak•freeand that carrier/reference gas sources are uncontaminated before turning on the detector. Do not use Teflon tubing, either as column material or as gas supply lines, because it is permeable to O2.
Since the TCD responds to any compound whose thermal conductivity is different from that of the carrier gas, H and He are most commonly used as carrier gases, with H giving somewhat greater sensitivity. However,
Hforms explosive mixtures with air (O ), and some components, particularly unsaturated compounds, may react with H . He produces almost as much sensitivity as H and is free from problems of reactivity with sample components or the filament. N or Ar may be used but give
lower response for most materials; however, they are useful if H or He is being analyzed.
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