Detector Systems
Thermal conductivity detector (TCD)
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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 O
2
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 O
2
.
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,
H
forms 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.
