Galvanic Applied Sciences AccuChrome MA2963, Operation Manual

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Revision 4 4 12/15/2018
2.2.3 Thermal Conductivity Detector
A thermal conductivity detector (TCD) is used to detect the amount of each individual
component as it elutes from the chromatograph columns (Figure 2-3). The TCD is housed
inside the chromatograph oven and is kept at a very stable temperature to minimize drift.
Figure 2-3: Thermal Conductivity Detector
The TCD consists of 2 thermistor beads housed in separate compartments, the Reference
Cell and the Measure Cell.
When there are no components eluting from the column(s), both of the thermistors are
exposed to carrier gas and will be at the same temperature and thus have the same
resistance. In this case the Wheatstone Bridge is balanced and the voltage from the circuit
will remain at zero. The outlet of the column is plumbed to the Measurement cell so that
when a compound elutes from the column it passes through the Measurement cell while the
Reference cell contains only carrier gas.
Heat from the Measure thermistor will be transferred away from the Thermistor by the gas
that passes through the Measure Cell. The amount of heat that is transferred will depend on
the amount of gas that is flowing past and that gas’s ability to conduct heat (its thermal
conductivity). The temperature of the Measure Thermistor, and thus its resistance, will
change. In this case the Wheatstone Bridge will become unbalanced and a voltage
deflection will be observed.