Thermal conductivity detectors work on the principle of heat transfer by convection (gas cooling). Here, the assumption is that sample compounds will have diﬀerent thermal properties than the carrier gas. Recall the dependence of a thermal mass ﬂowmeter’s calibration on the speciﬁc heat value of the gas being measured. This dependence upon speciﬁc heat meant that we needed to know the speciﬁc heat value of the gas whose ﬂow we intend to measure, or else the ﬂowmeter’s calibration would be in jeopardy.
Here, in the context of chromatograph detectors, we exploit the impact speciﬁc heat value has on thermal convection, using this principle to detect compositional change for a constant-ﬂow gas rate. The temperature change of a heated RTD or thermistor caused by exposure to a gas mixture with changing speciﬁc heat value indicates when a new sample species exits the chromatograph column.
A simpliﬁed diagram of a TCD is shown here, with pure carrier gas cooling two of the selfheated thermal sensors and sample gas (mixed with carrier gas, coming oﬀ the end of the column) cooling the other two self-heated sensors. Diﬀerences in thermal conductivity between gas exiting the column versus pure carrier gas will cause the bridge circuit to unbalance, generating a voltage signal at the output of the operational ampliﬁer circuit:
This type of chromatograph detector works best, of course, when the carrier gas has a signiﬁcantly diﬀerent speciﬁc heat value than any of the sample compounds. For this reason, hydrogen or helium (both gases having very high speciﬁc heat values compared to other gases) are the preferred carrier gases for chromatographs using thermal conductivity detectors.
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