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This universal detector, developed in the early days of GC for packed columns, is still in use now. Easy to build, it exists in a number of variants (Figure 2.13), including miniaturized forms (μ‐TCD) for capillary columns.

Its operating principle is based on the thermal conductivity of gas mixtures as a function of their composition. The main part of this detector is the katharometer, a thermostatted metal unit that is brought to a temperature slightly higher than that of the column and which includes thermistors located in tiny cavities. In the given example, the katharometer includes four thermistors, placed two‐by‐two and fed as indicated either with carrier gas sourced upstream from the injector or with the mobile phase downstream from the column. When a solute elutes, the conductivity of the mixture (carrier gas + compound) decreases with respect to that of the carrier gas alone. The thermal equilibrium is disrupted and this results in a variation in the resistance of one of the filaments, which is proportional to the concentration of the compound in the carrier gas. The dynamic range of this detector extends over some six orders of magnitude, and while its sensitivity is quite average (from ng to mg), it is being used more frequently thanks to the rise of micro‐GC (see Section 2.9.2).