Temperature and electrical resistance

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

A thermistor is made of a fused mixture of metal oxides. As the temperature of the thermistor increases, its electrical resistance decreases (as opposed to the bolometer). This relationship between temperature and electrical resistance allows thermistors to be used as IR detectors in the same way as bolometers. The thermistor typically changes resistance by about 5% per °C. Its response time is also slow. 

In general, a gas sensor can be defined as a device that informs about the composition of its ambient atmosphere (i.e., responds to the stimulus, Figure 22.1). More specifically, upon interaction with chemical species (adsorption, chemical reaction, and charge transfer), the physicochemical properties of the metal oxide sensitive layer (such as its mass, temperature, and electrical resistance) reversibly change. These changes are translated into an electrical signal such as frequency. 

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