Gas Chromatography Detectors – Early Gas Chromatography Detectors > The Flame Thermocouple Detector > Page 24

The Katharometer Detector

The first alternative GC detector to be devised was the katharometer introduced by Ray [7] (now known more prosaically as the hot wire detector (HWD)). It consists of two heated filaments, situated in the arms of a Wheatstone bridge, one suspended in the eluent gas from the column and the other in a pure reference stream of gas. In the presence of a solute, both the thermal conductivity and the heat capacity of the gas change changing the heat loss and, thus, the temperature of the filament and, consequently, its resistance The bridge is unbalanced and the out-of-balance signal is passed to a suitable monitoring device. This detector is relatively insensitive but responds to all solutes that differ in heat capacity and thermal conductivity from those of the carrier gas. This detector was used extensively in the early days of GC for the analysis of hydrocarbon gases. There was much discourse and dissent with regards to the exact mechanism of detection involved in the katharometer and even today it is considered to respond to a number of different physical properties of the eluent gas with no one property playing a major role.

The Flame Thermocouple Detector

The “flame thermocouple detector” was the next detector to be reported which was developed by Scott [8] and was, in fact, the forerunner to the flame ionization detector FID. A diagram of the flame thermocouple is shown in figure 8.

Figure 8 The Flame Thermocouple Detector