A filament carrying a current is situated in a tubular cavity through which flows the column eluent. Under equilibrium conditions, the heat generated in the filament is equal to the heat lost and consequently the filament assumes a constant temperature. The heat lost from the filament will depend on both the thermal conductivity of the gas and its specific heat. Both these parameters will change in the presence of a different gas or solute vapor and as a result the temperature of the filament changes, causing a change in potential across the filament. This potential change is amplified and either fed to a suitablerecorder or passed to an appropriate data acquisition system.As the detector filament is in thermal equilibrium with its surroundings and the device actually responds to the heat lost from the filament, the detector is extremely flow and pressure sensitive. Consequently, all katharometer detectors must be carefully thermostatted and must be fitted with reference cells to help compensate for changes in pressure or flow rate.
The Katharometer Detector
Figure 13. The Off-Line Katharometer Sensor
There are two types of sensor design, the "in-line" sensor where the column eluent actually passes directly over the filament (as shown in figure 12) and the "off-line" cell where the filaments are situated away from the main carrier gas stream and the gases or vapors only reach the sensing element by diffusion.(as shown in figure 13). Due to the high diffusivity of vapors in gases, the diffusion process can be considered as almost instantaneous. The filament wire is usually made from tungsten or platinum as both metals have high temperature coefficients of resistance and at the same time are relatively inert. The column and reference filaments are situated in the arms of a Wheatstone Bridge and a suitable current is passed through the filaments to heat them significantly above ambient temperature. To ensure temperature stability, the sensors and their conduits are installed in a high thermal conductivity metal block which is thermostatted by means of a separate oven. The performance of the in-line sensor is almost identical to that of the off-line sensor.
For maximum sensitivity hydrogen or helium is used as the carrier gas. The katharometer sensitivity is only about 10-6 g/ml (probably the least sensitive of all GC detectors) and has a linear dynamic range of about 500 (the response index being between 0.98 and 1.02)
Figure 14. The Separation of the Compounds of Hydrogen, Deuterium and Tritium
