Gas Chromatography Detectors - The Katharometer Detector > Page 33
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).
Courtesy of Supelco Inc.