Gas Chromatography - GC Detectors > The Katherometer Detector > Page 46

The katherometer may have an "in-line" sensor where the column eluent passes directly over the filament or an "off-line" sensor where the filaments are situated out of the main carrier gas stream and the gases or vapors reach the sensing element by diffusion. Due to the high diffusivity of vapors in gases, diffusion can be considered as almost instantaneous. The katherometer detector is very flow and pressure sensitive and the sensor must be carefully thermostatted and fitted with reference cells to compensate for changes in pressure or flow rate. The filaments of the reference and measuring cell are made to form two of the arms of a Wheatstone bridge and the out-of-balance signal amplified and fed to a recorder or computer data acquisition system.

Courtesy of the Supelco Inc.


Figure 30. The Separation of the Compounds of Hydrogen, Deuterium and Tritium

The maximum sensitivity will be realized if hydrogen is used as the carrier gas, but, to reduce fire hazards, helium is preferred and can be used with very little compromise in sensitivity. The katherometer sensitivity is about 10-6 g/ml with a linear dynamic range of about 500. Although the least glamorous, this detector can be used in most GC analyses that utilize packed columns and where there is no limitation in sample availability. The device is simple, reliable, and rugged and is particularly useful for those with limited experience in GC. It is also often the detector of choice for process monitoring. An example of the separation of the various compounds of hydrogen, deuterium and tritium, employing gas solid chromatography and using a katherometer detector is shown in figure 30. The stationary phase was activated alumina (treated with Fe(OH)2), and the column was 3 m long and 4 mm I.D. The carrier gas was neon, the flow rate 200 ml/min. (at atmospheric pressure) and the column temperature was -196oC.

The four detectors described are well established. reliable and generally simple to operate. They are also, probably the most popular. The FID, ECD, NPD and the katherometer are employed in over 90% of all GC applications. The FID is the most versatile, sensitive and linear, and probably the most generally useful. For details of other GC detectors see Gas Chromatography Detectors .