Gas Chromatography Detectors - Some Less Common GC Detectors > The Thermionic Ionization Detector > Page 83
The sensor consisted of a vacuum tube containing a filament, grid and anode, very similar in form to the thermionic triode valve. The tube was operated under reduced pressure and an adjustable leak was arranged to feed a portion of the column eluent into the gauge. The sensor was fitted with its own pumping system and vacuum gauge and the usual necessary cold traps. Helium was used as a carrier gas and the grid collector–electrode was set at +18 V with respect to the cathode and the plate at -20 V to collect any positive ions that are formed. As the ionization potential of helium is 24.5 volts, the electrons would not have sufficient energy to ionize the helium gas. However, most organic compounds have ionization voltages lying between 9.5 and 11.5 V and consequently would be ionized by the 18 V electrons and provide a plate current. The plate current was measured by an impedance converter in much the same way as the FID ionization current. The detection limit was reported to be 5 x 10-11 moles, but unfortunately the actual sensitivity in terms of g/ml is not known and is difficult to estimate. The sensitivity is likely to be fairly high, probably approaching that of the FID. The response of the detector is proportional to the pressure of the gas in the sensor from about 0.02 mm to 1.5 mm of mercury. In this region of pressure it was claimed that the response of the detector was linear (31). Hinkle et al. (32) who also examined the performance of the detector, suggested the sensor must be operated under conditions of molecular flow i.e. where the mean freepathofthe molecules is about the same as the electrode separation. Very pure helium was necessary to ensure a low noise and base signal. The detector had a "fast" response but its main disadvantage was the need to operate at very low pressures so that it required a vacuum pump; furthermore, forstability, thesensorpressureneeded to be very precisely controlled.