Gas Chromatography Detectors - Ionization Detectors > The Electron Capture Detector > Page 71


During the "off period" the electrons re-establish equilibrium with the gas. The three operating variables are the pulse duration, pulse frequency and pulse amplitude. The relationship between the number of electrons collected and the collecting time (the pulse width) is shown in figure 38. When no methane is present electron collection takes nearly 3 msec to complete. However, with 5% or 10% of methane present in the argon all the electrons are collected in less than 1 msec. This reflects the increased diffusion rates of the electrons in argon-methane mixtures. By appropriate adjustment of the pulse characteristics, the current can be made to reflect the relative mobilities of the different charged species in the cell and thus exercise some discrimination between different electron capturing materials. In general use, however, the pulse width is usually set at about 1 msec and the frequency of the pulses at about 1 kHz. Thisallowsabout 1 millisecondforthesensortore-establish equilibrium in the cell before the next electron collection occurs. The sampling and equilibrium periods are depicted in figure 39.



Figure 39 Common Pulse Pattern for the Electron Capture Detector Operated in the Pulsed Mode