Gas Chromatography - GC Detectors > The Electron Capture Detector > Page 43

Figure 26. Wave form of Electron Capture Detector Pulses

In the inactive period of the wave form, electrons having thermal energy only will attached themselves readily to any electron capturing molecules present in the cell with the consequent production of negatively charged ions. The negative ions quickly recombine with the positive ions (produced simultaneously with the electrons by the b particles) and thus become unavailable for collection. Consequently the standing current measured during the potential pulse will be reduced.

The period of the pulsed potential is adjusted such that relatively few of the slow negatively charged molecules (molecules having captured electrons and not neutralized by collision with positive ions) have time to reach the anode, but the faster moving electrons are all collected. 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. By appropriate adjustment of these parameters 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. A diagram of an electron capture detector is shown in figure 27.


Figure 27 The Electron Capture Detector