During the inactive period of the waveform, electrons having thermal energy only will be come readily attached to any electron capturing molecules present in the cell and produce negatively charged ions. The negative ions quickly recombine with the positive ions and, thus, become unavailable for collection. Consequently the signal received will constitute a reduction in the standing current. The period of the pulsed potential is adjusted such that relatively few of the slow negatively charged molecules have time to reach the anode, but the faster moving electrons are all collected. During the "off period" the electrons reestablish 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 64.

Figure 64. The Electron Capture Detector

There are a large number of different detector designs but the basic electron capture detector consists of a small chamber one or two ml in volume with two metal electrodes. The electrodes may be formed by concentric cylinders or by metal discs separated by a suitable insulator. The cell contains the radioactive source, usually electrically connected to the conduit through which the carrier gas enters and to the negative side of the power supply. A gauze diffuser "diffuser" is sometimes connected to the exit of the cell and to the positive side of the power supply. The electrode current is measured by a suitable amplifier and the data acquired by an appropriate computer.

The electron capture detector is extremely sensitive, probably the most sensitive GC detector available (ca. 10^-13 g/ml) and is widely used in the detection and analysis of halogenated compounds, in particular, pesticides. It will function using either helium, argon or argon/methane mixtures as carrier gases.