Gas Chromatography Detectors - Ionization Detectors > The Electron Capture Detector > Page 70

The detector can function in two ways: either a constant potential is applied across the sensor electrodes (the DC mode) or a pulsed potential is used (the pulsed mode). In the DC mode, a constant electrode potential of a few volts is employed that is just sufficient to collect all the electrons that are produced and provide a small standing current. If an electron capturing molecule (for example a molecule containing a halogen atom which has only seven electrons in its outer shell) enters the sensor, the electrons are captured by the molecules and the molecules become charged. The mobility of the captured electrons are much reduced compared with the free electrons and, furthermore, are more likely to be neutralized by collision with any positive ions that are also generated. As a consequence, the electrode current falls dramatically.

In the pulsed mode of operation, a mixture of methane in argon is employed as the carrier gas. Pure argon can not be used very effectively as the carrier gas as the diffusion rate of electrons in argon is ten times less than that in a 10% methane-90% argon mixture. The period of the pulsed potential is adjusted such that relatively few of the slow negatively charged molecules reach the anode, but the faster moving electrons are all collected.


Figure 38. Curves Relating Electron Collection to Pulse Width for Carrier Gases Containing Different Amounts of Methane