The Electron Capture Detector

The electron capture detector consists of a small volume cell which contains a low energy b-ray source, usually a Ni⁶³ source. The detector can be operated in two ways, either with a constant potential applied across the cell (the DC mode) or with a pulsed potential across the cell (the pulsed mode). In the DC mode, hydrogen or nitrogen can be used as the carrier gas and a small potential (usually only a few volts) is applied across the cell that is just sufficient to collect all the electrons available and provide a small standing current. If an electron-capturing molecule (for example a molecule containing an halogen atom which has only seven electrons in its outer shell) enters the cell, the electrons are captured by the molecule and the molecules become charged. The mobility of the captured electrons is much reduced compared with the free electrons and, thus, the electrode current falls dramatically. There are some disadvantages to the use of the DC mode of detection arising from the variation of electron energy with applied potential so the specific response of the detector to different molecules will depend on the applied potential

In the pulsed mode a mixture of 10% methane in argon is usually employed and the electron-capturing environment is quite different. The electrons generated by the radioactive source rapidly assume only thermal energy and, in the absence of a collecting potential, exist at the source surface in an annular region about 2 mm deep at room temperature and about 4 mm deep at 400 C. A short period square wave pulse is applied to the electrode collecting the electrons and producing a base current. The standing current, using 10% methane in argon is about 10^-8 amp with a noise level of about 5 x 10^-12 amp. The pulse waveform is shown in figure 63.

Figure 63. Waveform of Electron Capture Detector Pulses