Gas Chromatography - GC Detectors > The Electron Capture Detector > Page 42


The Electron Capture Detector

The electron capture detector contains a low energy b-ray source which is used to produce electrons for capturing by appropriate atoms. Although tritium adsorbed into a silver foil has been used as the b particle source, it is relatively unstable at high temperatures, the Ni63 source was found to be preferable. The detector can be used in two modes, 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 smaller than the free electrons and the electrode current falls dramatically. The DC mode of detection, however, has some distinct disadvantages. The most serious objection is that the electron energy varies with the applied potential. The electron capturing properties of a molecule varies with the electron energy, so the specific response of the detector will depend on the applied potential

Operating in the pulsed mode, a mixture of 10% methane in argon is employed which changes the nature of the electron capturing environment. 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 wave form is shown in figure 26.