Gas Chromatography coupled with an Electron Capture Detector is a specific technique for analyzing highly electronegative compounds in particular halogens.
The ECD detector uses a sealed source of 63Ni which is radioactive (beta-ray) to ionize the carrier gas. The fast beta particles generated by the source collide with the molecules of the carrier gas. Free slowly-moving electrons are produced by impact ionization and generate a measurable and constant current.
The electron current initiated by it is produced between two electrodes. It is powered by a potential difference and amplified by the make-up gas. The advantage of nitrogen as a make-up is that it takes little energy to extract an electron from this gas. This amplified current is then accelerated to an anode, measured and recorded.
A decrease in this background current is observed when electronegative components leaving the column pass through the EDC as they capture these electrons. This change is recorded as a positive peak and corresponds to the response of the detector.