The electron capture detector (ECD) is based on the fact that the current strength decreases when compounds in the effluent flow capture electrons.
In the ECD, the effluent gas is exposed to alpha or beta rays from a radioactive source. When the compounds in the effluent capture the free electrons there will be a decrease in current and this is measured in contrast to the measurement of increase in current in other ionization detectors.
The ECD is a modification of the ionization chamber used for detection of radiations. The effluent from the chromatographic column is exposed to slow electrons generated by the ionization of the carrier gas (which is either argon or nitrogen) by a constant flux of beta rays from a radioisotope.
When an electron capturing molecule in the effluent gas captures an electron the result is the formation of a negative ion of a much higher mass and a consequent reduction of current flow.
The ECD is a very sensitive detector for certain compounds such as halogenated organic compounds, but is insensitive to compounds like hydrocarbons. The detector is very useful in the detection of ultra trace amounts of metals. It is also used for the analysis of insecticides and pesticides.