Conductivity detectors for GC can only be used for those species which can be converted to a strong acid or base after an appropriate post column reaction. This is normally carried out by pyrolysis or catalytic oxidation/reduction in a low volume flow-through furnace which also receives a supply of oxygen or hydrogen, depending on the mode. The catalyst is usually a nickel wire at 500-1000 °C, and organic compounds are subsequently converted to small molecular weight fragments:
 
oxidation:
            halogen (X)        -->        HX
            nitrogen (N)       -->        NO₂ (low yield)
            sulphur (S)         -->        SO₂/SO₃
reduction:
            halogen (X)        -->        HX
            nitrogen            -->        NH₃
            sulphur              -->        H₂S 
In the ELCD the reaction products are swept from the reactor into a specially designed cell incorporating a gas-liquid contactor, where they are mixed with a solvent such as water or a low molecular weight alcohol; a separator where the liquid phase is separated from the insoluble gases and an electrical conductivity cell. The solvent is supplied in a recycling arrangement and the electrical conductivity is measured differentially, i.e. the difference between the conductivity of the pure solvent and the solvent +reactants is monitored.

The detector is capable of high sensitivity towards halogens (5x10^-13g Cl/s), nitrogen (10^-12 g N/s) and sulphur (10^-12 g S/s). It also has a wide linear range, in the region 10³-10⁵. The ELCD is used in a number of EPA methods for the analysis of volatiles in (waste) water, such as chlorinated solvents or pesticides.