A small radioactive source containing ³H or ⁶³Ni ionizes the molecules of the carrier gas (nitrogen or argon–methane), and a potential difference creates a small current. This current is reduced when an electronegative substance (such as a halocarbon) is introduced. The reduction in current is a measure of the concentration of the electronegative substance. The detection limit (threshold) varies greatly according to the substances to be analysed and can reach a mixing ratio of 10E-12. The linear dynamic range may be 10E4 but the maximum measuring value generally lies below 1 ppmv.