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This detector is considered to be selective because it is much more sensitive to halocarbon groups. A flow of nitrogen gas that has been ionized by electrons generated from a low‐energy β⁻ radioactive source (a few mCi of ⁶³Ni) passes between two electrodes maintained at a potential differential of around 100 V (Figure 2.14). At rest, a base current I₀ is generated, mainly due to free and very mobile electrons. If molecules (M) containing a halogen (F, Cl, Br) cross the zone between the two electrodes, they capture thermally excited electrons to form heavy negative ions, which by consequence are much less mobile.

The measured intensity decreases exponentially by following a law of the type I = I₀ exp[−kc]. The linear range is about four orders of magnitude with nitrogen as the make‐up gas. The presence of a radioactive source in this detector means that it is subject to special regulations (inspection, location and maintenance visits). This detector is often used for analyses of chlorinated pesticides and polychlorinated biphenyls.

Figure 2.14 (a) Electron capture detector (ECD) and (b) photo‐ionization detector (PID). The PID contains a filter, used to select the photon energy in order for analytes to be the only ionized species apart from carrier gas molecules (LiF at 11.8 eV, or sapphire at 8.4 eV). The ionization mechanism is reversible.

Make‐up gas. To provide satisfactory response and increase their sensitivity, the above two detectors described above should be supplied with a gas flow of at least 20 ml/min, which is far greater than that in capillary columns. This flow rate is attained by mixing a make‐up gas with the gas coming out of the column outlet. This make‐up gas is either identical to the carrier gas or different from it (inexpensive nitrogen gas).