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This detector is fairly selective but it has only a narrow range of applications. It is suitable for hydrocarbons as well as for sulfur or phosphorus derivatives. The operating principle consists in provoking ionization of the analytes by irradiation with a UV lamp emitting high‐energy photons (8.4–11.8 eV). Photo‐ionization occurs when the energy of the photon is greater than the first ionization energy of the compound (Figure 2.14). A photon of 9.6 eV can, for example, ionize benzene (PI₁ = 9.2 eV) but not isopropanol (PI₁ = 10.2 eV), which will therefore not be very visible on the chromatogram. Electrons released by an electrode connected to the terminal of an electrometer are collected for concentration measurements.

This detector can function at more than 400°C and is not destructive, as the ionization is reversible and affects only a small fraction of the molecules of each compound.