In the earlier form of the detector the column eluent entered a high vacuum chamber through a hollow anode. The chamber was provided with two electrodes at a sufficiently high potential difference to produce a glow discharge, and photons from the discharge had sufficient energy to ionize organic compounds present in the carrier gas.
This method of detection did not become popular because of the high vacuum at the end of the column and the resulting poor sensitivity, but later detectors were designed in which the detector cell was separated from the high vacuum energy source.

Photoionization detector

The microcell is now operated near to atmospheric pressure and the energy source is a UV lamp filled with xenon, krypton, or argon, depending on the ionization potential of the sample components. The principle is simple, depending on excitation of the molecules by photons from the energy source, followed by subsequent ionization of the exited molecules and amplification of the resulting current between the polarizing electrodes. All compounds having ionization potentials below the energy output from the UV source will give a response with a wide linear range.

The detector is non-destructive and considerably more sensitive than the FID for many compounds, particularly substituted aromatics and cyclic compounds. Response is poor towards permanent gases and low molecular weight alkanes.

An important advantage of the PID is its simplicity and the lack of any need for auxiliary gases except makeup flow when using capillary columns. It is therefore useful in hazardous situations where the use of flame detectors would be prohibitive or in portable GC systems. However, a very high purity of helium (5.0) must be used for a sufficiently low background signal and sensitivity.