Photoionization Detector - Principle

Principle

The first application of photoionization detection was as a gas chromatography (GC) ion detector. In a photoionization detector high-energy photons, typically in the ultraviolet (UV) range, break molecules into positively charged ions. As compounds elute from the GC's column they are bombarded by high-energy photons and are ionized when molecules absorb high energy UV light. UV light excites the molecules, resulting in temporary loss of electrons in the molecules and the formation of positively charged ions. The gas becomes electrically charged and the ions produce an electric current, which is the signal output of the detector. The greater the concentration of the component, the more ions are produced, and the greater the current.

The current is amplified and displayed on an ammeter. It is widely held that the ions recombine after passing the detector to reform their original molecules, however only a small portion of the airborne analytes are ionized to begin with so the practical impact of this (if it occurs) is probably negligible.