Photoionization detector

A photoionization detector or PID uses an ultraviolet (UV) lightsource to break molecules to positively charged ions that can easily be counted with a detector. Ionization occurs when a molecule absorbs high energy UV light, which excites the molecule, and results in temporary loss of a negatively charged electron and the formation of positively charged ion. The gas becomes electrically charged.

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In the PID, the charged particles produce a current that is amplified and displayed on a meter as "ppm" (parts per million) or "ppb" (parts per billion). The ions recombine after passing the detector to reform their original molecules.

Typical photoionization detectors measure volatile organic compounds and other toxic gases in concentration from 1ppb to 10,000ppm. The photoionizaton detector is the most efficient and inexpensive type of gas detector. They are capable of giving instantaneous readings and monitoring continuously. They are widely used in military, industrial, and confined working facilities for safety.

PIDs are used as monitoring solutions for:

• Lowest exposure level measurements
• Ammonia detection
• Hazardous materials handling
• Arson investigation
• Industrial hygiene
• Indoor air quality
• Environmental contamination and remediation
• Cleanroom facility maintenance

Principle

A PID is an ion detector which uses high-energy photons, typically in the UV range, to produce ions. As components elute from the GC's column they are bombarded by high-energy photons and are ionized. 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.

Application

PIDs are highly selective detectors. They will only detect components which have ionization energies similar to the energy of the photons the detector uses. This selectivity can be useful when analyzing mixtures in which only some of the components are of interest.

PIDs are non-destructive detectors. They do not destroy/consume the components they detect. Therefore they can be used before other detectors in multiple-detector configurations.