Photoionization detector

A photoionization detector or PID is a type of gas detector.

Typical photoionization detectors measure volatile organic compounds and other gases in concentrations from 1 parts per billion to 10 000 parts per million (ppm). 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:
* Lower exposure level measurements
* Ammonia detection
* Hazardous materials handling
* Arson investigation
* Industrial hygiene and safety
* 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 ultraviolet (UV) range, to break molecules to positively charged ions. As a compound elutes from the GC's column they are bombarded by high-energy photons and are ionized when a molecule absorbs high energy UV light. The UV light excites the molecule, and results in temporary loss of an electron and the formation of positively charged ion. 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 a meter. The ions recombine after passing the detector to reform their original molecules.

Application

As a stand alone detector PIDs are broad band detectors and not selective at all as these ionize everything with an ionization energy less than or equal to the lamp output. A PID is highly selective when coupled with some type of chromatographic technique or a pre-treatment tube such as a Benzene specific tube. The PID 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.