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 sub parts per billion to 10 000 parts per million (ppm). The photoionization detector is an efficient and inexpensive detector for many gas and vapor analytes. A PID may produce instantaneous readings and operate continuously. These hand-held, battery-operated detectors are widely used in military, industrial, and confined working facilities for safety.

PIDs are used as monitoring solutions for:

• Lower explosive limit measurements
• Ammonia detection
• Hazardous materials handling
• Arson investigation
• Industrial hygiene and safety
• Indoor air quality
• Environmental contamination and remediation
• Cleanroom facility maintenance

Principle

The first application of photoionization detection was as a gas chromatography (GC) ion detector.^[1] 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.

Application

As a stand alone detector PIDs are broad band detectors and are not selective, as these may ionize everything with an ionization energy less than or equal to the lamp output. A PID is highly selective when coupled with a 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 or lower than the energy of the photons produced by the PID lamp used in the detector. This selectivity can be useful when analyzing mixtures in which only some of the components are of interest.

The PID is usually calibrated using isobutylene, and other analytes may produce a relatively greater or lesser response on a concentration basis. Although many PID manufacturers provide the ability to program an instrument with a correction factor for quantitative detection of a specific chemical, the broad selectivity of the PID means that the user must know the identity of the gas or vapor species to be measured with high certainty. If a correction factor for benzene is entered into the instrument, but hexane vapor is measured instead, the lower relative detector response (higher correction factor) for hexane would lead to underestimation of the actual airborne concentration, and the user would not know that hexane had been measured instead of benzene.

PIDs are non-destructive detectors. They do not appreciably destroy/consume the components they detect. Therefore they can be used before other detectors in multiple-detector configurations. The signal produced by a PID may be quenched when measuring in high humidity environments,^[2] or when a compound such as methane is present in high concentration^[3] This attenuation is due to the ability of water, methane, and other compounds with high ionization potential (IP) values to absorb the photons emitted by the uv lamp without leading to the production of ion current. This reduces the number of energetic photons available to ionize target analytes.

References