Photo Ionization Detector
The Photo Ionization Detector (PID) is a portable vapor and gas detector that detects a variety of organic compounds. Photo ionization occurs when an atom or molecule absorbs light of sufficient energy to cause an electron to leave and create a positive ion.
The PID is comprised of an ultraviolet lamp that emits photons that are absorbed by the compound in an ionization chamber. Ions (atoms or molecules that have gained or lost electrons and thus have a net positive or negative charge) produced during this process are collected by electrodes. The current generated provides a measure of the analyte concentration. Because only a small fraction of the analyte molecules are actually ionized, this method is considered nondestructive, allowing it to be used in conjunction with another detector to confirm analytical results. In addition, PIDs are available in portable hand-held models and in a number of lamp configurations. Results are almost immediate.
Limitations and Concerns
The PID is not suitable for the detection of semi-volatile compounds.
The PID may give false positive readings for water vapor. Rain may also affect performance. High humidity can cause lamp fogging and decreased sensitivity. This can be significant when soil moisture levels are high or when a soil gas well is actually in ground water.
High concentrations of methane can hinder performance.
Rapid variations in temperature at the detector, strong electrical fields, and naturally occurring compounds, such as terpenes in wooded areas, may affect instrument response.
The PID must be re-calibrated frequently
Detection limits for most PIDs are in the parts per million range. Thus they are unsuitable for most vapor intrusion indoor air investigations, where screening or action levels are normally in the parts per billion range.
The PID is used mostly to detect VOCs in soil, sediment, air and water. It is often used to detect contaminants in ambient air and soil during drilling activities and during spills to identify potential problems.
Technology Development Status
The PID is commercially available and routinely used.
Other Resources and Demonstrations
Because this method is so widely used, numerous commercial sites are available. See, for example: http://www.afcintl.com/gasdet/rae/ultrarae3000.htm, http://www.raesystems.com/products/minirae-3000