The Photo-Ionization Detector (PID) is a 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.