PIDs produce instantaneous readings, operate continuously, and are commonly used as detectors for gas chromatography or as hand-held portable instruments.
Their primary use is for monitoring possible worker exposure to volatile organic compounds (VOCs) such as solvents, fuels, degreasers, plastics and their precursors, heat transfer fluids, lubricants, etc.
Portable PIDs are used for monitoring: In a photoionization detector, high-energy photons, typically in the vacuum ultraviolet (VUV) range, break molecules into positively charged ions.
A few of them may recapture an electron within 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 usually negligible.
With a gas chromatograph, filter tube, or other separation technique upstream of the PID, matrix effects are generally avoided because the analyte enters the detector isolated from interfering compounds.
[4] At the higher concentrations, response gradually deviates from linearity because of recombination of oppositely charged ions formed in close proximity and/or 2) absorption of UV light without ionization.