Photoionization Detector - Application

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, or when a compound such as methane is present in high concentration 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.