What is Photo Ionization Detector (PID)?

Photoionization Detector (PID)

A Photoionization Detector (PID) consists of an ultraviolet (UV) lamp of a specific energy and an ionization chamber. Compounds passing through the chamber are excited by photons of UV energy and ionized according to the following equation:

R + hυ → R+ + e–

where
R = most organic/inorganic compounds
These ions are attracted to a collecting electrode, producing a current proportional to the concentration of the compound.
Whether or not a compound can be detected by a PID depends upon the energy required to remove an electron from the compound (its ionization potential). If the lamp energy is greater than the compound’s ionization potential, the PID will detect it. The standard lamp in the TVA-1000B is 10.6 eV. Other lamps (9.6 and 11.8 eV) are also available. The 11.8 eV lamp permits detection of many compounds not ionized by the standard lamp. The lower energy (10 eV) lamps, however, allow more selectivity by not responding to undesired compounds with a higher ionization potential.

NOTE: Refer to MI 611-183 for information and guidance on proper use of the 11.8 eV lamp.

Because of its smaller dynamic range (0-2000 ppm), the PID is not the detector of choice for measuring high concentrations of vapors. A PID is also more susceptible to interference from water vapor than a FID. However, as a PID does not require hydrogen or oxygen, it is the detector of choice when fuel is limited or unavailable, or when ambient oxygen concentrationsare low. The PID is also very sensitive to aromatic and chlorinated compounds, and can even measure some inorganic compounds that the FID does not detect at all (ammonia, carbon disulfide, carbon tetrachloride, chloroform, ethylamine, formaldehyde, and hydrogensulfide, to name a few).

Benefits of Photoionization Detection

♦ High sensitivity to aromatics, unsaturated hydrocarbons and chlorinated hydrocarbons
♦ Ability to measure some inorganic gases
♦ Very simple operation
♦ No support gases required
♦ Non-destructive detector allows sample to be recovered