Photoionization detectors (PIDs)
use a high energy ultraviolet (UV) light source to ionize chemicals in an air
stream. The charged molecules are collected on a charged surface which
generates a current which is directly proportional to the concentration of the
chemical in the air being sampled.
The ability of a chemical to be ionized is a function of its ionization potential (IP). If the energy of the UV lamp is greater than or equal to the IP of the chemical being sampled, then the chemical will be detected. Typically, PID detectors will come equipped with a UV lamp at 10–10.6 electron volts (eV). Tables listing the IP for chemicals and their relative sensitivity are generally available from the manufacturer. Higher energy lamps (11.7 eV for the Photovac Model 2020Pro) are available to detect chemicals which have high IPs. For example, methylene chloride requires use of the 11.7 eV lamp for detection because the IP for methylene chloride is 11.35 eV. In general, these higher energy lamps have a much shorter lifetime than the 10.6 eV lamps.
In general, aromatic hydrocarbons such as benzene, toluene and xylene provide a sensitivity of approximately 0.1 ppm with photoionization detection. Unsaturated hydrocarbons, alcohols, ethers, and chlorinated hydrocarbons have intermediate sensitivity by PID, and saturated hydrocarbons such as n-hexane tend to be the least sensitive. For example, n-hexane is approximately 1/10 as sensitive as benzene by PID. While it might be expected that the sensitivity of a chemical would be related to its IP, this is not always the case. For example, benzene with an IP of 9.245 eV, and which has a relatively high sensitivity by photoionization detection, is actually slightly less sensitive than vinyl bromide with an IP of 9.80 eV.
In many instances a reference gas is used to calibrate the PID. Frequently, isobutylene gas in air is used as a calibration gas. The meter can then be used to read directly in isobutylene units. If gases other than isobutylene are measured, the isobutylene units can be converted using the appropriate response obtained from the instrument manual for the PID meter used. For example, if the response factor listed in the manual for benzene (relative to isobutylene) is 0.5 and if a meter which had been calibrated with isobutylene was used to measure benzene, the actual benzene concentration in air will be one half of the meter reading. Thus, if the meter reads 5.8 ppm isobutylene in a benzene atmosphere, the benzene concentration is actually 2.9 ppm. Similarly, if the meter reads 10 ppm isobutylene in an atmosphere of ethyl acetate, the ethyl acetate concentration is 38 ppm because the response factor for ethyl acetate is 3.8.
Many PID meters are programmed with internal response factors based upon isobutylene gas and the instrument can be set up to read ppm for the gas of interest. Direct calibration of the instrument, or verification of the calibration if stored response factors are used to calibrate the instrument, is desirable. This can be done by testing a known concentration of an atmosphere containing the chemical of interest prepared in a gas bag.
Photoionization sensitivity is dependent upon the age of the lamp and cleanliness of the lamp window. Over time, the output of the lamp will be reduced and also the accumulation of organic deposits on the surface of the lamp will reduce sensitivity. A buildup of film on the lamp will reduce the sensitivity of the meter. The meter also has a reduced sensitivity in high humidity. One manufacturer (RAE Systems) reports up to a 30% reduction in response for measurements in high humidity air when compared to calibration of the same chemical in dry air. For the most accurate results, it is best to calibrate the meter using representative air.
MicroRAE also reports that a "quenching effect" can be observed in which the UV lamp light rays are scattered by the presence of non-ionizable gas molecules. Water vapor, carbon dioxide, methane, and carbon monoxide can all produce a low reading for the gas of interest if present in the air being sampled.
Follow the manufacturer's recommendations for maintaining the detector in optimal condition. This will include routine cleaning of the UV lamp and frequent replacement of the dust filter. Because of the fragile nature of the lithium fluoride window on the 11.7 eV lamps, special precautions must be followed and cleaning should only be done using Freon or chlorinated solvents. The exterior of the instrument can be wiped clean with a damp cloth and mild detergent, if necessary. Keep the cloth away from the sample inlet and do not attempt to clean the instrument while it is connected to a power source.