Photoionization detectors are very fast to respond. They detect many gases that the Broad Range Electrochemical sensor cannot such as benzene, ammonia, acetone, formaldehyde, etc. Their main applications range from confined space entry to protection of the environment. These detectors are not specific and will respond to all gases that have different ionization potentials to the lamp output. PIDs are primarily used to detect volatile organic compounds in parts per billion concentrations up to 1% depending on the gas.

The diagram above depicts the main items in a Photoionization detector (PID). The PID lamp looks very similar to a medical ampoule filled with inert gas such as krypton at low pressure. When the krypton gas is energized inside the lamp ultraviolet (UV) light energy (Electron volts eV) is produced. The most common lamp emits 10.6eV. The window is made from special material and is expensive. It acts as a filter and only allows the wave frequency of ultraviolet light to pass. The electrodes are biased with stable DC volts and the output signal will change whenever there are changes in the electrical field. As the gas enters the detector in front of the window the gas molecules are excited producing positively charged ions. These charged ions change the electrical field and produce an electrical current which is amplified and made directly proportional to the volume of gas. (Parts per billion). All gases have different ionization potentials (IP) and gases with IPs at equal or below the electron voltage (eV) output of the lamp will be detected. For example benzene has an IP of 9.24eV and will be detected by the 10.6 eV lamp which is mostly used as it detects nearly all the other VOCs. By using a lamp with a lower eV output it is possible to limit the number of gases that can be detected hence increasing its selectivity which can be an advantage in some cases over a higher rated lamp that has more of a broader range. As PIDs are not specific to any gas correction factors may be applied in many respects similar to that shown above for catalytic sensors. Therefore should the PID be calibrated to a specific gas if the correction factor is known then the actual concentration of the targeted gas can be calculated by the user or software within the instrument.