Cross Sensitivity Of Photoionisation Detectors
Every sensor has some cross-sensitivity, this is when the sensor responds to non-target gasses, causing a false reading. It is very important to be aware of potentially cross-sensitive chemicals when choosing as instrument and interpreting the resulting data.
Photo Ionisation detectors (PID) are non selective and non-discriminatory, so are unable to measure individual components with in a gas mix. PIDs can measure a broad range of volatile organic compounds (VOCs) and some inorganic compounds in the parts-per-million (ppm) to parts-per-billion (ppb) range. However not all molecules can be detected by PID and the major components of air,
i.e., nitrogen, oxygen, carbon dioxide, argon, etc., do not cause a response, but most VOCs do give a response. Any VOCs in the area which are exposed to the PID, either intentionally (when then instrument is running as designed) or unintentionally (such as a cleaning activity near the PID) will cause a response.
Suppression Of The Signal
Some gases, such as water vapour, methane (CH4), and carbon dioxide (CO2) can cause a suppression of the ionising light effecting the instrument reading. Methane is what is used to heat most home so is commonly found in the power industry where other harmful VOCs are found and need to be measured.
How Does a PID Work?
A UV lamp generates high-energy photons, which pass through the lamp window into the sensor chamber. Sample gas passes over the sensor and about 1% of it through a membrane filter into sensor chamber.
The inset on the lower right shows what happens on a molecular level. When a photon with enough energy strikes a molecule M, an electron (e–) is ejected. The M+ ion travels to the cathode and the electron travels to the anode, resulting in a current. This current is proportional to the gas concentration. The electrical current can be displayed as a ppm or ppb concentration.
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