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Air monitoring for gases and vapours often plays an important role in occupational health and safety during exploration and remediation work in contaminated areas. In addition to monitoring acute hazards such as fire, explosion and oxygen deficiency with multi-gas measuring devices, chronic hazards for employees must also be assessed using suitable measuring devices.
One way of monitoring breathing air quality is to use a photoionisation detector (PID). In addition to an internal pump for drawing in the ambient air to be tested, this measuring device has a UV light source and a so-called ionisation chamber as a measuring cell. Here, the pollutant gases drawn in from the ambient air are ionised by the UV light and thus become electrically conductive. This enables an electric current to flow through the measuring cell, the strength of which depends on the number of ionised particles. This current flow is electrically amplified and can be shown on the device's display as a pollutant gas concentration using suitable calibrations. Calibration is usually carried out with a non-hazardous substance (usually isobutylene). A substance-related adjustment can be carried out using conversion factors, some of which are also stored in internal data beacons. However, this is only useful if the individual substances are known.
The components of the uncontaminated ambient air (nitrogen, oxygen, carbon dioxide, noble gases) are not ionised by the UV source, but organic solvents such as hydrocarbons can be ionised very well. The strength of the UV source is decisive for the number of detectable substances. Not all organic gases and vapours can be detected with commercially available UV sources (9.8 eV = electron volt, 10.6 eV or 11.7 eV), whereby the required ionisation energy (in eV) is a physical substance constant. It is not possible to identify substances with a PID.
However, a large number of organic chemicals can already be detected or measured with a 10.6 eV lamp, including alcohols, aromatics, iodides and bromides, amines, mercaptans, aldehydes, ethers, acrylates and esters, etc.
Among other things, the PID can be used for continuous air monitoring during remediation work, but it only shows sum signals for the substances detected. To assess the resulting hazard situation, the percentage composition of the individual components must be determined in advance in the laboratory in order to calculate threshold values above which further protective measures, such as dilution of the pollutants by blowers, are required. PID measurements should only be carried out by trained personnel, as the measurement results must be expertly evaluated.
Other possible applications include, for example, checking drill cores for solvent contamination during remediation investigations of contaminated sites, leak detection in industrial plants or the localisation of damaged areas in the event of chemical accidents.
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