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Photoionization Detector (PID) For On-Site Environmental Testing

AEL believes in using the right tools when approaching an environmental site investigation. For sites with volatile organic compounds (VOCs) in the soil or groundwater, which could result from several potentially contaminating activities like dry cleaning or using certain solvents, this can involve the use of a photoionization detector (PID) for on-site environmental testing.

On-site testing is useful for a variety of environmental impacts, including VOCs (using a PID), metals (using XRF), and PHCs (using UVF). These tools give environmental researchers necessary information when directing site investigations and remediations.

VOCs are unstable compounds, usually originating from solvents, and when present in soil and exposed to oxygen, they readily off-gas into air. A PID uses an ultraviolet light source to break down VOCs into positive and negative ions. The charge of the ions is measured by the PID, and this is represented as a measure of the concentration of VOCs in air. PIDs are non-selective, meaning that all VOCs are detected, and cannot be separated out in the testing results. Laboratory analysis is required to determine concentrations of individual VOC compounds, but on-site testing can give a good screening for samples that should be tested further.

Each VOC has a different minimum energy required to break down the gas, called the ionization potential. In order for the VOC to be detected, the ionization potential must be less than the energy of the light emitted by the PID. Depending on the VOCs to be detected, different lamps can be used, each using a different energy. There is a trade-off between the lamps – although a higher energy lamp may be able to detect most VOCs, it has a short lifespan, and can be much more costly. At the opposite end, a low energy lamp will last much longer, but will be limited in the number of VOCs it can detect.

The process of detecting VOCs in soil through on-site testing requires collecting VOCs in an enclosed space, such that they can be produced but not escape before they can be detected. This is generally done by placing a small amount of soil in a jar or plastic zip-close bag, about half full. If a jar is used, aluminum foil is placed over the top. The container is then completely sealed and the sample is allowed to sit for approximately 10 to 15 minutes, were VOCs will off-gas into the closed bag or jar (headspace development). The PID is then quickly inserted into the bag or jar, and a measure of the concentration of VOCs is taken.

Alternatively, a PID may be run over a soil core immediately after it has been removed from the ground, although this exposes the PID to outside sources of VOCs (E.g. exhaust fumes from a drill rig) or may underestimate the concentration of VOCs if the soil sample is allowed to sit exposed for a length of time. Another consideration is temperature. VOCs will not readily off-gas in cold temperatures, generally below 10C. In cold temperatures, soil samples need to be gently warmed before testing. Caution must be used, as if the soil is warmed to quickly, VOCs can be destroyed.