Photoionization detectors (PIDs) are often employed to measure VOC concentrations in water or soil. For example, hydrologists and environmental engineers often use PIDs to monitor chlorinated solvents (e.g., percholorethylene) and fuels (e.g., gasoline, diesel) in soil or groundwater during clean-up operations such as excavations and soil vapor extractions.
PIDs can also be used to monitor water stripper effluents and off-gases from waste waters. Although PIDs cannot measure directly in these media, they can indirectly measure the vapours emitted from them. For example, Hewitt and Lukash* reported linear correlations between headspace PID response and soil concentrations of benzene, toluene, xylenes, dichloroethylenes, trichloroethylene and perchloroethylene.
Through the use of PIDs it is possible to attain an idea of the presence and levels of volatile organic compounds VOCs in both soils and water. For identification of the presence of VOCs we use headspace screening for semi-quantification of VOCs, we use headspace analysis. Both procedures are very similar but with more stringent controls on the latter.
Soil Headspace Screening
In order to perform a headspace screening, a soil or groundwater sample is placed in an airtight container, typically a glass jar or polyethylene bag, leaving one-half to one-third empty.
The concentration in the headspace, measured in ppmv, does not equal the soil or water concentration, measured in mg/kg or mg/L. The vapour concentration depends on such factors as soil-to-headspace weight and volume ratio, soil permeability, affinity of the compound to the soil, temperature, equilibration time, and dilution during the measurement procedure. Therefore, it is important that these factors are controlled as closely as possible if quantitative soil or water concentrations are desired.
In a typical procedure, a sample of soil or water is filled approximately halfway into a mason jar with a ring-type lid. Enough air space (“headspace”) above the sample is allowed for sampling.
A piece of aluminum foil is placed over the mouth of the jar and held in place with the lid ring.
The jar and its contents are brought to room temperature and shaken to mix the soil sample with the air in the headspace.
The influent probe and effluent line of the portable PID are then poked through the foil and the VOC concentration measured in the headspace of the jar.
A similar procedure can be performed using a plastic bag instead of a jar, but the headspace volume is not as reproducible, and thus the results are less quantitative. A sampling septum can be made easily for virtually any hard container with aluminium foil held in place by a rubber band around the rim of the vessel. When sampling with the PID, ensure that dirt and water are not sucked into the PID probe.
As a result, headspace screening provides more consistent readings that can be used for estimating relative concentrations. However, readings remain relatively inconsistent, because volatilisation of contaminants is affected by:
Ambient air dilution into jar;
Temperature variations; and
Time to prepare and analyse sample.
The lower detection limit is generally around 100 ppmv but may be significantly lower under ideal conditions (e.g., no wind, no humidity, high O2 levels). Headspace screening measurements are also only qualitative, however, their detection limits are generally between 10 and 100 ppm.
Head Space Analysis
Headspace analysis is similar to headspace screening but the procedures are more controlled and the results more accurate. A measured quantity of a soil or groundwater sample is placed in a polyethylene bag. For soil samples, a specified volume of deionized water is also placed in the bag in order to facilitate a more consistent partitioning of organic vapors. The bag is then inflated and the sample is agitated. After a specified time, an FID or PID is used to measure the headspace. QC procedures include the development of a calibration curve using field standards. These standards help in the interpretation of instrument responses and provide a comparison with samples of known concentrations.
Headspace analysis is classified as a semi-quantitative, method because it provides an order of magnitude indication of contamination, but it does not provide information about the concentration of specific constituents. The lower detection limit with this method may be as low as 0.1 ppm for gasoline in water, but it is generally above 1 ppm. For all methods, soil samples that are clay rich or contain high organic content may provide inconsistent results. In addition, gasoline should be relatively fresh or only slightly weathered for useful results.
For known contaminants it is also possible to get a greater idea of vapour concentrations through the application of correction factors. For more information on this please visit our PID correction factors page.
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