Detailed description of filter techniques

The concentrations of organic pollutants in air are much smaller than in water or soil, because the respective equilibrium partition constants are far on side of the liquid or solid phases. Consequently, it is often necessary to sample several hundred cubic meters of air to extract enough pollutants for quantification. These are directly sucked through a filter (typically a polyurethane foam or XAD) with a high-volume sampler. Hence, in this special case sampling and extraction are done in combination.

Depending on the type of pollutant a certain fraction of it may be sorbed to aerosols while the rest is in the gas phase. In order to separate both fractions in the sampling procedure a special particle filter (e.g. a glass or quartz fiber filter) can be used in front of the PU or XAD filter.

Similar to other filter techniques, it is important to adjust the type and amount of filter to the sampled volume of air to rule out breakthrough of the compound through the filter. The appropriate ratio of sample to filter volumes can be calculated if the partition constants at the sampling temperature are available (see Box 10 and Box 11). However, in practice this is still often done by trial and error. To this end, a second PU or XAD filter (a so-called back-up filter) is placed behind the first one. Both filters are then analyzed separately in order to check whether breakthrough has occurred. If no breakthrough occurred through the first filter, the analyte concentration in the backup filter must be close to zero and the backup filter is no longer necessary.