Concentrations of some substances of interest in air pollution contexts cannot be measured directly. Instead, the substances needs to be sampled and accumulated for subsequent analysis. This may be the case for example when the levels of the sought-for substances are very low, when no direct measurement techniques are readily available, and/or when it is a group of substances with varying optical and/or chemical properties that are to be monitored as a single entity.
In ambient air quality monitoring, sampling can be applied to determine particulate matter concentrations, and also to investigate the particulates with respect to metals. A low and well-controlled stream of air flows through a filter, and the filter can then be analysed with respect to the matter on the filter surface. Using beta attenuation
, the weight of the sampled particles can be meaured. The particulate matter concentration can then be determined by dividing the mass on the filter by the air volume having passed the filter during the sampling time. The filter can also be investigated by X-ray fluorescence (XRF) at a laboratory to determine the specific concentrations of for example lead in the particles.
In emissions monitoring, the sampling flow rate cannot be fixed, but it must be the same as the primary flow in the gas duct which may vary by time. The accumulated sample would otherwise not be representative for the actual average conditions in the duct. The sampling flow is therefore controlled by a signal coming from a flow meter in the duct. This is called isokinetic sampling.
After having been extracted from the duct, the sample is lead directly to and through a sorbent container within which a material suitable for adsorption of the sought-for substances resides. By example, when sampling dioxins and furans, the sorbent material is specified to be a solid resin such as XAD-2. After the sorbent container, the sample gas passes further cooling and drying stages before it reaches a flow control valve and a flow meter, and finally the pump driving the sample flow through the system. The sampled gas can then be emitted back to the duct or to the open air.
A sorbent container can sit in the sampling flow for some hours up to multiple weeks, depending on the sought-for substances and the regulations controlling the sampling. Immediately after the sampling period, the container is removed and quickly replaced by a new one. The old container is sent to a laboratory for analysis while the new container is used for the next sampling period. At the laboratory, the sorbent is resolved and heated, and the released fumes including the adsorbed sought-for substance can be analysed by for example high-resolution gas chromatography and/or mass spectrometry, GC/MS.
The result from the laboratory analysis is the total weight of the sought-for substance captured in the sample container. This is divided by the normalised gas volume having passed the sorbent container while it was sampling. Thanks to the isokinetic sampling, this yields the true, normalised average concentration of the sought-for substance during the sample period.