The trapping and collection of airborne contaminants in the form of gases, vapours or aerosols is of importance in a number of fields. For example, contaminants may be collected to determine the downwind spread of herbicides or pesticides from a cross-spraying operation, to determine industrial compliance with air quality standards, to monitor workplace air quality or for controlling the use of toxic materials and to monitor and identify toxic substances released during military encounters.
The current techniques for collecting air samples are:
1. By using liquid-filled impingers or bubblers. Air is drawn ("bubbled") through the liquid which partially or completely dissolves the contaminants and thus removes them from the air stream. The solution is subsequently analyzed by injection into, e.g., a gas chromatograph.
2. By using tubes packed with a solid, porous absorbent. The solid material retains contaminants by, for example, physical inclusion in pores or electrostatic attraction at active polar sites on the surface of the solid when air is drawn through the tubes. The solid material is subsequently solvent extracted and the extracts analyzed, or the material is heated to thermally desorb the trapped contaminants into an appropriate apparatus for analysis.
3. By collecting sample volumes of the air in suitable containers such as large bags or gas syringes. The air is withdrawn from the collection container using a small gas tight syringe and injected into an analysis apparatus.
4. By drawing air continuously through analyzers which monitor contaminant concentration in real time or near real time.
All of these techniques suffer from certain disadvantages. For example, bubblers are relatively large and cumbersome devices that are difficult to use in field trials or large scale sampling networks. In addition, only a very small aliquot of the trapping solvent or liquid charge of the bubbler is needed to perform an analysis. This severely restricts the sensitivity of analysis methods based on bubbler sample collection.
While sample tubes packed with a solid adsorbent overcome many disadvantages of the bubbler sampling system, they suffer from other disadvantages. Standard solid sorbent tubes are of a relatively large size, which is dictated by the requirement to either take air samples over long periods of time or to sample relatively high concentrations of contaminants. A large quantity of solid adsorbent is required to prevent exceeding the retention capacity of the adsorbent. Effective thermal desorption of such tubes is difficult to achieve since the sheer bulk of the asorbent material prevents the material from being heated in a uniform fashion. Uneven heating produces non-instantaneous desorption of the sample components and relatively broad chromatographic "peaks". In addition, with current solid adsorbent packed tube technology, the outer surfaces of the sample tubes are usually exposed to the ambient atmosphere. This can create problems with respect to trace analysis as any contaminant retained on the outer surface of the tube from handling or exposure may enter into the analytical process.
A field sampling system based on large volume air collection bags is bulky, labour intensive in preparation and operation and requires manual techniques in transferring air samples from the collection devices to the analytical instrumentation.
Real time monitors are generally single point samplers. While they can be used with a manifold to sample a few points in sequence, for a large field trial or ambient air quality study, many such devices would be required to simultaneously sample the air at many different locations. The cost of this would be prohibitive. These devices are relatively sophisticated and will require attention from skilled operators to ensure their proper functioning.
The objective of the present invention is to provide an air sampler for field use that is relatively simple and cost effective in large scale operations and which overcomes many of the disadvantages associated with using solid sorbent tubes.