Sampling and Recovery Techniques for the Determination of Gases and Vapors in Air

Abstract

AbstractIt is possible to determine many gases and vapors in workplace air through the use of direct‐reading instruments. However, these instruments are often expensive, and not suited to personal monitoring as required by standards and regulations. They also may be insufficiently selective to monitor atmospheres contaminated by a mixture of components, or insufficiently sensitive to determine the low concentrations of, for example, regulated carcinogens. A more common technique is to remove a sample of air for subsequent analysis, typically in a laboratory, but sometimes in the field. This article covers sampling techniques for the determination of gases and vapors in air. It also covers recovery techniques that are applicable to the laboratory. It is not possible to completely divorce these subjects as the selection of sampling method is often dependent on the choice of recovery technique, and vice versa. Taking a sample of air improves the sensitivity of analysis, as well as integrating the result over the time‐period of sample collection. Exposure limit (EL) values are normally given as time‐weighted average concentrations over work‐shifts or shorter periods. It is possible in most cases to choose a sampling method to cover a full work‐shift with a single sample. By far the commonest technique is to concentrate the sample by collecting the contaminants of interest using a solid or liquid sorbent, where the mechanism of collection is adsorption, chemical reaction, or solution. The collection procedure involves either pulling air through the sorbing medium (active sampling) or allowing the molecules to diffuse to the medium under certain conditions (passive, or diffusive, sampling).The commonest technique for recovery is solvent desorption. An aliquot of the solution is then presented for analysis. For volatile organic compounds (VOCs), the commonest technique is to use a charcoal sorbent medium contained in a tube, through which air is drawn, followed by desorption using carbon disulfide, either alone or mixed with other solvents. Probably more than 75% of workplace air samples for VOCs are taken this way. Thermal desorption, using heat to release the sampled VOCs, rather than a solvent, is of rapidly growing importance because of its higher sensitivity. The selection of sorbent is more critical, and most recent protocols are using multiple beds of different sorbents. Whole‐air samples in polymer bags have been taken for industrial monitoring of permanent gases, whereas whole‐air samples in passivated metal canisters have been used commonly in ambient air monitoring of VOCs. Similar, but smaller, metal canisters may also be used in future for industrial hygiene work. Industrial hygiene air sampling will exist for as long as chemicals are used in the workplace, to determine the existence of hazards and the extent of risks, as well as to ensure the effectiveness of engineering controls. In many countries, the use of air sampling is a requirement of health and safety programs in order to compare workers' exposures to regulatory guidelines or legal limits.