Diffusive samplers workplace monitoring

For workplace monitoring two main types of diffusive sampler have been used for monitoring of indoor air ... 

A diffusive end cap which contains a stainless steel mesh screen can also be used and this can prevent air movement within the diffusive air gap when exposed to high air velocities (Brown, 1993). Evaluation of the sampler for workplace monitoring found that the diffusive uptake rate of the sampler was not influenced by air velocities as low as 0.007ms-1. 

The first protocols developed for evaluation of the performance of diffusive samplers were based on workplace applications. The European standard EN838 1995 (EN (1995)) is an example. This approach has been adapted to provide a protocol for the evaluation of the performance of diffusive samplers for ambient air monitoring (EN, 2004a). It describes a series of tests that enable a calculation of the measurement uncertainty. The key sampler related factors assessed are ... 

Diffusive samplers originally developed for workplace monitoring have been applied to a wide range of studies of VOCs in non-industrial indoor air through a modification of sampling times and analytical methods. The main types of... 

Compared with methods for the workplace, methods for diffusive sampling of non-occupational indoor air and ambient air have been less well developed. The range of concentrations and environmental conditions used to evaluate samplers for workplace monitoring is not directly applicable to non-occupational environments. However, diffusive monitors are Ending increasing use in non-occupational environments. This chapter discusses the principles governing diffusive sampling and the factors that can influence sampler performance, and reviews studies that have applied the technique for the measurement of VOCs in indoor air. 

The exposure periods required to obtain sufficient sensitivity to measure VOCs in low pg/m concentrations using diffusive samplers are typically several days or weeks. It would be a major task to apply the laboratory and field tests required by protocols to evaluate diffusive monitors used in the workplace for these long exposure periods. At present the uncertainty associated with measurement by diffusive samplers requires further investigation, but a number of studies have shown them to be applicable to the measurement of VOCs in indoor air. 

As for workplace monitoring, there have been two main types of diffusive sampler used for monitoring of indoor air (i) badge type samplers containing a strong adsorbent such as charcoal that requires solvent desorption for GC analysis and (ii) tube type samplers with weaker adsorbents such as the porous polymer Tenax that can be thermally desorbed. The samplers most widely used have been developed for monitoring of workplace atmospheres and applied to indoor air through modification of the exposure period and the analytical method. 

The most widely used badge type sampler for indoor air studies has been the OVM 3500. This is a circular badge with a 1-cm diffusion length containing a charcoal wafer. Desorption of VOCs is carried out within the monitor itself by the addition of carbon disulfide. Exposure periods applied have ranged from 24 h to 3 weeks. The diffusive uptake rates reported by the manufacturer for 8-h exposure periods are about 30 ml/ min, but actual values are compound specific. For the monitoring of hexane in the workplace, the diffusive uptake rate is not significantly affected by ambient air movement, provided that there is a minimum air velocity of about 0.1 m/s (HSE, 1992). 

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