A new laboratory test chamber for the determination of diffusive sampler uptake rates

Diffusive samplers are today widely used for air quality control in indoor and outdoor environments and for personal exposure studies as well. The uptake rate is a fundamental parameter of a sampler for the calculation of the concentration of the substance to be monitored depends directly on it. Uptake rates can be affected by numerous factors. Their values and the range of validity have to be determined in an appropriate exposure chamber. The laboratory test chamber presented here consists of a loop made of glass, stainless steel and PTFE containing the generated atmosphere in which diffusive samplers can be tested. It is possible to accommodate several samplers simultaneously and simulate various environmental conditions such as temperature, wind speed, wind direction, humidity, atmospheric composition, total pressure and exposure duration. All working parameters have to be monitored including the concentrations of the components of the generated atmosphere. It is shown that the expected concentration of a volatile compound like benzene can be reached very quickly whereas those of less volatile compounds like toluene or xylenes takes longer. The procedure for overcoming this difficulty is described in this paper. By means of an application it is also shown that exposure duration can unequally affect diffusive sampler uptake rates depending on sampler geometry and the nature of the adsorbent. It appears that a radial high-uptake rate diffusive sampler packed with a thermally desorbable material may be unsuitable for long-term monitoring of a volatile compound like benzene. However an axial low-uptake rate diffusive sampler, also packed with a thermally desorbable material, seems more appropriate for this application due to the higher stability of its uptake rate.