Strategies for the simultaneous collection of vapours and aerosols with emphasis on isocyanate sampling

Abstract

Workplace air frequently contains hazardous substances that may be present as vapours or as aerosols with a wide range of particle sizes. Depending upon a chemical species' volatility and use, it may be present in significant amounts in both the vapour and particulate phases. Unfortunately, the mechanisms by which vapours and particles are removed from an air stream during pumped sampling are substantially different. Collection of vapour molecules relies on their diffusion to a surface during their residence time in a sampler. Once in contact with a surface, vapour molecules are trapped either by adsorption onto a solid surface, absorption by a liquied, or by reaction with the medium or chemicals in the medium. Aerosol particles are most frequently collected by filtration or inertial impaction. However, if it is necessary to collect both phases simultaneously, a sampler with two stages is generally required. The exact nature of the sampler depends upon the size of the aerosol particles and the physical and chemical characteristics of the species of interest. A number of recent projects undertaken by researchers at the National Institute for Occupational Safety and Health have dealt with development of sampling and analytical methods for compounds present in workplace air as both vapour and aerosol particles. One strategy invoked in several instances consisted of a filter for particle collection followed by an appropriate second stage for vapour collection. For organophosphorus pesticides, the second stage was a sorbent tube. For gaseous hydrogen fluoride, it was an alkaline-impregnated back-up pad. For formaldehyde, the second stage was an impinger containing an aqueous solution of sodium hydrogensulfite. Sampling of isocyanates requires a somewhat different strategy. Isocyanate aerosol cannot simply be collected on a filter because the isocyanates can be lost through reaction with other compounds present in the aerosol particle or simultaneously collected on the filter. It is necessary to derivatize the isocyanate species rapidly on collection. Filters and sorbents impregnated with derivatizing reagent as well as impingers and bubblers containing solutions of derivatizing reagent have all been used for the collection of isocyanate aerosol. But neither filters nor impingers appear to adequately sample for the entire range of isocyanate aerosol likely to be encountered in the workplace. This is because particles of less than about 2 µm in diameter are not efficiently collected by an impinger and isocyanate species present in large particles are not efficiently derivatized when collected on reagent-coated filters. However, the combination of an impinger followed by a reagent-coated filter should satisfactorily collect isocyanate aerosols and vapours.