STRATEGIES FOR THE SIMULTANEOUS COLLECTION OF VAPORS AND AEROSOLS WITH EMPHASIS ON ISOCYANATE SAMPLING

Workplace air frequently contains hazardous substances that may be pre sent as vapours or as aerosols with a wide range of particle sizes. De pending upon a chemical species' volatility and use, it may be present in significant amounts in both the vapour and particulate phases. Unf ortunately, 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 surf ace, vapour molecules are trapped either by adsorption onto a solid su rface, absorption by a liquid, or by reaction with the medium or chemi cals in the medium. Aerosol particles are most frequently collected by filtration or inertial impaction. However, if it is necessary to coll ect both phases simultaneously, a sampler with two stages is generally required. The exact nature of the sampler depends upon the size of th e aerosol particles and the physical and chemical characteristics of t he species of interest. A number of recent projects undertaken by rese archers at the National Institute for Occupational Safety and Health h ave dealt with development of sampling and analytical methods for comp ounds present in workplace air as both vapour and aerosol particles. O ne strategy invoked in several instances consisted of a filter for par ticle collection followed by an appropriate second stage for vapour co llection. For organophosphorus pesticides, the second stage was a sorb ent tube. For gaseous hydrogen fluoride, it was an alkaline-impregnate d back-up pad. For formaldehyde, the second stage was an impinger cont aining an aqueous solution of sodium hydrogensulfite. Sampling of isoc yanates requires a somewhat different strategy. Isocyanate aerosol can not simply be collected on a filter because the isocyanates can be los t through reaction with other compounds present in the aerosol particl e or Simultaneously collected on the filter. It is necessary to deriva tize the isocyanate species rapidly on collection. Filters and sorbent s impregnated with a derivatizing reagent as well as impingers and bub blers containing solutions of derivatizing reagent have all been used for the collection of isocyanate aerosol. But neither filters nor impi ngers appear to adequately sample for the entire range of isocyanate a erosol likely to be encountered in the workplace. This is because part icles of less than about 2 mu m in diameter are not efficiently collec ted by an impinger and isocyanate species present in large particles a re not efficiently derivatized when collected on reagent-coated filter s. However, the combination of an impinger followed by a reagent-coate d filter should satisfactorily collect isocyanate aerosols and vapours .