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 recommended for sampling. Also, the larger the particle size, the more strongly an impinger is recommended. An impinger is recommended for all aerosols having particle diameters greater than 2 µm because it is believed that the poor mixing on filters results in poor derivatization efficiency. Based on the thin-walled sampler model of Vincent,75 estimation of the aspiration efficiency of the impinger indicates reasonably close agreement with the inhalability convention for particles smaller than 20 µm when ambient wind speed is low (as is typical for indoor workplace environments76). For larger particle sizes and relatively high wind speeds, the impinger is expected to undersample relative to the inhalability convention. Filters are recommended for sampling particles smaller than 2 µm because these particles are known to be collected inefficiently by impingers and because smaller particles, by requiring much less reagent than large particles, are less susceptible to local depletion of reagent. For environments that are likely to have isocyanate species present as both large and small particles, a sampling train consisting of an impinger followed by a reagent-coated filter is recommended.2,49 There are other factors in addition to accuracy that must be considered in choosing a sampler. The use of an impinger is considerably less convenient than use of a filter. Impingers may even be deemed inappropriate for personal sampling. NIOSH Method 552177 (which is adapted from United Kingdom Health and Safety Executive Method MDHS 2578) uses an impinger containing a solution of reagent in toluene. Because the toluene vaporizes extensively during sampling, NIOSH does not recommend this method for personal sampling. NIOSH Method 552279 uses an impinger containing a solution of dimethyl sulfoxide (DMSO). Because DMSO is readily absorbed through the skin, NIOSH recommends that DMSO impingers be used for area air sampling only. The new NIOSH method under development uses impingers containing a solution of reagent in butyl benzoate. Since butyl benzoate is a non-volatile solvent, sampling with it generates minimal vapor. It does not elicit the same concern as DMSO in terms of dermal absorption.

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