Dichotomous sampling of vapor and aerosol of methylene-bis-(phenylisocyanate) [MDI] with an annular diffusional denuder.

A dichotomous sampler was developed for speciation of airborne methylene-bis-(phenylisocyanate) (MDI) into vapor and aerosol fractions. The sampler consisted of an impactor inlet and two sections of annular diffusional denuder, in series with a glass fiber filter. The denuders and filter were coated with N-4-nitrobenzyl-N-1-propylamine for collection and derivatization of MDI followed by high performance liquid chromatography analysis. The sampler was evaluated in the laboratory with test atmospheres of MDI over a range of 24 to 355 micrograms/m3 at an average temperature of 26 degrees C. The total amounts of MDI collected by the dichotomous sampler were not significantly different from a reference sampler over the entire range of MDI concentrations. The first denuder collected MDI vapor with an average efficiency of 91.6 +/- 5.2%, in comparison to a theoretical efficiency of 94.1%. MDI existed solely in the vapor state up to approximately 75 micrograms/m3 and was collected on the denuders. Above this, MDI partitioned into vapor and aerosol, and the device efficiently separated the sample into the vapor fraction, found on the denuders, and the aerosol fraction, which penetrated to the filter. While this work was done with condensation aerosol, appropriate modification of the sampler inlet should allow use of the device for sampling atmospheres containing larger aerosols.