Personal size-separating impactor for sampling microbiological aerosols

A commercially-available personal impactor was altered to sample viable microorganisms onto a semisolid, moist, gelatin medium rather than onto a stainless steel or filter surface. The Marple personal cascade impactor is an eight-stage sampler with predicted cut-offs of 20, 15, 10, 6, 3.5, 2, 1, and 0.61 microns for stages one to eight, at a flow rate of 2 L/min. The possibility was examined that using trays containing a small amount of gelatin medium in place of the thin, flat filters of the original design would alter the impactor's performance. A polydisperse aerosol of di-2-ethylhexyl phthalate (DEHP or DOP) was sampled directly into an aerodynamic particle sizer and through a stage of the personal impactor. The aerosol particles penetrating the tested stage were sized and counted, and the counts compared with those in the total aerosol. With a Mylar medium filter as the collecting substrate, the measured particle cut-offs (D50) for stages four to seven were 5.2, 3.4, 1.4, and 1.0 microns. With a tray containing gelatin as the collecting substrate, the D50 were 5.9, 4.0, 1.6, and 1.0 microns. The size separation of the personal sampler for ambient bacterial and fungal aerosols compared well with that of the Andersen microbiological impactor. The use of a moist collecting surface, compared to a dry surface, can be expected to enhance recovery of viable airborne microorganisms sensitive to dehydration.