Collection efficiency of a personal sampler for microbiological aerosols

A modified personal impinger (MPI) for sampling airborne microorganisms was tested for collection efficiency with the jet nozzle placed at various positions above and below the liquid surface. The sampler was operated with 10 mL of water and sampling rates between 1.08 and 3.4 L/min. The collection efficiencies for a polydisperse aerosol of diethylhexyl phthalate (DEHP) and monodisperse aerosols of polystyrene latex particles (ULP) were determined with an optical particle counter. The results show that a test aerosol generated from a suspension, such as ULP, gives test results that agree better with theoretical predictions than a polydisperse oil aerosol such as DEHP. The measured aerodynamic 50% cutoff diameters (D50) agreed with those predicted from impaction theory with the jet nozzle 4 mm from the flask bottom. For preservation of viability during sampling of microorganisms, it is common to use impingers with the jet nozzle above the liquid surface. These tests showed that if the MPI is operated with the jet nozzle above the liquid surface, D50 will be displaced toward larger particle sizes because the jet-to-plate distance/jet diameter ratio increases substantially when a soft impaction surface such as the liquid is used. The increased D50 could to some extent be compensated for by increasing the flow rate. An increased flow rate will, however, result in more losses because of aerosol regeneration.