A slit impactor utilizing a transpirated impaction surface for collection of bioaerosols

Abstract

In recent years, there has been a growing concern of possible attacks against the United States by other countries using biological weapons. In response to this concern, the U.S. government has increased its defensive efforts against such threats. The Aerosol Technology Laboratory at Texas A&M University has been participating in the effort to evaluate and upgrade biological particle samplers that are to become part of an integrated detection system. One goal of the effort by the Aerosol Technology Laboratory has been to explore the feasibility of utilizing the principal of inertial impaction to design a sampler that has a cut-point of 1 []m, low power consumption, heating capabilities, and sampling rate of several hundred L/min. The sampler also needs to transfer aerosol particles to the hydrosol state in order to be compatible with real-time analyzers and subsequent off-line schemes that use organism growth to characterize a threat. To fulfill these requirements, a bioaerosol sampler, which sampled air at a flowrate of 500 L/min through eight rectangular impaction nozzles with an air flow power requirement of 6.5 watts, was designed and evaluated. The particles were then impacted onto a porous stainless steel surface and entrained in a liquid, which was transpirated through the pores. This hydrosol was transported to an analysis system. This particle sampler was tested with liquid oleic acid particles and solid polystyrene latex particles used to simulate biological particles. Tests were conducted in either an aerosol wind tunnel or a flow-through system. Results showed that the sampler had a sharp efficiency curve with a 50% cut-point of 0.84 []m. Particles greater than 1.2 []m were collected with at least 94% efficiency. The transmission of 2.3 []m aerodynamic diameter polystyrene particles from aerosol to hydrosol was determined to be 90% at an effluent liquid flow rate of 4.5mL/min, and 63% at 1.0 mL/min. Also, the sampler time constants were the same for a step increase or decrease in solid aerosol concentration. The time constants were 1 and 2.5 minutes for liquid flow rates of 4.5 and 1.0 mL/min, respectively. Preliminary tests with Bacillus globigii (BG) spores gave results of 25% to 35% collection efficiency at an effluent liquid flow rate of 4.5 mL/min. The difference in aerosol-to-hydrosol transmission was partially due to the small size of the BG spores (~0.7 []m AD). Other possible reasons included particle adhesion to the impaction surface and degradation of viability in the aerosol-to-hydrosol transfer process.