New bio-aerosol collector using a micromachined virtual impactor

For collection and concentration of bioaerosols, we designed and evaluated a single stage virtual impactor, which was fabricated by micro-electro-mechanical systems (MEMS) process. The cut-off diameter of 1 μm was selected, since 1 μm is the lowest size as used in the US Government Joint Biological Point Detection System [Haglund, J. S., & McFarland, A. R. (2004). A circumferential slot virtual impactor. Aerosol Science and Technology, 38, 664–674; Moshier, T., & Buonaugurio, T. (2000). Joint Biological Point Detection System (JBPDS) requirements and design interplay. Proceedings of the First Joint Conference on Point Detection for Chemical and Biological Defense, October 23–27, 2000, Williamsburg, VA.] The design value of a 1 μm cut-off diameter required a nozzle width and thickness of 880 and 200 μm, respectively. The virtual impactor was evaluated for physical and biological collection efficiencies. For the performance evaluation of physical collection efficiency and wall loss, polystyrene latex (PSL) particles were generated from an atomizer and their size distribution was measured using an aerodynamic particle sizer (APS, TSI model 3321) and a scanning mobility particle sizer (SMPS, TSI model 3936). The measured cut-off diameter was 0.95 μm, which agreed with the calculated results (=0.94 μm) determined with a commercial computational fluid dynamics (CFD) package, FLUENT, and the measured wall loss was below 33.5%. For the performance evaluation of biological collection efficiency, Staphylococcus epidermidis bioaerosols were dispersed into air by a nebulizer. The bioaerosols were measured using APS and sampled with a bioaerosol sampler. The overall physical collection efficiency based on the number concentration was 73.8±3%, which was similar to the one based on the number of colonies (=76.7±7%). We found that most of the bioaerosols collected and concentrated by our virtual impactor were viable.