Evaluation of XMX/2L-MIL Virtual Impactor Performance and Capture and Retention of Aerosol Particles in Two Different Collection Media

The United States Air Force uses the XMX/2L-MIL (XMX) high volume air sampler to collect samples for biological analysis. The XMX uses a virtual impactor to concentrate particles 1.0 to 10 μm in size into a secondary flow prior to sample collection using a liquid impinger in a collection tube. There are no known published studies regarding virtual impactor inter-instrument variability, effect of reducing the secondary flow on particle concentration, or capture and retention efficiency (CRE) of particles in the collection media performance characteristics when using the XMX. These performance characteristics were evaluated by lofting test aerosols of Arizona Road Dust or fluorescent polystyrene latex (FPSL) spheres into a 14 m3 test chamber, measuring the chamber and post-virtual impactor particle concentrations using aerodynamic particle sizers, and measuring the concentration of FPSL spheres captured and retained in the collection media using a fluorometer. Notable findings include detection of significant inter-instrument virtual impactor variability, significant difference in particle concentration at reduced secondary flow, and significant differences in CRE due to particle size and secondary flow. This research demonstrates that when using an XMX limit of detection precision is suspect and the importance of collecting and analyzing multiple samples for improved risk assessment