Performance Characteristics of the Aerosol Collectors of the Autonomous Pathogen Detection System (APDS)

Abstract

This research analyzes the physical performance characteristics of the aerosol collectors of the autonomous pathogen detection system (APDS) that was recently developed by the Lawrence Livermore National Laboratory. The APDS is capable of continuous and fully autonomous monitoring for multiple airborne threat organisms and can be used as part of a monitoring network for urban areas and major public gatherings. The system has already been successfully tested with airborne Bacillus anthracis and Yersinia pestis biowarfare agents. The APDS aerosol collection system consists of a PM-style cap to remove large particles and a low-pressure drop virtual impactor preconcentrator positioned in front of a wetted-wall cyclone. The aerosol collectors operate at flow rates as high as 3750 l/min and collect airborne particles into 4 ml of liquid for subsequent detection. In our tests we determined the overall collection efficiency of the system by measuring the difference between inlet and outlet particle concentrations. The tests were performed with polydisperse oleic acid and monodisperse polystyrene latex (PSL) particles (0.6–3.1 µ m), and for three values of the major air flow rates in the virtual impactor (1760, 2530, and 3300 l/min), two values of the product, or cyclone, flow rates (375 and 450 l/min), and two different volumes of collection liquid (4 and 6 ml). We found that the cutoff size (d₅₀ ) of the entire collection system varied from 1.5 to 2.0 µ m when collecting PSL particles, with 3.1 µ m PSL particles being collected with efficiency of approximately 85%. When collecting oleic acid particles the d₅₀ of the entire system varied from 1.1 to 1.6 µ m. The concentration rates of the aerosol collection system were found to increase with increasing overall collection flow rate and approached one million per minute at the highest tested flowrates. Such high concentrating rates and high air sample volumes make the APDS collection system highly suitable for detecting low concentrations of airborne pathogens.