Particle sampling from ambient air is a well-developed technology described in many texts (e.g., Vincent 1989; Willeke and Baron 1993; Hinds 1999). However, there are many cases in which the concentration of airborne particles is so low that it becomes necessary to sample for extended periods of time to collect enough mass for chemical or biological characterization of the collected sample. One of these cases is the identification of biological aerosols by mass spectrometry (Barshick et al. 1999: Hart et al. 2000). For such applications, the aerosol sampler delivers a representative sample of the airborne particles to a pyrolysis tube where, on a periodic basis, material is converted thermally and chemically to components that are analyzed by mass spectrometry. The mass rate of delivery of the material to the pyrolysis tube must be sufficient to allow the detection of biological aerosols with a concentration of about I particle per liter of air. A response time on the order of several minutes is desired. For this application it becomes necessary to concentrate particles from several hundred liters per minute of air to a small flow rate compatible with that of a pyrolysis tube followed by a mass spectrometer.
Methods for concentrating airborne particles in the 1.0 µm to 10 µm diameter range are limited. We chose to use virtual impaction because of the extensive theoretical and experimental background information available (Marple and Chien 1980; Chen and Yeh 1987; Loo and Cork 1988; Sioutasetal. 1994; Li and Lundgren 1997) and the relatively long history of its use for this specific application.
Virtual impactors are well known in the aerosol community and have typically been employed for ambient, workplace, and high-altitude sampling (Anderson et al. 1993; Marple et al. 1995: Lundgren et al. 1996; Laucks and Twohy 1998). These devices split the airflow into two streams, one of which is particle rich and one, particle lean. The device works on the same principle as a conventional impactor, namely, the inertia of the particles causes the particles to leave the fluid streamlines, allowing for a separation of the particles from the airflow. In a conventional impactor. particles with sufficient inertia or 'size' strike a collection plate where they remain for later analysis. In a virtual impactor. particles are drawn through acceleration nozzles and upon exiting these nozzles, encounter a receiving tube. Nearly a stagnation point occurs at the entrance to the receiving tube, and particles with sufficient inertia (above the cutpoint) continue forward with a small portion of the flow into the receiving tube. The How going into the receiving tube is called the 'minor' flow and is typically 5-25% of the flow entering the acceleration nozzle. The rest of the flow, called the 'major' flow, carrying only small particles (below the cutpoint), diverts around the receiving tube and is discarded. The large particles contained in the minor How are now concentrated by approximately the ratio of the total flow rate to the minor flow rate. However, a fraction of particles smaller than the cutpoint. with the same concentration as the ambient air, passes through the receiving tube with the minor flow concentrated large particles.
A high-performance aerosol concentrator for biological agent detection