Design and performance of a pumped counterflow virtual impactor

Counterflow virtual impaction uses a flow of gas in a direction opposite the motion of the particles to separate them from gas and smaller particles. In the past such devices have used aircraft flight or wind tunnel flow to impart momentum to the particles. Here we describe the design and performance of an apparatus, termed a pumped counterflow virtual impactor (PCVI), which uses a vacuum pump to provide the flow. We show that this device is capable of inertial separation with a sub-micrometer cutoff diameter, particle enhancement approaching the ratio of the output to the input flow, and replacement of the ambient gas in the output flow with another gas that may be more suitable for downstream analysis techniques. Rejection of input gas and particles smaller than the cutoff diameter can exceed 99.9%.