When a monodisperse aerosol expands through a thin-plate circular orifice into an evacuated chamber, the particles tend to focus at a downstream axial location that is dependent upon the Stokes number. Since viscous drag is negligible within the chamber, the particles continue on straight-line trajectories beyond the focal point forming a cone. This behavior has been exploited for size-selective transmission of ultrafine aerosol into a high vacuum chamber for chemical analysis. In this paper, we present an experimental and theoretical characterization of this behavior. The entire flow is modeled using a potential flow solution upstream of the orifice coupled with a paraxial model employing previously obtained empirical expressions for the centerline Mach number within the orifice. Particle trajectories are calculated from the resulting flow and compared to experimental measurements. Limiting particle trajectories are experimentally determined from measurements of the size of the particle beam cross section. Particle transmission efficiency is measured using a single-particle mass spectrometer. Despite the assumption of inviscid flow, the model provides reasonable agreement with the experimental measurements, suggesting that inertial separation is not strongly influenced by the boundary layer developing along the orifice walls. (C) 2004 Elsevier Ltd. All rights reserved.