Particle deposition in different regions of a critical orifice assembly was studied numerically and experimentally. The investigated orifice is an O'Keefe E-9 (O'Keefe Control Co.) orifice whose diameter is 0.231 mm and critical flow rate is 0.455 slpm. The orifice assembly has an inlet tube (inner diameter = 10.4 mm, length = 90 mm) and outlet tube (inner diameter = 6.2 mm, length = 60 mm). In the numerical study, axisymmetric, laminar flow field of the orifice assembly was obtained first by solving the Navier-Stokes equations. The diffusion loss of nanoparticles was then calculated by solving the convection-diffusion equation. Inertial impaction and interception loss of 2-10 mu m particles was calculated by tracing particle trajectories in the flow field. In the experimental study, monodisperse NaCl (20-800 nm in aerodynamic diameter) and fluorescein-containing oleic acid (2-10 mu m in aerodynamic diameter) particles were used to test particle loss in both diffusion- and inertial impaction-dominated regimes. The numerical results were compared with the experimental data and good agreement was obtained with the maximum deviation smaller than 10.4%. (C) 2007 Elsevier Ltd. All rights reserved.