A review of a number of widely used impactors suggests that the poorer performance often seen in some of the stages is due to the onset of turbulence in the jet at too large values of either the Reynolds number Re, or the nozzle-to-collector distance L. This phenomenon is studied here by measuring the collection efficiency versus Stokes number curves eta(S) of a low-pressure thin-plate-orifice impactor as a function of Re and L (measured in units of the orifice diameter d(n)). A drastic broadening of the eta(S) curve is observed in the vicinity of a critical Reynolds number Re*(L/d(n)). Re* increases at diminishing L/d(n), taking values near 800 and 400 for L/d(n) of 3.1 and 4, respectively. No transition is seen at L/d(n) = 1 or 2, even at Re as high as 2700. This transition in the jet modifies both the high and the low S tails of the eta(S) curve. It should be distinguished from a previously studied turbulent transition of the boundary layer near the collector plate, which arises at much larger Reynolds numbers, changes only the low Stokes number tail of the eta(S) curves and disappears when using small collector plates. A specialized experimental apparatus is used to provide an initial jet with very low turbulence level, as well as to isolate incipient turbulence effects from other mechanisms leading to broadening of the eta(S) curves. The particles are brought very close to the axis via aerodynamic focusing, while particle capture by Brownian diffusion is offset with a repulsive electric field. Free-stream turbulence ahead of the impactor nozzle is eliminated by passing only a small fraction of the flow through the critical orifice and the focusing lenses. The remaining gas required to attain jet Reynolds numbers up to 3700 is introduced laminarly and axisymmetrically as sheath air through an outer porous wall right before the impactor nozzle. At Re in the range of a few tens, a strong increase of the critical Stokes number with increasing L/d(n) is observed. (C) 2002 Elsevier Science Ltd. All rights reserved.