A microslot impactor has been designed with the objective of collecting large, concentrated samples suitable for chemical analysis of trace organic constituents. It employs a single slotted orifice per stage, with orifice widths as small as 0.1 mm. Using a relatively small pump (13 kg), five cutpoints from 0.1 to 3.5 mu m are obtained at st flow rate of 36 l min(-1) (6 x 10(-4) m(3) s(-1)). The microslot impactor operates with incompressible how and minimal pressure drops for the upper four stages, and sonic flow through the last (fifth) stage. Measured cutpoints for the incompressible flow stages are somewhat higher than predicted, with critical Stokes numbers corresponding to 10 to 30% greater than theory.