This study investigates the effect of the nozzle throat length on the resolution of the low pressure impactor (LPI). Two basic nozzle geometries, rectangular slit and round nozzle with 25 nm cutpoint were investigated both experimentally and by numerical simulation. A new impactor stage with variable nozzle throat length and jet-to-plate distance was designed, built and tested. The impactor was calibrated at four different configurations with monodisperse dioctyl sebacate (DOS) aerosol. A very good agreement between the simulated and the experimental resolutions and cutpoints were found. The main conclusions are that (a) the length of the nozzle is a crucial parameter affecting the resolution and (b) it is possible to achieve a better resolution with a rectangular slit type impactor than with a round nozzle type LPI. The best observed resolution was achieved with the slit type LPI with a nozzle throat length to width ratio of only 0.33. Compared to similar cutpoint impactor stages of commercially available LPIs, the resolution of the new stage is by far the highest. (c) 2012 Elsevier Ltd. All rights reserved.