Measurement techniques which allow the detection of airborne nanoparticles are of great interest for e.g. exposure monitoring and quality control during nanoparticle production. An increasing number of commercial devices use a unipolar diffusion charger to charge the particles and determine the nanoparticle concentration and sometimes size. The analysis however may be biased by the presence of large particles. We therefore developed a preseparator that removes particles larger than 450 nm, i.e the minimum in the range of particle lung deposition curves, but only causes a low pressure drop. The preseparator uses a total flow rate of 2.5 L/min and consists of two stages. The first stage is a virtual impactor that removes particles larger than approximately 1 µm with a minor flow of 1 L/min. Particles above 450 nm are removed from the remaining 1.5 L/min in the cyclone of the second stage. The combination of a cyclone with a virtual impactor was shown to reduce the pressure drop of the preseparator from 8.1 to 5.6 kPa compared with a cyclone alone and improve the sharpness of the separation curve for cut-off diameters around 450 nm. Furthermore the virtual impactor extends the cleaning intervals of the preseparator, because large particles are no longer deposited in the cyclone. Eventually the preseparator was tested with an opposed flow diffusion charger and it was shown that particle charging is not affected by the pressure drop.