SIZING NANOPARTICLES WITH A FOCUSING IMPACTOR - EFFECT OF THE COLLECTOR SIZE

Aerosol impactors generally use a collector with a diameter d(c) consi derably larger than that of the nozzle d(n). Focusing impactors, howev er, bring all the aerosol very near the central streamline and may ope rate at d(c) < d(n). Some advantages of this possibility are investiga ted here including: (i) reduction of subcritical particle deposition r ate by Brownian motion (proportional to d(c)(2)) and concomitant incre ase in resolution without the need for electrostatic blowing; (ii) red uction of the Stokes number S at 50% collection efficiency, which low ers the minimum particle size that the impactor can measure. This seco nd advantage is limited, however, because the reduction in S hurts th e efficiency of the focusing lenses, broadening the aerosol beam. When this increasing beam width exceeds the decreasing collector diameter, impactor resolution is drastically reduced. Based on measurements wit h mobility diameters as small as 9.7 nm, the minimum diameter of unit density spheres analyzable with this impactor is found by extrapolatio n to be 4.1 nm with electrostatic repulsion, and 3.13 nm without it. ( C) 1998 Elsevier Science Ltd. All rights reserved.