Impactors used in tandem with a DMA measure particle mass independently of density and shape. When operated hypersonically at pressure ratios of several thousands, they appear to be capable of analyzing unit density particles smaller than 2 nm. Because calibration techniques with oil drops below some 8 nm are difficult, the calibration particles used in this study are singly charged commercial mass standards of Polyethylene glycol (PEG) molecules with diameters from 2.1 to 9.8 nm. They are formed from singly dissolved PEG molecules and their clusters (up to the hexamer) by electrospray followed by charge-reduction. Several impactors running at pressure ratios as high as 3600 are tested. Their capture efficiency curves show well defined steps in all cases, but with a tendency to become broader at the smallest particle sizes. The critical nozzle to plate distance L required for impaction becomes anomalously small at decreasing particle size. This trend is provisionally attributed to rarefaction effects in the post-shock region, due to internal relaxation. An impactor with a larger pressure ratio should hence be able to analyze unit density particles smaller than 2 nm prior to the onset of translational non-equilibrium. (C) 2003 Elsevier Ltd. All rights reserved.