The differential mobility analyzer (DMA) commercialized by RAMEM under the nickname Rio Arriba has been evaluated with an emphasis on its transonic operation. The DMA is designed to have a very low pressure drop, with (i) an inner electrode supported upstream in a region of wide cross section, and (ii) a fully axisymmetric exhaust system including a diffuser and no constriction at all. The low flow resistance design is successful, as the instrument attains a flow rate of 2200 l/min (a Reynolds number of 62,000 based on mean speed and gap between the two cylindrical electrodes) as well as sonic conditions. A record resolution of 106 (full width at half height, FWHH = 0.0094) is seen at the sonic point with ions 1 nm in diameter. Remarkably, this performance is very close to the ideal limit set by Brownian motion alone, implying that the DMA is exceptionally well centered. Peak widths are non-ideal below the transonic region, perhaps due to upstream radiation of unsteady flow perturbations from the exhaust region to the working section. (C) 2005 Elsevier Ltd. All rights reserved.