Resolution improvements of a nano-DMA operating transonically

The differential mobility analyzer (DMA) commercialized by RAMEM under the nickname Río 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=0.0094FWHH=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.