Sizing nanoparticles and ions with a short differential mobility analyzer

An analysis of the effects of diffusion in differential mobility analyzers (DMAs) at high Peclet number (Pe) shows that the associated peak broadening may be greatly reduced when the axial separation between entrance and exit slits is comparable to the inter-electrode gap. This prediction is verified in a shortened version of Reischlʹs DMA using molecular ions from an electrospray source. Gaussian peaks with relative full width at half maximum as small as 0.066 are observed at a DMA Reynolds number of 1190 in the mobility spectra of (butyl)4N+ ions in air (Pe = 104). After correcting for the broadening effect of the aerosol flow rate and the sampling slit width, the predicted diffusive peak widths agree well with those observed, but only at the highest resolutions. The DMA described here is the first one able to measure with excellent resolution the mobilities of particles down to 1 nm in diameter, and even of molecular ions.