Resolution of the radial differential mobility analyzer for ultrafine particles

The resolution of the radial differential mobility analyzer (radial DMA) for particles in 3–60 nm diameter range is probed through tandem radial DMA measurements employing identical radial DMAs. The observed broadening of the range of transmitted particles with decreasing particle Peclet number was shown to be consistent with Stolzenburgʹs (1988, Ph.D. Thesis, University of Minnesota) model of diffusion broadening of the transfer function, although the broadening was somewhat greater than predicted. A similar, but smaller, deviation is seen in Stolzenburgʹs data obtained using a cylindrical DMA. The enhanced broadening is thought to result from flow disturbances within the DMA. Diffusional deposition of particles in the radial DMA for two different sheath flow rates correlated well with Pe−2/3, while electrophoretic particle losses in the transition from high voltage at the outlet of the DMA to grounded tubing are shown to be independent of the particle Peclet number. The latter effect is, however, small for the radial DMA. Consistent with observations previously made using cylindrical DMAs, the voltage corresponding to the peak in the number concentration is slightly higher for the second DMA than for the first one. This apparent decrease in mobility correlates with Pe−1.