Differential mobility analysis of molecular ions and nanometer particles

Differential mobility analyzers (DMAs) can classify a steady and narrowly monomobile stream of charged particles from a continuum spectrum of electrical mobilities. This feature has been widely exploited in aerosol research involving particles larger than some 10 nm. But high losses and low resolution due to Brownian diffusion have traditionally impaired DMA use for smaller particle sizes. These aerosol instruments have thus been poor competitors with time of flight drift tubes for mobility-based analysis of molecular ions and clusters. Recently, however, these problems have been overcome. Specially designed and run DMAs can now supply a pure and steady stream of molecular ions or clusters in a gas by classification from a complex ion mixture. The potential of such devices for investigations involving ions and nanoparticles is illustrated through various examples of recent work.