Continuous-flow differential mobility analysis of nanoparticles and biomolecules

The differential mobility analyzer (DMA) is a powerful instrument that continuously separates aerosol particles according to their migration velocities in an electric field with high resolution. Because of the low fields employed, the mobility can be related to particle size or ion cross section. Combined with a sensitive detector, such as a continuous-flow condensation particle counter, the DMA enables differential size distribution measurements to be made within minutes to seconds. Over the past few decades, these capabilities have made the DMA a central tool for aerosol characterization in the 10-1,000-nm size range. DMAs have been adapted recently for measurement of particles as small as 1 nm and are now contributing to our understanding of nucleation, nanotechnology, and gas ions. Moreover, the opposed migration classifier, a new approach to differential mobility analysis, expands the dynamic range and shows promise both for increasing resolution beyond present levels and for changing the way that instruments are built. Thus, the DMA continues to advance methods and capabilities for physical characterization at transition from molecules to clusters to particles.