Differential Mobility Analysis for optimum separation of ions and charged particles
The Differential Mobility Analyzer, DMA is a peculiar IMS device combining a flow field U and an electric field E between two parallel plates. It disperses a beam of ions into a fan, such that only the ions with a well defined electrical mobility are transmitted into an outlet slit leading to the MS inlet. The main advantage of the DMA over other more familiar IMS instruments is the simplicity of its integration as a front-end into a preexisting MS with an atmospheric pressure source. The integration to form a DMA-MS instrument requires essentially no change in the vacuum system of the MS. The DMA measures true mobility, generally in air, N2 or CO2). The DMA is also unique in enabling mobility determination of ions as produced by the ES source, without the structural modifications that tend to arise when an ion crosses a vacuum interface, an ion guide, or is injected into a drift cell.
Figure 1: Operation of the differential mobility Analyzer (DMA). Ions are fed through a slit on the upper plate, and dispersed into a fan by combined fluid flow (rightwards) and electric (downwards) fields. Ions of a pre-selected electrical mobility are drawn through an exit hole into the mass spectrometer.