Various novel mobility analyzer (MA) designs useful for the separation of gas phase ions and charged particles according to their electrical mobilities are discussed. Traditional differential mobility analyzer (DMA) designs have mostly been restricted to two-elements (electrodes) of either parallel or coaxial cylindrical geometries, with the inlet and the outlet on different elements, between which is maintained a large voltage difference. Calculations of the performance of several MA designs free from some such restrictions are presented in the case of potential flows. They include, for example, devices in which (i) the inlet and outlet are on the same element; (ii) the inlet and outlet are at the same voltage ("isopotential devices"); (iii) more than two-elements exist; (iv) a fair fraction of the sheath gas flow passes through one of the elements; (v) all ions within a finite mobility range originating at an inlet point are focused on the outlet (mobility focusing). These latter devices may be more properly described as "Ion Filters" because only ions within a tunable mobility range can reach the outlet. By restricting the "bandwidth" through additional elements or auxiliary sheath gas flow suction/injection, these filters can be used as high resolution DMAs. Isopotential aspiration counters have been used previously as atmospheric ion counters. (C) 2005 Elsevier Ltd. All rights reserved.