A High Field Asymmetric Waveform Ion Mobility Spectrometry (FAIMS) apparatus comprises (a) a first and a second gas inlet; (b) an expansion chamber receiving ions from an ion source and the first and second gas flows from the first and second gas inlets, respectively; (c) an outer electrode having a generally concave inner surface and comprising: (i) an ion inlet operable to receive, from the expansion chamber, the ions and a combined gas flow comprising portions of the first and second gas flows; and (ii) an ion outlet; and (d) an inner electrode having a generally convex outer surface that is disposed in a spaced-apart and facing arrangement relative to the inner surface of the outer electrode for defining an ion separation region therebetween, wherein the combined gas flow and a portion of the ions travel through the ion separation region from the ion inlet to the ion outlet.