A partial ovoidal FAIMS apparatus for separating ions is presented with inner and outer electrodes that have geometrically matched convex and concave partial ovoidal surfaces, respectively, that share at least one common defining axis and display a nearly constant displacement between the opposing surfaces. The ovoidal surfaces can be that of an egg shaped ovoid, an ellipse, or a sphere. The apparatus can have at least one ion inlet and an ion outlet where the outlet is on or as near as possible to a point on the defining axis in the outer electrode and the inlet or inlets are on, near, or symmetrically disposed about the defining axis of the ovoidal surfaces. Electrical contacts to the electrodes permit the application of an asymmetric waveform and a compensation voltage to at least one of the electrodes. The electrodes are positioned, stabilized, and insulated from each other by a base plate.