A high field asymmetric waveform ion mobility spectrometer (FAIMS) includes at least two focusing electrodes that are separated one from the other by a gap, which decreases in width along a direction of ion flow. Within the gap is disposed an electrode assembly including a first electrode and a second electrode, approximately flat surfaces of the first and second electrodes facing one another so as to define a space of approximately uniform thicknesses therebetween. During use electrical signals are applied via an electrical contact on at least one of the first electrode and the second electrode, and on each one of the at least two focusing electrodes. The electrical signals are for establishing electrode electric field conditions within the space between the first electrode and the second, for selectively transmitting ions therethrough and for directing the selectively transmitted ions in a direction away from one of the at least two focusing electrodes and toward a central portion of the space.