A differential mobility apparatus comprising: a housing having an entrance and an exit; at least two parallel plate electrodes disposed within said housing separated from one another by a fixed distance, the volume between the two parallel plate electrodes defining an ion path through which ions flow from the entrance to the exit, the ion path having a cross-sectional area normal to the direction of ion flow; a voltage source for providing RF and DC voltages to at least one of the parallel plate electrodes to generate an electric field, the electric field configured to pass though selected ion species based on mobility characteristics, the electric field including an asymmetric field waveform applied between the at least two parallel plate electrodes; a drift gas supply for supplying a gas that flows through the entrance to the exit; and at least one entrance electrode plate, the at least one entrance electrode plate having an aperture for allowing the traversal of ions and the gas into the housing; wherein the drift gas flowing through the entrance to the exit is at ambient pressure; the differential mobility apparatus further comprises a vacuum source positioned downstream from said parallel plate electrodes; and the at least one entrance electrode plate is sealingly engaged to the entrance so as to prevent inflow or outflow of gas other than the gas flowing through the aperture or through the exit, and the at least one entrance plate electrode is electrically separated from the parallel plate electrodes, and the cross-sectional area of the aperture is less than the cross-sectional area of the ion path a housing having an entrance and an exit; at least two parallel plate electrodes disposed within said housing separated from one another by a fixed distance, the volume between the two parallel plate electrodes defining an ion path through which ions flow from the entrance to the exit, the ion path having a cross-sectional area normal to the direction of ion flow; a voltage source for providing RF and DC voltages to at least one of the parallel plate electrodes to generate an electric field, the electric field configured to pass though selected ion species based on mobility characteristics, the electric field including an asymmetric field waveform applied between the at least two parallel plate electrodes; a drift gas supply for supplying a gas that flows through the entrance to the exit; and at least one entrance electrode plate, the at least one entrance electrode plate having an aperture for allowing the traversal of ions and the gas into the housing; wherein the drift gas flowing through the entrance to the exit is at ambient pressure; the differential mobility apparatus further comprises a vacuum source positioned downstream from said parallel plate electrodes; and the at least one entrance electrode plate is sealingly engaged to the entrance so as to prevent inflow or outflow of gas other than the gas flowing through the aperture or through the exit, and the at least one entrance plate electrode is electrically separated from the parallel plate electrodes, and the cross-sectional area of the aperture is less than the cross-sectional area of the ion path