The present invention relates to an apparatus and method for focusing, separating, and detecting gas-phase ions using the principles of electrohydrodynamic quadrupole fields at high pressures, at or near atmospheric pressure. Ions are entrained in a concentric flow of gas and travel through a high-transmission element into a RF/DC quadrupole, exiting out of the RF/DC quadrupole, and then impacting on an ion detector, such as a faraday plate; or through an aperture or capillary tube with subsequent identification by a mass spectrometer. Ions with stable trajectories pass through the RF/DC quadrupole while ions with unstable trajectories drift off-axis collide with the rods and are lost. Alternatively, detection of ions with unstable trajectories can be accomplished by allowing the ions to pass through the rods and be detected by an off-axis detector. Embodiments of this invention are devices and methods for focusing, separating, and detecting gas-phase ions at or near atmospheric pressure, when coupled to mass spectrometers.