An ion separation device is configured to operate at sub-ambient or below atmospheric pressure. The device comprises an ion entrance aperture 2 and an ion exit aperture 8 which are spatially separated in a first direction 14 and second direction 16. A driving force urges ions through the device in the first direction. A separating force urges the ions in the second direction and causes ions to separate according to a physicochemical property, such that only ions having a particular physicochemical property value exit the device through the exit aperture. An exit aperture 30 may also be separated from the entrance aperture in a third orthogonal direction 28, and a different separating force may urge the ions in the third direction to cause ions to separate according to a different physicochemical property. The separation techniques may be selected from ion mobility separation, low electric field ion mobility separation, high electric field ion mobility separation, differential mobility separation, or mass to c
