Apparatus for analyzing sample gas consisting of a trace gas contained in a carrier gas. The sample gas is carried through a cylindrical duct; ions from the trace gas are formed, and are subjected to a transverse electric field which concentrates the flux of desired ions into an axial stream located away from the duct walls. The electric field may be produced by a combination of the duct walls and a central axial needle electrode in the duct, or by conical focussing screens or by cylindrical lens elements. The concentrated flux of ions passes through spaced interface and orifice plates containing a gas curtain between them and then passes into a vacuum chamber containing a mass spectrometer or other detector. The curtain gas, which blocks flow of carrier gas into the vacuum chamber, is cryo pumpable (e.g. CO.sub.2). The vacuum chamber contains surfaces cooled so as to cause deposit of the curtain gas thereon, thereby maintaining a vacuum. The cryo pumpable curtain gas arrangement may also be used to transfer other species or materials, such as materials arranged in a continuous strip, between two gaseous media, one at lower pressure than the other, without transfer of one such medium into the other.