Abstract: The combination of an electrically conductive hollow body, a step-down transformer operable at a frequency greatly in excess of 1 kHz and solid phase temperature depressing means is used to realize the invention in its method and apparatus aspects. The temperature depressing means serve to bring the hollow body to a given depressed temperature so that when a gas component entrained in a gas carrier with which it forms mixture is passed through the hollow body for a period of, say, a few minutes the gas component condenses in the hollow body. The condensed component may later be freed and re-injected into the carrier by a process of thermal desorption which is achieved by direct ohmic heating of the hollow body via of the step-down transformer. The desorption time would typically extend over some ten seconds or so, as against the condensation time of several minutes, with the result that the gas component is concentrated in time, i.e.