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The shock wave system of the underexpanded free jet, in a region where the pressure is in the range of 10−2 up to 1 torr, is partly invaded by background molecules, these latter being pumped and carried by the supersonic flow. When a gas mixture is introduced into the nozzle chamber and outside the shock barrel, this aspiration acts very selectively on the light molecules, and a mixture enriched in lighter species is attainable by skimming the free jet. This new aerodynamic separation effect, which is applicable to isotopic mixtures, is very strong in comparison to the efficiency of an elementary gaseous diffusion membrane.