A filter including multiple tanks connected in series by connecting ducts. In the filter a flow of gaseous flows encounters multiple obstacles in it, an effect of which is efficiently to block constituents with heaviest molecular weight, without substantially hindering diffusion of lighter constituents. This may be obtained by internal baffles, or by a bank of finer ducts, for example at an inlet of the filter. Such a filter may find application to separation of hydrogen and radon produced by nuclear waste.