A molecular separator was specifically designed to maintain GC column efficiency with high resolution columns. The separator uses a porous silver membrane and has a flow path free of unswept pockets with a low internal volume (15 µl). In the flow rate range of 2–7 cm3/min the separator demonstrated yields of 25–60% and enrichments from 2 to 10. A comparison of GC peak widths from the flame detector and the total ion monitor showed no detectable peak spreading in the molecular separator.