GC-MS analysis apparatus has an interface section between GC and MS sections, which is located with respect to the direction of an analyte flow downstream of the GC section and upstream of the MS section. The interface section comprises at least one membrane with at least one orifice capable of establishing a molecular flow condition in the analyte passing between the GC and MS sections through the membrane. The membrane is subjected to a pressure differential such that the pressure pa in a region located upstream of the membrane is higher than the pressure pb in a region located downstream of the membrane.