Abstract: A membrane-introduction mass spectrometer (MIMS) device has a sample inlet assembly provided with a membrane held by an outer retaining ring across the entrance of a central passage, a porous metal frit backing the membrane, and a cylindrical support piece supporting the frit. In a preferred embodiment, the membrane is a 10-micron thickness layer of silicone evenly coated upon an inert polymer backing material. The porous frit is a titanium or steel sponge metal. The cylindrical support piece is made of titanium with small, drilled thru-holes to allow passage of gases and volatile organics into the mass spectrometer. The sample inlet assembly includes a high-pressure temperature probe for sensing fluid temperature to correct for temperature variations in membrane diffusion rates. The sample inlet assembly is provided as a front end to an underwater sampling probe.