The disclosed method and apparatus couple a membrane interface directly to a mass spectrometer at atmospheric pressure. The membrane may be in capillary or sheet form and allows the introduction of a liquid or gaseous sample to one side of the membrane while the other side of the membrane is bathed with a solution that can easily be used in an atmospheric pressure ionization source. Volatile molecules permeate through a suitable membrane such as poly-dimethyl silicone (PDMS), mix into the appropriate solvent, and are ionized. Because of the rules governing abstracts, this abstract should not be used in construing the claims.