The present invention relates to a mass spectrometer that includes an ionization source having a chamber for ionizing a fluid sample. The ionization chamber has surfaces to reduce the overall interaction with reactive samples. The inner surface walls of the ionization chamber may be formed from an inorganic conductive nitride or disulfide material or may be applied to a substrate as a coating. The invention also includes a method for reducing the interaction of a reactive analyte with the inner wall of the chamber by application or coating the inner wall of the chamber with an inert conductive material.