Abstract: The invention provides a mass spectrometer in an optical emission spectrometer having a glow discharge source which is suitable for the analysis of solid non-conducting samples. The source comprises a chamber into which an inert gas is introduced and maintained at a pressure substantially below atmospheric pressure, and two electrodes between which a glow discharge is struck by application of a direct or RF voltage to the electrodes. The cathode electrode comprises the solid non-conducting sample and an auxiliary electrode disposed between the discharge and the sample. The auxiliary electrode comprises an aperture through which the sample is exposed to the discharge. In the case of a mass spectrometer, ions formed in the discharge pass through an aperture in the chamber to a mass analyzer, while in the case of an optical emission spectrometer the chamber is sealed by a window through which radiation generated in the discharge can pass to an optical spectral analyzer.