The invention provides a mass spectrometer for the analysis of a sample dissolved in a super-critical fluid. The fluid is expanded directly into an ionization chamber without a liquid-gas transition and the sample is ionized by means of a glow discharge established in the chamber. Preferably the electrodes between which the discharge is struck comprise the wall of the ionization chamber and the tube through which the super-critical fluid enters the chamber. The spectrometer is primarily intended for use with a super-critical fluid chromatograph.