Abstract: A mass spectrometer (1) suitable for the analysis of the eluent of a liquid or supercritical fluid chromatograph is disclosed. In order to maximize the flow rate of fluid which can be accepted into its ionization chamber (13), aperture closing means (13) are provided between an electron or particle source (27) and an entrance aperture (40) into the ionization chamber through which electrons or particles enter the chamber when required to ionize the sample therein. The aperture closing means are operable to close said aperture when such ionization is not required, thereby increasing the maximum flow rate into the ionization chamber. Sample ionization may then be effected by means of a glow discharge in the chamber or by a liquid ionization process such as thermospray ionization.