In or for a dual source mass spectrometry system (10), an ion source housing (16) for detachable connection to a mass spectrometer of the system. The ion source housing comprises a source chamber (22) having an outlet port for connection to a vacuum region of a mass spectrometer, a sample port for receiving a gas chromatography [GC] column and means for charging analyte molecules discharged from said GC column, wherein the housing comprises a docking means by which a GC interface probe can be releasably engaged with the housing.