In a mass spectrometer, ions from an ion source pass through an inlet aperture into a vacuum chamber for transmitting prior to mass analysis by the mass analyzer. The configuration of the inlet aperture forms a sonic orifice or sonic nozzle and with a predetermined vacuum chamber pressure, a supersonic free jet expansion is created in the vacuum chamber that entrains the ions within the barrel shock and Mach disc. Once formed, an ion guide with a predetermined cross-section to essentially radially confine the supersonic free jet expansion can focus the ions for transmission through the vacuum chamber. This effectively improves the ion transmission between the ion source and the mass analyzer.