An ion source and method for providing ionized particles to a molecular/atomic analyser, such as a mass spectrometer, are disclosed. The ion source includes a vessel defining a channel; a gas inlet extending from the gas source into the channel, for introducing a gas flow into the channel; a sample inlet extending into the channel for introducing sample within the channel; and an ionizer to ionize the sample in the channel. The vessel is sufficiently sealed to allow the channel to be pressurized, at a pressure in excess of 100 Torr. At least one gas source maintains the pressure of the channel at a pressure in excess of 100 Torr and the pressure exterior to the channel at a pressure in excess of 0.1 Torr and provides a gas flow that sweeps across the ionizer to guide and entrain ions from the ionizer to the outlet.