An ion source for a mass spectrometer which operates at low pressure, the ion source comprising: a plurality of sample ionisers operative at atmospheric pressure to provide a plurality of streams of sample ions; an interface chamber, operable at a pressure lower than atmospheric pressure, having a plurality of entrance orifices located at a first position to collect sample ions into the interface chamber from said streams of sample ions and an exit orifice at a second position downstream of the said first position of the entrance orifices, for said sample ions to exit the interface chamber to the mass spectrometer, the interface chamber defining a plurality of separate ion paths for ions to travel between a respective one of the entrance orifices and the exit orifice; and ion control means, located downstream of the said entrance orifices, and arranged selectively to prevent ions from passing along a chosen one or ones of the ion paths to the mass spectrometer, the ion control means including gating means which, when open, permits passage of a selected one or ones of streams of sample ions to the mass spectrometer, the gating means being provided within the interface chamber between the said first and second positions or ion trapping means arranged selectively to prevent ions entering it from exiting therefrom; and wherein the interface chamber defines a plurality of interface channels (60, 70) each in communication with a corresponding one of the entrance orifices, each interface channel in turn constraining a corresponding one of the said streams of sample ions to follow a corresponding one of the said ion paths and wherein at least some of the interface channels (60, 70) are adapted so as to prevent a direct line of sight between their corresponding entrance orifice and the said exit orifice. a plurality of sample ionisers operative at atmospheric pressure to provide a plurality of streams of sample ions; an interface chamber, operable at a pressure lower than atmospheric pressure, having a plurality of entrance orifices located at a first position to collect sample ions into the interface chamber from said streams of sample ions and an exit orifice at a second position downstream of the said first position of the entrance orifices, for said sample ions to exit the interface chamber to the mass spectrometer, the interface chamber defining a plurality of separate ion paths for ions to travel between a respective one of the entrance orifices and the exit orifice; and ion control means, located downstream of the said entrance orifices, and arranged selectively to prevent ions from passing along a chosen one or ones of the ion paths to the mass spectrometer, the ion control means including gating means which, when open, permits passage of a selected one or ones of streams of sample ions to the mass spectrometer, the gating means being provided within the interface chamber between the said first and second positions or ion trapping means arranged selectively to prevent ions entering it from exiting therefrom; and wherein the interface chamber defines a plurality of interface channels (60, 70) each in communication with a corresponding one of the entrance orifices, each interface channel in turn constraining a corresponding one of the said streams of sample ions to follow a corresponding one of the said ion paths and wherein at least some of the interface channels (60, 70) are adapted so as to prevent a direct line of sight between their corresponding entrance orifice and the said exit orifice.