A mass spectrometer system , comprising: an ion source housing defining an ionization chamber , the ionization chamber comprising a curtain plate defining a curtain plate aperture through which ions generated in the ionization chamber from a sample can be transmitted to one or more downstream mass analyzers; an orifice plate defining a sampling orifice, said orifice plate being separated from the curtain plate so as to define a curtain chamber therebetween through which ions from the curtain plate aperture can be transmitted to the sampling orifice, wherein the system is configured to provide a pressure differential between either side of the orifice plate such that the transmitted ions undergo a vacuum expansion at the sampling orifice; a power supply electrically coupled to the curtain plate and the orifice plate for providing electrical signals thereto; and a controller operatively coupled to the power supply, said controller configured to control the electrical signals applied to the curtain plate and orifice plate so as to modulate the electric field within the curtain chamber from a first configuration during a first period in which ions are preferentially transmitted through the sampling orifice and a second configuration during a second period in which ions are substantially prevented from being transmitted through the curtain chamber to the sampling orifice; characterized in that the system further comprises: a curtain gas supply for flowing curtain gas into the curtain chamber, wherein the curtain gas flow is effective to prevent at least a portion of molecules within the sample from transiting to the sampling orifice. an ion source housing defining an ionization chamber , the ionization chamber comprising a curtain plate defining a curtain plate aperture through which ions generated in the ionization chamber from a sample can be transmitted to one or more downstream mass analyzers; an orifice plate defining a sampling orifice, said orifice plate being separated from the curtain plate so as to define a curtain chamber therebetween through which ions from the curtain plate aperture can be transmitted to the sampling orifice, wherein the system is configured to provide a pressure differential between either side of the orifice plate such that the transmitted ions undergo a vacuum expansion at the sampling orifice; a power supply electrically coupled to the curtain plate and the orifice plate for providing electrical signals thereto; and a controller operatively coupled to the power supply, said controller configured to control the electrical signals applied to the curtain plate and orifice plate so as to modulate the electric field within the curtain chamber from a first configuration during a first period in which ions are preferentially transmitted through the sampling orifice and a second configuration during a second period in which ions are substantially prevented from being transmitted through the curtain chamber to the sampling orifice; characterized in that the system further comprises: a curtain gas supply for flowing curtain gas into the curtain chamber, wherein the curtain gas flow is effective to prevent at least a portion of molecules within the sample from transiting to the sampling orifice. <EOS>