Cold cathode ion source

A mass spectrometer comprising an ion source, a collector, and an electrostatic mass filter interposed between said source and collector, the ion source comprising:

• two cathodes spaced apart on the axis of the source, an anode interposed between the cathodes, said anode being of generally annular cross-section transverse to said axis, and magnet means for generating a magnetic field aligned in the direction of said axis and extending between said cathodes, said cathodes defining therebetween a space within which said anode is positioned and into which gas or vapour is introduced for generation of said ions, one of said cathodes being apertured for therethrough of said ions from said space;
• said ion source further comprising a first electrode, a second electrode and a third electrode, said first electrode being positioned on said axis adjacent said one cathode so that said one cathode is between said first electrode and said space, said second electrode having an axially aligned first aperture and being positioned adjacent said first electrode to define therewith a generally annular opening between the peripheries of said first electrode and said first aperture, said third electrode being disposed axially away from said second electrode and farther away from said space than is said second electrode, said third electrode having a second axially aligned aperture therein, whereby, upon application to said first and second electrodes of a positive potential and to said third electrode of a potential which is negative relative to that applied to said second electrode, there is created an electrostatic field at or around said first electrode and between said second and third electrodes and directed so as to cause high energy negative ions exiting from said opening to hit the first electrode and so as to cause low energy negative ions around said first electrode to be deflected through said first aperture and thence through said second aperture;
• the mass filter having predetermined electrical potentials applied thereto to cause said filter to generate an electrostatic field such that ions from said ion source and of particular energy are passed along a predetermined path from the filter to the collector;
• and
• said collector comprising:
• a collector member disposed away from said path and having a potential applied thereto so as to attract, from ions passing through the filter, only ions having energy levels in a particular range for which the filter is effective and such that ions of energy levels substantially above the particular range are not deflected from said path sufficiently to be collected.