Patent ID: 7427751

Claim:
A mass spectrometer comprising: a main magnet including spaced-apart polepieces which define a gap, the main magnet producing a main magnetic field in the gap; an ion source to generate ions and to accelerate the ions into the main magnetic field in the gap, the ion source located outside the gap, wherein the ion source comprises: an extractor electrode that defines an elongated extractor slit; a reference electrode that defines an elongated reference slit; at least one filament positioned adjacent to the extractor slit to generate ionizing electrons; a repeller electrode spaced from the extractor electrode, wherein the ionizing electrons generated by the filament are accelerated from the filament to an ionization region near the extractor slit for ionization of a gas, and ions generated in the ionization region are extracted through the extractor slit and are accelerated through the reference slit into the gap between the polepieces of the main magnat; and a source magnet including spaced-apart polepieces that produce a source magnetic field to constrain trajectories of the ionizing electrons; and an ion detector to detect a selected species of the ions generated by the ion source and deflected by the main magnetic field, the ion detector located in the gap at a natural focus point of the selected species of the ions, wherein the dimensions and spacings of the extractor electrode, the reference electrode and the repeller electrode are configured to form a electric field that focuses ions generated in the ion source toward the extractor slit so that the main magnet deflects the ions of the selected species to the ion detector without use of a physical entrance slit.