A multipurpose apparatus is useful in either prefiltering ions or in forming ions from residual gases, prior to the analysis of the prefiltered or formed ions in a quadrupole type mass spectrometer. The apparatus includes a cylindrical grid, an annular entrance aperture wherein a center circular baffle has a unidimensional electrode extending along the axis of the grid, and a circular exit aperture, the axis of the grid being in line with the axis of a quadrupole type mass analyzer. The application of appropriate potentials to these members allows only low energy ions to pass therethrough and into the quadrupole analyzer. High energy ions or neutral particles having an axial trajectory are blocked by the baffle, while divergent high energy charged particles are insufficiently deflected and hence do not pass through the exit aperture. The apparatus further includes a filament positioned outside the grid, which, when energized, emits electrons. The electrons are accelerated toward the grid, passing therethrough whence they ionize residual gas atoms therein, which ionized atoms are extracted through the exit aperture and pass into the quadrupole analyzer.
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