A method and apparatus are provided for reducing undesired isotopes of ion implanted species from an ion beamline. The apparatus described herein is a mass spectrometric variable exit aperture that selectively reduces the size of the exit aperture as perceived by the ion beam. In one embodiment, the variable mass analyzing exit is located in the mass analyzer at the upstream position of the decomposition aperture and is effective to size the exit aperture to allow passage of the desired injected isotope (s) while blocking the passage of unwanted injection isotopes . In one particular embodiment, the mass spectrometry variable exit aperture is mounted to the beam guide, which is eventually mounted between the poles of the AMU magnet (s). The mechanical drive mechanism enables the shield structure to be moved into the beam path in an incremental manner as guided by a control unit tied to the beam properties.
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