Abstract: A quadrupole mass spectrometer wherein the quadrupole and the ion detector are not in axial alignment with each other, with an exit apertured plate having an exit aperture disposed adjacent the exit end of the quadrupole and an entrance apertured plate having an entrance aperture disposed in front of the ion detector. An electrode in the form of a hollow cylinder or a plate having a circular aperture formed therein is interposed between the exit apertured plate and the entrance apertured plate so that the central axis of the aperture of the electrode is aligned with that of the entrance aperture to the ion detector. A controller is provided to apply a voltage individually to the exit apertured plate and the interposed electrode so that the action of the fringing electric field about the exit end of the quadrupole to cause divergence of the ion beam is substantially suppressed.