In mass spectrometry, ion optics process a received ion beam into an output ion beam travelling in an output direction and having a spatial distribution in a plane perpendicular to the output direction elongated in one dimension of the plane relative to the other dimension of the plane and defines an axis of elongation thereby. A quadrupole ion optical device comprises first and second pairs of opposing elongated electrodes, receiving the output ion beam travelling along the output direction and defining an acceptance axis in a plane perpendicular to the direction of elongation of the first and second pairs of opposing elongated electrodes. The acceptance axis is an axis on which maximum acceptance of ions to the quadrupole ion optical device is attained. The first and second pairs of opposing elongated electrodes are oriented substantially to match the acceptance axis to the axis of elongation defined by the spatial distribution.