Abstract: Apparatus and method to improve the filtering action of a quadrupole mass filter by reducing a precursor fault caused by an asymmetric electrical field between a pair of opposing electrode rods in a y-axis of the quadrupole mass filter. Processing means for processing detector data determine the filtering action of the quadrupole mass filter by checking for a precursor fault. If a precursor fault is detected then power supply control means introduce an AC potential difference across the electrode rods in the y-axis in order to reduce asymmetry in the electrical field in the y-axis.

Abstract: A flexible coupling for connecting a pair of rotating elements, comprising a body part (10) adapted at each end to receive part of the rotating elements, and comprising four slits (20a) each slit being longitudinally and angularly spaced from the other slits, each slit terminating in a concave face (24). The invention provides advantages over known flexible couplings since the provision of arcuate and concave end faces on each slit produces an increased material overlap with adjacent slits which decreases variations in torsional stiffness but does not affect the flexibility of the coupling.