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.