A mass analyser comprises a pair of electrode arrays. Each array has a set of focusing electrodes which are supplied, in use, with voltage to create an electrostatic field in a space between the electrode arrays causing ions to undergo periodic, oscillatory motion in the space, ions passing between electrodes of the sets of focusing electrodes and being repeatedly focused at a center plane, mid-way between the electrode arrays. At least one electrode of each set of focusing electrodes has an electrode surface closer to the center plane than the electrode surfaces of other electrodes of the same set. The analyzer may be an ion trap mass analyser or a multi-turn ToF mass analyzer.