An electrostatic ion trap for mass analysis includes a first array of electrodes and a second array of electrodes, spaced from the first array of electrode. The first and second arrays of electrodes may be planar arrays formed by parallel strip electrodes or by concentric, circular or part-circular electrically conductive rings. The electrodes of the arrays are supplied with substantially the same pattern of voltage whereby the distribution of electrical potential in the space between the arrays is such as to reflect ions isochronously in a flight direction causing them to undergo periodic, oscillatory motion in the space, focused substantially mid-way between the arrays. Amplifier circuitry is used to detect image current having frequency components related to the mass-to-charge ratio of ions undergoing the periodic, oscillatory motion.