A method of separating charged particles using an analyzer is provided, the method comprising: causing a beam of charged particles to fly through the analyzer and undergo within the analyzer at least one full oscillation in the direction of an analyzer axis (z) of the analyzer whilst orbiting about the axis (z) along a main flight path; constraining the arcuate divergence of the beam as it flies through the analyzer; and separating the charged particles according to their flight time. An analyzer for performing the method is also provided. At least one arcuate focusing lens is preferably used to constrain the divergence, which may comprise a pair of opposed electrodes located either side of the beam. An array of arcuate focusing lenses may be used which are located at substantially the same z coordinate, the arcuate focusing lenses in the array being spaced apart in the arcuate direction and the array extending at least partially around the z axis, thereby constraining the arcuate divergence of the beam a plurality of times as it flies through the analyzer.