A mass spectrometer comprising: an ion energy filter 14 arranged and configured to filter ions according to their kinetic energy and so as to only transmit ions having a component of kinetic energy in a first dimension (z-dimension) that is within a selected range; and a multi-reflecting time of flight mass analyser or mass separator 1 having an ion accelerator 6, and two gridless ion mirrors 2 that are elongated in the first dimension (z-dimension) and configured to reflect ions multiple times in a second orthogonal dimension (x-dimension), wherein the ion accelerator 6 is arranged to receive ions from the energy filter 14 and accelerate the ions into one of the ion mirrors 2.