A method for reducing the energy spread of ions over a specific and limited mass to charge ratio range is disclosed, along with an ion deceleration arrangement for implementing such a method. An electric field, having an electric field strength E is generated by a deceleration electrode arrangement (250). Ions of a specific and limited mass to charge ratio range, but having a spread of energies, are directed into the decelerating electric field generated by the deceleration electrode arrangement (250). The decelerating electric field is then removed, once substantially all of the ions of the specific mass to charge ratio range have entered the decelerating electric field. By matching the electric field strength E to the energy spread of the ions upon entry into the electric field, the energy spread of the said ions is reduced. Preferred embodiments of the invention employ energy dispersion upstream of the ion deceleration arrangement. For example, an ion mirror arrangement (200) may be used, the ions reflecting off an ion mirror (220) within that ion mirror arrangement (200) to promote energy defocusing.