A method of obtaining a mass spectrum of elements in a sample is disclosed. Sample precursor ions having a mass to charge ratio M/Z are generated, and fragmented at a dissociation site, so as to produce fragment ions of mass to charge ratio m/z. The fragment ions are guided into an ion trap of the electrostatic or “Orbitrap” type, the fragment ions entering the trap in groups dependent upon the precursor ions M/Z. The mass to charge ratio of each group is determined from the axial movement of ions in the trap. The electric field in the trap is distorted. Ions of the same m/z, that are derived from different pre-cursor ions, are then separated, because the electric field distortion causes the axial movement to become dependent upon factors other than m/z alone.