A linear ion trap (6, 7, 8)is disclosed comprising a central quadrupole rod set (6)and a post-filter quadrupole rod set (8). A 180° phase difference is maintained between axially adjacent rod electrodes of the central quadrupole rod set (6) and the post-filter quadrupole (8) so that an axial pseudo-potential barrier is created between the central quadrupole rod set (6) and the post-filter quadrupole (8). A supplementary AC voltage is applied to the rods of the central quadrupole (6) in order to radially excite ions which are desired to be ejected from the ion trap. The ions are ejected from the ion trap (6, 7, 8) non-adiabatically in an axial direction.