The invention is directed to mass spectrometer comprising an extraction system for secondary ions. The system (18) comprises: an inner spherical deflecting sector (42); an outer spherical deflecting sector (44); a deflecting gap (46) formed between the sectors (42; 44); a housing (38) in which the sectors (42; 44) are arranged. The deflecting sectors (42; 44) are biased at retarding potentials in order to reduce the energy of the ion beam (16) entering the deflecting gap (46). The system (18) further comprises an exit disc electrode (64) with an exit through hole (66) centred about the exit axis (24), said intermediate electrode (64) being biased at an intermediate voltage between the voltage of the housing (38) and the average voltage of the sectors (42; 44). The trajectories of the secondary ions become more parallel to the exit axis (24) and become closer to said axis.