The invention is directed to mass spectrometer comprising an extraction system for secondary ions. The system comprises: an inner spherical deflecting sector; an outer spherical deflecting sector; a deflecting gap formed between the sectors; a housing in which the sectors are arranged. The deflecting sectors are biased at retarding potentials in order to reduce the energy of the ion beam entering the deflecting gap. The system further comprises an exit disc electrode which is biased at the midvoltage of the average voltage of the sectors, and two side plates both facing the spherical sectors, the side plates being biased in order to create an electrostatic field perpendicular to the exit axis.