The invention relates to a method and device for the fragmentation of macromolecules in linear quadrupole RF ion traps according to Wolfgang Paul. The invention consists in fragmenting the ions by the capture of low energy electrons (ECD), injected into the linear RF ion trap. One way of doing this is to inject low energy electrons through the gap between the pole rods. Another possibility is to inject the electrons through an opening in one of the pole rods carrying a RF voltage, the electron source being kept at the highest positive potential which is achieved on the center axis of the ion trap during the RF period. Both methods can be improved by pulse-shaped RF voltages, offering longer periods for electron capture. The electron beam can be guided by a magnetic field.