The invention relates to methods and devices for the effective introduction of ions, which are stored in an RF ion guide into a quadrupole ion trap. The invention consists of arranging a switchable ion lens between the RF ion guide and the quadrupole ion trap, and introducing the ions into the quadrupole ion trap by a suitable connection of the ion lens only during the filling period, while otherwise the ions are reflected back into the RF ion guide. The filling period can be divided up and limited to the capture intervals of the quadrupole ion trap during each RF period. Measurement of the filling rate can be made by switching open the ion lens longer then the admission interval of the quadrupole ion trap, and measuring the flow of the ions passing through on the detector.