In a mass spectrometer with an electrostatic ion trap, the electrostatic ion trap has an outer electrode with an ion-repelling electric potential applied to it and a plurality of inner electrodes with ion-attracting potentials applied to each inner electrode. The outer electrode and the inner electrodes are shaped and arranged in such a way that a harmonic electric potential is formed in one spatial direction and, perpendicular to this spatial direction, an electric potential is formed in which ions move in stable, radial trajectories.