The present invention relates to a method and device for measuring m/z ratios of ions in ion cyclotron resonance (ICR) mass spectrometry. The described ion traps for ICR mass spectrometry are distinct from the previous configurations by having one or many narrow aperture (flat) detection electrodes that could be moved radially inward the ICR trap, for example on the plane where radiofrequency excitation potential is minimal, closer to the post-excitation ion trajectories.