Electrostatic ion trap

An ion trap includes an electrode structure, including a first and a second opposed mirror electrodes and a central lens therebetween, that produces an electrostatic potential in which ions are confined to trajectories at natural oscillation frequencies, the confining potential being anharmonic. The ion trap also includes an AC excitation source having an excitation frequency f that excites confined ions at a frequency of about twice the natural oscillation frequency of the ions, the AC excitation frequency source preferably being connected to the central lens. In one embodiment, the ion trap includes a scan control that mass selectively reduces a frequency difference between the AC excitation frequency and about twice the natural oscillation frequency of the ions.