Patent ID: 7714283

Claim:
An electrostatic ion trap for a mass spectrometer, comprising: an electrode arrangement defining an ion trapping volume; the electrode arrangement being arranged to generate a trapping field defined by a potential U′(r,φ,z)=U(r,φ,z)+W, where U(r, φ,z) is an ideal potential which traps ions in the Z-direction of the trapping volume so that they undergo substantially isochronous oscillations and where W is a perturbation to that ideal potential U(r,φ,z); wherein: the geometry of the electrode arrangement generally follows one or more lines of equipotential of the ideal potential U(r,φ,z) but wherein at least a part of the electrode arrangement deviates to a degree from that ideal potential U(r,φ,z) so as to introduce the perturbation W into the said trapping field, the degree of deviation from the ideal potential U(r, φ,z) being sufficient to result in the relative phases of the ions in the trap shifting over time such that at least some of the trapped ions have an absolute phase spread of more than zero but less than about 2π radians over an ion detection period T m .