US Patent 7842918 Chemical structure-insensitive method and apparatus for dissociating ions

In a method for exciting a precursor ion in an ion trap, the ion is trapped in a nonlinear trapping field that includes a quadrupolar field and a multipole field. The quadrupolar field is generated by applying a radio-frequency (RF) trapping voltage to the ion trap at a trapping amplitude and trapping frequency. A supplemental alternating-current (AC) voltage is applied to the ion trap at a supplemental amplitude and supplemental frequency. The supplemental amplitude is low enough to prevent ejection of the ion from the ion trap, and the supplemental frequency differs from the secular frequency of the ion by an offset amount. One or more operating parameters of the ion trap are adjusted, such that the ion absorbs energy from the supplemental field sufficient to undergo collision-induced dissociation (CID) without being in resonance with the supplemental field.