LOW-ENERGY COLLISION-INDUCED DISSOCIATION - PHASE-SHIFTING EXCITATION CONTROL OF ION KINETIC-ENERGY IN ION-CYCLOTRON RESONANCE SPECTROMETRY

A new method of effecting collision induced dissociation (CID) of molecular ions for use in conjunction with Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is reported. Using an electronic circuit external to the ICR hardware and software, ions are continuously accelerated and decelerated for hundreds of milliseconds by repeatedly shifting the phase of the radiofrequency excitation driving signal by 180-degrees. At low kinetic energies, a succession of collisions which transfer small amounts of energy favors the lowest energy fragmentation pathway and allows ions with large activation energies to fragment at internal energies within a few kilocalories per mole of threshold. This is demonstrated by the sole production of m/z 92 in the fragmentation of n-butylbenzene cation and by abnormally large secondary isotope effects in the dissociation of tert-butoxide-d6 anion.