CONTROL OF ION KINETIC-ENERGY IN ION-CYCLOTRON RESONANCE SPECTROMETRY - VERY-LOW-ENERGY COLLISION-INDUCED DISSOCIATION

A novel means of effecting collision-induced dissociation 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 over a period of hundreds of milliseconds by repeatedly shifting the phase of the radio frequency 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 kcal/mol of threshold. This is demonstrated by the sole production of m/z 92 in the fragmentation of n-butylbenzene cation and by extremely large secondary isotope effects in the dissociation of t-butoxide-d6 anion.