Appropriate choice of event length in sustained off-resonance irradiation collision-induced dissociation (SORI-CID) experiments: Activated ion collision-induced dissociation

Sustained off-resonance irradiation collision-induced dissociation (SORI-CID) is used in Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) to activate ions through multiple, low-energy collisions. This is done by irradiating the ions with RF energy with a frequency offset, Δf, from their resonant frequency. This causes the orbit radius and kinetic energy of the excited ions to fluctuate with a periodicity of 1/Δf. In this paper we demonstrate that the extent of ion activation (and subsequent dissociation) is strongly influenced when SORI-CID event lengths are not integer multiples of 1/Δf. This is due to residual kinetic energy in the ions following the SORI event. These effects are seen over a range of Δf values. Small variations in SORI-CID event lengths can therefore be used to enhance or suppress dissociation, and care must be taken to account for these effects when comparing threshold dissociation energies obtained using these methods.

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► Ion kinetic energy varies periodically during sustained off-resonance irradiation.
► Fractional SORI cycles leave kinetic energy in the system at the end of the event.
► Residual kinetic energy strongly affects the extent of ion activation.