Application of sustained off-resonance irradiation infrared multiphoton dissociation tandem mass spectrometry to the analysis of biomolecules.

A modified pulse sequence for infrared multiphoton dissociation (IRMPD) experiments on a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer in conjunction with sidekick trapping is presented. For IRMPD tandem mass spectrometry experiments gated trapping is normally applied. It ensures that the ions remain on-axis and, thus, cross the laser beam which is aligned on-axis in commercially available instruments. Sidekick trapping is used to capture more ions in the ICR cell in order to increase the signal intensity. However, it may lead to off-axis ion motion, which reduces or even excludes interaction with the laser beam. In this contribution sustained off-resonance irradiation (SORI) was applied to overcome this disadvantage of sidekick trapping. SORI is normally used in conjunction with collision-induced dissociation (CID) experiments to increase the kinetic energy of the ions. Here, SORI is used to influence the cyclotron motion during the laser irradiation time, which leads to temporary intersection of the ion trajectory with the laser beam. With this easy-to-handle experimental setup, IRMPD of ions captured with sidekick trapping leads again to the generation of fragment ions as is demonstrated with several biologically relevant samples like peptides, lipids and glycolipids.