Amplification of infrared multiphoton dissociation efficiency in a quadruple ion trap using IR-active ligands

A strategy for increasing the efficiency of infrared multiphoton dissociation (IRMPD) in a quadrupole ion trap (QIT) is described. IR-active ligands (IRALs) are incorporated into noncovalent complexes of the type [M2+(analyte) IRAL]+, where M is a transition metal such as copper or cobalt and IRAL is an auxiliary ligand with an IR-active phosphonate functional group. The complexes are formed via self-assembly in solution directly prior to ESI-MS analysis. We demonstrate this new IRMPD approach for the structural characterization of flavonoids. The fragment ions obtained by IRMPD are similar to those obtained by CAD and allow facile isomer differentiation of flavonoids. Fourier transform infrared absorption attenuated total reflectance (FTIR-ATR) and energy-variable CAD experiments indicate that the high IRMPD efficiencies stem from the very large IR absorptivities of the IR-active ligands.