Enhanced detection of flavonoids by metal complexation and electrospray ionization mass spectrometry

Metal complexation with the use of an auxiliary ligand is explored as an alternative to conventional protonation or deprotonation for analysis of a series of flavonoids by electrospray ionization mass spectrometry. Use of a neutral auxiliary ligand, 2,2'-bipyridine, results in formation of [MII(flavonoid - H)bpy]+, ternary complexes with intensities that are 2 orders of magnitude greater than the corresponding protonated flavonoids and up to 1.5 orders of magnitude greater than the deprotonated flavonoids, based on confirmation by collisionally activated dissociation patterns. The formation of ternary complexes with six divalent transition metals, Co2+, Ni2+, Cu2+, Zn2+, Mn2+, and Fe2+ were compared. Cu2+ resulted in the most intense complexes and simplest mass spectra, while Co2+ gave the second most intense spectra and also produced two key products that could be useful for a selected ion monitoring strategy. Complexation with iron(III) bromide is also investigated to explore the feasibility of using triply charged metals.