Electrospray mass spectra of solutions of Fe2+/Fe3+quercetin and (10 μM each, 1 : 1) in methanol/water (1 : 1, v/v). (A), with Fe2+, the inset is the isotopic pattern of the peak at m/z = 375.1; (B), with Fe3+, the inset is the isotopic pattern of the peak at m/z = 658.