Characterization of methylation site of monomethylflavan-3-ols by liquid chromatography/electrospray ionization tandem mass spectrometry

Liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI‐MS/MS) was used for the structural characterization and differentiation of four isomeric O‐monomethylated catechins (on phenolic positions) by the analysis of the fragmentation behaviour of catechin. The catechin fragmentation routes were rationalized and it is shown that several diagnostic ions such as 1,3A+, 1,2B+, and 1,4B+ allow the unambiguous identification of the methylated ring. The precise position of the methyl group on each ring is determined by the difference in the relative intensities of the diagnostic ions. Isomeric O‐methylepicatechins were also differentiated using this methodology. Copyright © 2000 John Wiley & Sons, Ltd.