Effects of metal ions (Cu2+, Fe2+ and Fe3+) on HPLC analysis of catechins

► Metal ions (Cu2+, Fe2+ and Fe3+) seriously affected HPLC analysis of catechins. ► Metal ions bind to catechins and oxidize them or catalyze their oxidation by oxygen. ► An optimum condition using EDTA was proposed for HPLC analysis. Catechins [(−)-epicatechin (EC), (−)-epigallocatechin (EGC), (−)-epicatechin gallate (ECG) and (−)-epigallocatechin gallate (EGCG)] were analysed by HPLC using an ODS column, an electrochemical detector (0.75V vs. Ag/AgCl) and an eluting solvent composed of water containing buffer (84% v/v), acetonitrile (12% v/v) and ethylacetate (4% v/v) in the presence of metal ions (Cu2+, Fe2+ and Fe3+). HPLC peaks were affected by metal ions: the peak intensities of ECG and EGCG decreased, but the peak intensities of EC and EGC were not affected seriously. Fe2+ most markedly decreased the peak intensities of EGCG. EDTA was added to mask metal ions and an optimum condition was proposed. The effects of the metal ions on HPLC analysis are discussed from the viewpoints of metal complex formation with catechins and oxidation of catechins on the basis of ultraviolet–visible (UV–vis) spectrophotometry, electrospray ionisation mass spectrometry (ESI-MS) and cyclic voltammetry (CV).