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(-)-Epigallocatechin gallate (EGCG) of catechins changes from non-colored at around neutral pH to yellow at higher pH region in aqueous solution. The pH-dependent oxidation of EGCG was analyzed by liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS). LC/MS/MS analysis of EGCG and its related compounds, (-)-epicatechin gallate (ECG) and (-)-epigallocatechin (EGC), successfully elucidated the structure relationship of EGCG solution involving the color change reaction at different pH conditions. The oxidation species produced at alkaline pH was detected at a different retention time from EGCG in the chromatograms of the EGCG sample. The oxidation species was found to correspond to M+14 (where M is the molecular weight of EGCG), which has two hydrogen atoms removed and addition of one oxygen atom to the gallyl moiety in the B-ring of EGCG.

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Much attention has been focused on the strong antioxidant activity and antitumorigenic property of catechins extracted from green tea. However, not many studies on the oxidation process of catechins have been conducted. We employed an on-line electrochemistry/electrospray ionization mass spectrometry (EC/ESI-MS) technique to identify unstable oxidation products of catechins. We have succeeded in detecting dimer products formed by the electrolysis of ( )-epigallocatechin, ( )-epicatechin gallate and ( )-epigallocatechin gallate. EC/ESI-MS was a useful technique to study electrochemical oxidation process of antioxidants.