Characterisation of catechins in green and black teas using square-wave voltammetry and RP-HPLC-ECD

Eight green and three black teas have been analysed using square-wave voltammetry at a glassy-carbon electrode, at pH 2 and square-wave frequency 100 Hz. The SWV response of teas consisted of two partially resolved anodic peaks: peak 1 at around 0.372 V, and peak 2 around 0.491 V. First anodic peak was due to oxidation of pyrogallol catechins, i.e. epigallocatechin gallate (EGCG) and epigallocatechin (EGC), and peak 2 can be associated with oxidation of ortho-hydroquinone and galloyl groups in catechins, gallic acid, theaflavins, and other catechol-type phenolics present in tea. Peak 1 has been chosen for quantification purposes, and concentrations of catechins were calculated from the calibration plot constructed for EGC used as external standard. Amounts of catechins in green teas obtained by SWV were in good agreement with those obtained using reverse-phase high-performance liquid chromatography with electrochemical detection (RP-HPLC-ECD). For black tea samples the detector response of peak 1 was diminished due to interferences of coexisting polyphenols, and SWV obtained results were something lower than those obtained by HPLC-ECD.