Voltage dependency in coulometric analysis of tea polyphenols and isoflavones in foodstuffs

To establish an analytical method for phenolic compounds in foodstuffs and tissue samples, we determined five simple polyphenols, five tea polyphenols and five isoflavones using high performance liquid chromatography equipped with a coulometric array detector. When simple polyphenols were detected with eight coulometric array detectors at 350 to 700 mV, trihydroxyphenols gave the highest signal at 350 mV, o- and p-diphenols at 400 mV, and m-diphenol at 700 mV. In tea polyphenols with m-diphenolic A ring and di- or triphenolic B ring, strong signals were detected at 400 and 700 mV. In isoflavone aglycons having monophenolic or m-diphenolic A ring, monophenolic B ring, and C ring with 4-carbonyl group, a strong signal was observed at 500 or 550 mV and at 700 mV. However, equol without 4-carbonyl group gave a single peak at 650 mV. These results suggest that the signal at 350-400 mV is due to the conjugated OH groups present in o- or p-derivative, the signal at 500-550 mV is due to the 4-carbonyl group and the signal at 700 mV to isolated OH groups present in monophenols or m-diphenols.