Herein, a high-performance liquid chromatography-diode array detection method has been developed for the simultaneous determination of 15 phenolic antioxidants: flavan-3-ols, (-)-epigallocatechin, (+)-catechin, (-)-epigallocatechin gallate, (-)-epicatechin, (-)-epicatechin gallate, (-)-gallocatechin, a phenolic acid (gallic acid), a hydroxycinnamic acid (chlorogenic acid), flavones (apigenin), flavonols (kaempferol, quercetin, and myricetin), and purine alkaloids (caffeine theophylline, theobromine) in different herb extracts, tea, and coffee varieties. The developed method was validated and successfully applied in order to determine the polyphenolic content to estimate the antioxidant activity of the Sideritis species commonly known as Greek mountain tea. To the best of our knowledge, this is the first report on the quantitative determination of catechins and other polyphenols in Greek mountain tea. Acidic hydrolysis was necessary for the simultaneous determination of the aglycones of the target analytes. According to our results, chlorogenic acid, myricetin, apigenin, catechin, and epicatechin gallate are found in the Sideritis species.
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