Online extraction–HPLC–FRAP system for direct identification of antioxidants from solid Du-zhong brick tea

Abstract Du-zhong brick tea (DBT), a new post-fermented functional tea, is marketed in China for its prevention of diseases from oxidative stress. However, antioxidant compounds in DBT are not clear. Here, a novel online extraction–HPLC–ferric reducing antioxidant power (OLE–HPLC–FRAP) has been developed to screen antioxidants in DBT. For OLE, guard column is packed with DBT (1.0 mg), then compounds are extracted directly by HPLC mobile phase for HPLC–FRAP analysis without sample pretreatment procedures. Eleven antioxidants have been screened and identified, three of which are detected in Du-zhong for the first time. Contributions of eleven antioxidant compounds to total antioxidant activity are evaluated using chlorogenic acid as antioxidant marker. Results indicate that the developed OLE–HPLC–FRAP system is a simple and powerful tool to analyze antioxidant compounds from solid complex mixtures, which also contribute to the enhancement of the value of DBT as a good functional tea.