Determination of Tannin in Green Tea Infusion by Flow-Injection Analysis Based on Quenching the Fluorescence of 3-Aminophthalate

A flow-injection analytical system was developed to determine tannin content in green tea infusions. The flow-injection system is based on measuring the quenching effect of tannin on the fluorescence of 3-aminophthalate. Fluorophore was obtained by auto-oxidation of luminol during solution preparation. System performance was satisfactory for routine analysis (sample throughput >20 h-1; linear dynamic range for tannic acid, 0.005−0.3 mg/mL; linear dynamic range for green tea tannin, 0.02−1.0 mg/mL; CV < 3%). The flow-injection method is immune from interference by coexisting ascorbate in green tea infusion. Analytical results were verified by the ferrous tartrate method, the Japanese official analytical method. Keywords: Flow-injection; tannin; fluorescence; quenching; luminol; 3-aminophthalate; green tea