Determination of Tannin and Amino Acids in Tea by Fluorescent Flow-Injection Analytical System

A fluorometric flow-injection analytical system for determining both tannin and total amino acid content is proposed. The sensing principles are based on the quenching effect of tannin on the fluorescence of 3-aminophthalate and the fluorogenic reaction of amino acids with o-phthalaldehyde (OPA), respectively. Both reactions were integrated into a fluorometric flow-injection system with the same excitation and emission wavelength (340/425 nm). A three-way valve was introduced to switch between the reagents for sequential determination of tannin and amino acids. Parameters of operation condition were optimized, and interference of tannic acid on the fluorescence intensity of OPA-amino acids reaction was investigated. The linear dynamic range for tannic acid and theanine were 50-250 μg mL-1 and 0.1-1 μg mL-1 (CV < 5%, n = 3), and the sample throughout was 20 samples h-1. Effects of plucking position of tea plants, withering time and steeping method on tannin and amino acids in consumer tea brew of Pouchong tea in Taiwan were analyzed by the proposed system. Analytical results of commercial oolong tea products in Taiwan were compared to their market prices.