Determination of total phenols in tea infusions, tomato and apple juice by terbium sensitized fluorescence method as an alternative approach to the Folin-Ciocalteu spectrophotometric method.

A fast screening of total phenols in tea infusions, tomato and apple juice samples using terbium sensitized fluorescence is described. The proposed method is based on the fluorescence sensitization of terbium (Tb(3+)) by complexation with flavonols (quercein as a reference standard) (at pH 7.0), which fluoresces intensely with an emission maximum at 545nm when excited at 310nm. Quercetin and terbium cations (at pH 7.0) form a stable complex and the resulted emission at 545nm can be used for the determination of the total phenols concentration expressed in terms of "quercetin equivalent". Based on the obtained results, a sensitive, simple and rapid spectrofluorimetric method was developed for the determination of total phenols. In the optimum conditions, the calibration graph was linear from 0.01 to 2μgmL(-1), with the limit of detection of 0.002μgmL(-1). The relative standard deviation values were in the range of 0.75-2.3%. The total concentrations of quercetin equivalent in five tested samples were found in the range of 6.6-27.9μgmL(-1) and the results compare favorably with those obtained by spectrophotometric method (r=0.999).