Luminescent determination of ascorbic acid in dietary supplements

The article presents the results of the ascorbic acid (AA) determination with using a complex of an Tb (III) ion with ciprofloxacin (CF) as a lanthanide luminescent marker. The luminescent properties of the Tb (III)-CF complex in the presence of AA were studied. The excitation and luminescence spectra, triplet level of ligand, the kinetics of the luminescence decay of the Tb(III)-CF complex in the presence of AA were analyzed. The excitation spectrum of the Tb (III)- CF complex has broad bands with maxima at 302 and 355 nm that corresponding to the n→π* electronic transition in the absorption spectrum of ligand. The luminesce spectra demonstrate the emission transitions arising from 5D4 energy level to 7Fj multiplet ground state. The luminescence of the Tb(III)-CF complex was found to be quenched by AA. It was established that the lifetime of the excited 5D4 state of the Tb (III) ion decreases with AA concentration increasing up to 0,25 mg/сm3. Luminescence quenching of the Tb (III)-CF complex by AA follows the Stern-Volmer relationship of AA. The Stern-Volmer constant K is 2478 dm3/mol. The biomolecular quenching rate constant kq is 1,25∙107 dm3/mol. The effect of luminescence quenching of the Tb (III)-CF complex was used to developing the procedure for determining of AA in the dietetic additives «Asvitol» and «Ascorbic acid». The linear calibration plot for AA was obtained over the concentration range of 0,02 to 0,25 mg/сm3.