Selective determination of nicotinic acid and nicotinamide using terbium(III) sensitised luminescence

This paper reports a rapid and selective method based on the reaction of formation of a luminescent chelate between terbium(III) and nicotinic acid. The determination of nicotinamide is also possible by its previous conversion into nicotinic acid by an alkaline hydrolysis step. Stopped-flow mixing technique has been used in order to develop the corresponding equilibrium and kinetic methods, which allow analytical data to be obtained in only ca. 2 and 0.2s, respectively. Calibration graphs are linear in the ranges 0.81-122 and 0.81-162µM, for equilibrium and kinetic methods, respectively, and the detection limits are 0.27 and 0.24µM, respectively. The relative standard deviation is close to 3%. Both methods have been applied to the determination of nicotinic acid and nicotinamide in food and pharmaceutical samples. The recoveries ranged from 91.5 to 106.1%.