Kinetic determination of salicylic acid, diflunisal and their mixture based on lanthanide-sensitized luminescence

The combined use of kinetic and equilibrium measurements, obtained with the aid of the stopped-flow mixing technique, allowed the development of a simple, fast direct method for the simultaneous determination of salicylic acid and diflunisal in untreated serum samples that can be readily applied to routine analyses. The method is based on the formation of ion-association complexes between the cationic surfactant cetyltrimethylammonium bromide and the ternary anionic chelates formed by salicylate and diflunisal with terbium(III) and EDTA in a basic medium. The intramolecular energy transfer from salicylate or diflunisal to terbium(III) gives rise to luminescence emission by the lanthanide ion. The calibration graphs thus obtained are linear over a wide concentration range and the detection limits are in the nanogram per millilitre region. Within-assay and between-assay relative standard deviation ranged from 2.6% to 3.6% and from 1.3% to 8.5%, respectively. Mixtures of salicylic acid and diflunisal in ratios between 6:1 and 1:12 can be satisfactorily resolved. The proposed method was applied to the direct determination of these compounds in serum with recoveries of 92.7-109.0% for salicylic acid and 86.4-110.9% for diflunisal.