Evaluation of the terbium(III)-sensitized luminescence with benzenepolycarboxylic acids - Determination of terephthalic acid in drink samples

The reaction of terbium(III) with several benzenepolycarboxylic acids, viz. phthalic, isophthalic, terephthalic, hemimellitic, trimellitic and trimesi c acids, was systematically studied in order to determine the effect of the geometry of these compounds on the luminescence intensity obtained. This s tudy showed that terephthalic acid gave a very high luminescence signal wit h terbium(III) in the presence of trioctylphosphine oxide as synergistic ag ent and Triton X-100 as micellar medium, and by using the time-resolved mod e. The system was used for the development of a very sensitive and fast met hod for the determination of terephthalic acid, which can be applied to mig ration studies of this compound released to drinks stored in plastic bottle s of polyethylene terephthalate. Initial rate and equilibrium signal measur ements were obtained in only 0.2 and 2 s, respectively, by using the stoppe d-flow mixing technique, although the features of the kinetic method were b etter than those of the equilibrium method. The calibration graph of the ki netic method was linear over the range 0.01-3.0 mg ml(-1) and the detection limit was s ng ml(-1). The relative standard deviation was close to 2%. Th e analytical recoveries obtained by applying the method to the analysis of drink samples ranged from 92.5 to 107.0%. The method can be extended to the determination of terephthalic acid esters, such as dimethyl terephthalate, after their alkaline hydrolysis. (C)2000 Elsevier Science B.V. All rights reserved.