Long-wavelength fluorescence polarization immunoassay for surfactant determination

A new homogeneous fluoroimmunoassay method based on the use of dynamic long-wavelength fluorescence polarization is presented here for the first time. This methodology, which is applied to the determination of linear alkylbenzenesulfonates (LASs) in water samples, involves the use of a new long-wavelength tracer synthesized from the oxazine dye Nile Blue (NB) via a carbodiimide method. This tracer exhibits fluorescent properties at lambda(ex) 626 and lambda(em) 674nm. The variation of fluorescence polarization with time is followed using the T-format configuration of the spectrofluorimeter and the analytical parameter used is the initial rate, which is measured in only 0.7s. The dynamic range of the calibration graph is 0.05-4.7mg/L, with a detection limit of 0.03mg/L. The precision, expressed as relative standard deviation was assayed at 0.05 and 1mg/L, giving values in the range 7.6-9.1%. Other anionic, cationic and non-ionic surfactants were tolerated at much higher concentration levels than that of the analyte. The method has proven its practical usefulness for the analysis of water samples, in which only a solid phase extraction step is necessary. Recoveries ranged from 80.8 to 119.8%, with a mean value of 100.8%.