Determination of surfactant concentration using micellar enhanced fluorescence and flow injection titration

Flow injection titration was used for the determination of anionic, cationic, nonionic and zwitterionic surfactants. The procedure was based on the micellar-enhanced fluorescence of 1,8-anilino-naphthalene sulfonate (ANS). Samples were injected into a carrier stream of phosphate buffer and 1.0 mol l(-1) NaCl. The sample then passed through a mixing chamber which generated the exponential peak shape needed for the titration as well as diluted the sample in the carrier stream to control the pH and ionic strength of the sample. The peak width was linearly related to the logarithm of the surfactant concentration. The minimum detectable concentration was governed by the critical micelle concentration for anionic, zwitterionic and nonionic surfactants, but below the critical micelle concentration for cationic surfactants. The linear range extended for approximately 1.5 orders of magnitude. Reproducibility ranged from 12% at the lower end of the calibration range to 1.1% at higher concentrations. For SDS recoveries of 82-108% were achieved in matrices as concentrated as 1 mol l(-1) in NaCl or Na(2)SO(4).