The anionic surfactant, dodecylbenzenesulfonate (DBS), reacts with 3,6-bis (dimethylamino)-10-dodecylacridinium bromide (AO-10-D) to quench the fluorescence of AO-10-D. When the cationic surfactant was added to the mixed solution of AO-10-D and DBS, the fluorescence intensity increases with the increase in the concentration of cationic surfactant because added cationic surfactant preferentially forms the ion associate with the DBS. The detection of increase in the fluorescence intensity was applied to the flow injection determination of a cationic surfactant. Cationic surfactants such as Zephiramine could be determined in the concentration range from 1 x 10-6 to 4 x 10-5 mol/l by the proposed method. The sampling rate was ca. 10 samples h-1. The coexistence of a non-ionic surfactant, tetramethylammonium chloride and tetraethylammonium chloride at 5, 25 and 25 times excess to a cationic surfactant did not interfere with the determination of the cationic surfactant.
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