A multicommuted flow system with liquid–liquid microextraction for determination of anionic surfactants in freshwaters

Abstract

Anionic surfactants are usually determined by the spectrophotometric methylene blue method that is exhaustive, time-consuming and requires large amounts of sample and organic solvent. In this work, a flow system with liquid–liquid microextraction was developed for the environmentally friendly determination of anionic surfactants in natural waters. A CHCl₃ aliquot was introduced at the front of the sample zone to form a film of the organic phase in the inner wall of a hydrophobic (PTFE) reactor, followed by sample and reagent aliquots inserted in tandem. Then a second CHCl₃ aliquot removed the ion-pair for detection at a glass tube without phase isolation by using a fiber-optic spectrophotometer. A linear response from 0.05 to 2.0 mg L⁻¹, detection limit of 0.02 mg L⁻¹ (99.7% confidence level), coefficient of variation of 1.5% (n = 15, 1.0 mg L⁻¹ SDS) and sampling rate of 18 determinations per hour were achieved. In each determination, only 44 μL of CHCl₃ was consumed, which is 2200-fold lower than the amount required in the official procedure. The proposed procedure was successfully applied for determination of anionic surfactants in freshwaters, with results in agreement with those obtained by the official method at the 95% confidence level. The proposal is thus a fast, reliable, inexpensive and greener alternative for determination of anionic surfactants in natural waters, with reduction in the solvent and reagent consumption as well as waste generation.