In-syringe magnetic stirring assisted dispersive liquid–liquid micro-extraction with solvent washing for fully automated determination of cationic surfactants

Abstract

An automated simple analyzer system for the extraction of cationic surfactants as an ion-pair with disulfine blue dye is described based on the technique in-syringe magnetic stirring-assisted dispersive liquid–liquid micro-extraction. The use of chloroform as an extraction solvent denser than water required the operation of the syringe pump upside-down. The remaining air cushion inside the syringe allowed emptying the syringe completely and reducing the dead volume significantly compared to previous studies. Since the stirring bar placed inside the syringe to obtain a closed yet size-adaptable mixing chamber remains at the same position, the former magnetic stirring bar driver was simplified. The new system configuration further enabled automated in-syringe washing of the organic phase with water and barium acetate solution to minimize interference. High signal repeatability with <5% RSD was achieved both for extraction as well as for double organic phase washing. Only 220 μL of the extraction solvent and 4 mL of the sample were required for simple extraction achieving a detection limit below 30 nmol L⁻¹ and a linear response up to 1 μmol L⁻¹ of cetyltrimethylammonium bromide. The time of analysis was 240 s for simple extraction. Considerable reduction of interference was achieved by extract washing up to 545 s. Analyte recovery in real water samples was 95.6 ± 7.0% on applying extract washing.