Microporous membrane liquid-liquid extraction coupled on-line with normal-phase liquid chromatography for the determination of cationic surfactants in river and waste water

Membrane-based continuous liquid-liquid extraction combined on-line with no rmal-phase liquid chromatography is proposed for the determination of catio nic surfactants in complex aqueous samples. The technique has the potential for complete automation. Selective enrichment of cationic surfactants from spiked river water and waste-water samples with simultaneous removal of ma trix constituents, followed by a quantitative transfer of the extract onto a liquid chromatographic column and separation of the surfactant homologues yielding low detection limits, has been realised. The homologues of alkyld imethylbenzylammonium chloride (Dodigen 226) were chosen as model compounds in the method development. Dodigen homologues were ion-paired with heptano ic acid and extracted into chlorobutane by means of microporous membrane li quid-liquid extraction. It was thereby possible to attain an enrichment of over 250 times for one of the homologues, viz. the concentration in the org anic liquid is 250 times higher than in the original sample. Detection limi ts for the three best-detected homologues of the mixture were in the range 0.7-5 mu g/l in spiked river water samples. Ion-pair normal-phase liquid ch romatography, again with heptanoic acid as counter-ion, gave the necessary separation of the surfactant homologues. (C) 2000 Elsevier Science B.V. All rights reserved.