Cloud-point extraction of nodularin-R from natural waters

A cloud-point extraction (CPE) technique for the determination of a cyanobacterial hepatotoxin, nodularin-R, in aqueous media using a cationic surfactant, tricaprylmethylammonium chloride (Aliquat-336), was developed. Cloud-point phase separation of Aliquat-336 at ambient temperature was induced by the addition of sodium sulfate. The Aliquat-336/Na2SO4 CPE system displayed large preconcentration factor, FC, for nodularin-R. At the operational CPE conditions, FC of 709.2 was achieved. Distribution constant, KD, of the distribution of nodularin-R between the surfactant-rich and aqueous phases of the CPE system was estimated to be (4.94(plus-minus)0.8)×10^3. Coupled to liquid chromatography with UV detection, the CPE method offered a detection limit of 330 pg ml^-1 (in freshwater)/1.3 ng ml^-1 (in seawater) and a repeatability of 6.4% (in freshwater) (n=7, P<0.05) for nodularin-R in a sample of 25 ml. The CPE is a rapid process and no cleanup step is required. Effects of pH, natural abundant anion (chloride) and dissolved organic matters (DOM, humic acid, HA) on the extraction efficiency were evaluated. A double CPE technique was developed to overcome interferences encountered in the analysis of environmental samples. Applicability of the new method to the determination of nodularin-R in real coastal marine water samples has also been demonstrated.