A new cloud-point extraction and preconcentration method using a cationic surfactant, Aliquat-336 (tricaprylyl-methylammonium chloride), has been developed for the determination of cyanobacterial toxins, microcystins, in natural waters. Sodium sulfate was used to induce phase separation at 25 ℃. The phase behavior of Aliquat-336 with respect to concentration of Na{sub}2SO{sub}4 was studied. The cloud-point system revealed a very high phase volume ratio compared to other established systems of nonionic, anionic, and cationic surfactants. At pH 6-7, it showed an outstanding selectivity in analyte extraction for anionic species. Only MC-LR and MC-YR, which are known to be predominantly anionic, were extracted (with averaged recoveries of 113.9 ± 9 and 87.1± 7, respectively). MCRR, which is likely to be amphoteric at the above pH range, was not detectable in the extract. Coupled to HPLC/ UV separation and detection, the cloud-point extraction method (with 2.5 mM Aliquat-336 and 75 mM Na{sub}2SO{sub}4 at 25 ℃) offered detection limits of 150 ± 7 and 470 ± 72 pg/ mL for MC-LR and MC-YR, respectively, in 25 mL of deionized water. Repeatability of the method was 7.6 for MC-LR and 7.3 for MC-YR. The cloud-point extraction process can be completed within 10-15 min with no cleanup steps required. Applicability of the new method to the determination of microcystins in real samples was demonstrated using natural surface waters collected from a local river and a local duck pond spiked with realistic concentrations of microcystins. Effects of salinity and organic matter (TOC) content in the water sample on the extraction efficiency were also studied.
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