Abstract
Microcystins, cyclic heptapeptide hepatotoxins, are the most frequently detected from freshwater cyanobacteria around the world. WHO recommended a guideline value of microcystin－LR that the dosage should be below 1 μg・L－1 in drinking water. To monitor microcystins concentration in water, LC－MS/MS has been used. However, ionization efficiencies of internal standards are different from those of microcystins. As the results, the accuracy of microcystin determination is not enough for the monitoring.To improve the accuracy, we developed a simple and precise method using stable isotope (15N)－labeled microcystin. As procedures, a frozen water-sample (0.5 mL) was thawed and added 15N－labeled microcystin with 0.5 mL of MeOH. After ultra-sonication for 10 min of the treated sample, the sample solution was filtered using 0.22 μm membrane filter. The filtrate was injected to LC－MS/MS and quantified. The limit of quantification was around 0.1 μg・L－1 depended on the instrument sensitivity. Using this method, high accurate quantified results of microcystins in water sample were obtained.
