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Various microcystins (MCs) were isolated from extracts of Microcystis aeruginosa grown in lake Thang Cong, Hanoi, Vietnam. MC-RR, MC-YR, and MC-WR were determined by liquid chromatography with mass spectrometric detection (LC-MS). Besides these major microcystins five minor compounds were identified as microcystins using fragmentation mass spectrometry (LC-MS-MS). Two minor compounds were identified as desmethyl variants of MC-WR and MC-RR, respectively, and a third as an MC-LR variant. Two components showed characteristic fragmentation profiles of MC but could not be characterised exactly owing to their low concentrations. Three other components (900–1100 dalton) proved not to be microcystins. [ABSTRACT FROM AUTHOR]