Identification of microcystin-RR and [Dha7]microcystin-RR in commercial standards by electrospray ionization mass spectrometry

Three different commercial standards of microcystin-RR were assessed for purity by the liquid chromatography coupled with electrospray ionization mass spectrometry (LC/ESI/MS) technique. Although the liquid chromatograms with photodiode array detector for each standard looked, virtually identical, the analysis of corresponding mass spectra revealed that only one of them contained microcystin-RR per purity as say. The second standard was a mixture of microcystin-RR, and its demethyl variant identified as [Dha(7)]microcystin-RR, and the third one contained [Dha(7)]microcystin-RR only. We strongly recommend applying LC coupled with MS for purity assay of microcystin standards.