Liquid chromatography coupled to quadruple time-of-flight tandem mass spectrometry for microcystin analysis in freshwaters: method performances and characterisation of a novel variant of microcystin-RR

Cyanobacteria, also called blue-green algae, occur worldwide within water blooms in eutrophic lakesand drinking water reservoirs, producing several biotoxins (cyanotoxins). Among these, microcystins(MCs) are a group of cyclic heptapeptides showing potent hepatotoxicity and activity as tumourpromoters. So far, at least 89 MCs from different cyanobacteria genera have been characterised.Herein, ion trap, matrix-assisted laser desorption/ionisation time-of-flight (MALDI-ToF) and quadrupletime-of-flight (Q-ToF) mass spectrometry (MS)-based methods were tested and compared foranalysing MCs in freshwaters. Method performances in terms of limit of detection, limit ofquantification, mean recoveries, repeatability, and specificity were evaluated. In particular, a liquidchromatography/electrospray ionisation (LC/ESI)-Q-ToF-MS/MS method was firstly described toanalyse MCs in freshwaters; this technique is highly selective and sensitive, and allowed us tocharacterise the molecular structure of an unknown compound. Indeed, the full structural characterisationof a novel microcystin variant from a bloom of Planktothrix rubescens in the Lake Averno,near Naples, was attained by the study of the fragmentation pattern. The new cyanotoxin wasidentified as the 9-acetyl-Adda variant of microcystin-RR.