Quantitative liquid chromatography–tandem mass spectrometry method for determination of microcystin-RR and its glutathione and cysteine conjugates in fish plasma and bile

A rapid and sensitive liquid chromatography–mass spectrometry (LC–MS) method was developed and validated for the simultaneous determination of microcystin-RR (MC-RR) and its glutathione and cysteine conjugates (MC-RR-GSH and MC-RR-Cys, respectively) in fish plasma and bile. The analytes were extracted using methanol, followed by an Oasis mixed-mode cation-exchange polymeric sorbent. The separation was performed on a reversed-phase Waters XBridge C18 column with the gradient mobile phase, consisting of water and acetonitrile (both acidified with 0.5‰ formic acid). Mean recoveries of MC-RR, MC-RR-GSH and MC-RR-Cys ranged from 80.7 to 93.7%, 81.1 to 93.1% and 80.3 to 93.2%, respectively, at three concentrations (0.2, 1.0 and 5.0 μg mL−1). Limits of detection (LODs) for MC-RR, MC-RR-GSH and MC-RR-Cys were 6, 12 and 9 ng mL−1, respectively. Limits of quantification (LOQs) were 15, 30 and 22.5 ng mL−1 for MC-RR, MC-RR-GSH and MC-RR-Cys, respectively. This method makes it feasible for the identification and quantification of MC-RR, MC-RR-GSH and MC-RR-Cys in limited and complex biological fluid samples (such as plasma and bile, typically 50 μL), which were previously excluded or difficult to study due to the relatively large sample volumes.