Improved solid-phase extraction protocol and sensitive quantification of six microcystins in water using an HPLC-orbitrap mass spectrometry system

Abstract

A solid-phase extraction (SPE) protocol was developed and combined with HPLC-ESI-MS for the quantification of common cyanobacterial toxins, microcystins (MCs), in water. An HPLC binary gradient was established for the separation of MC-LR, MC-YR, MC-RR, MC-LA, MC-LW, and MC-LF, while quantification of MCs in MC-spiked HPLC-grade water, tap water, and lake water was achieved in selected ion monitoring (SIM) mode using a high-resolution Orbitrap Fusion Tribrid mass spectrometer. All MC ions were detected with high mass accuracy of ≤1.61 ppm using the orbitrap mass analyzer, and MCs were quantified using external calibration based on LC-SIM-MS peak areas of the monoisotopic MC ions. The LOQs of MC-LA, MC-LW, and MC-LF in water using LC-SIM-MS were ∼25 ng L⁻¹ while LOQs of MC-RR, MC-LR and MC-YR in water were ∼10 ng L⁻¹. After the purification and preconcentration of MCs by SPE, LOQs of MC-LA, MC-LW and MC-LF were improved to ∼1 ng L⁻¹ and LOQs of MC-RR, MC-LR and MC-YR were improved to ∼600 pg L⁻¹ in tap water and lake water. The percent recoveries of MCs after SPE and sample preconcentration were in the range from 97.1% to 100.9% with a relative standard deviation (RSD) of ≤5.0%, indicating that this is one of the most efficient and reproducible SPE procedures for recovery of MCs from aqueous solutions. Additionally, a parallel LC-tandem mass spectrometry (MS/MS) method was established for the structural identification of MCs. Overall, the MC purification and preconcentration method developed and LC-SIM-MS enable sensitive, selective, and reproducible identification and quantification of MCs in water using the MS and MS/MS capabilities of a high-mass accuracy orbitrap mass spectrometer.