Determination of trace amount of microcystins in water samples using liquid chromatography coupled with triple quadrupole mass spectrometry

Abstract

Microcystins, a group of cyclic heptapeptide heaptoxins and tumor promoters, are bio-generated by cyanobacteria occurring in eutrophic lakes, rivers and reservoirs. In the present work, a novel liquid chromatography electrospray ionization (ESI-MS) tandem triple quadrupole mass spectrometry method was developed to determine the trace amounts of major microcystin variants in waters. Solid phase extraction (SPE) consuming 10 mL of water samples was used for sample cleaning-up and analyte enrichment. A reverse phase LC separation system consisting a symmetry 300 C-18 column (4.6 mm × 75 mm i.d., 3.5 μm, pore size 300 Å) and binary gradient methanol-water mobile phase was applied for the separation of microcystin variants (MC-RR, -LR, -LW and -LF). Formic acid was spiked into the mobile phase to enhance the ionization efficiency. Tandem MS/MS analysis was performed in multi-reaction monitoring mode (MRM). Product-ion traces at m/ z 519.9 → 135.0 for MC-RR, 498.4 → 135.0 for MC-LR, 1025.8 → 891.7 for MC-LW and 986.8 → 852.5 for MC-LF was used for quantification of the corresponding microcystin variants, and trace at m/ z 556.1 → 278.0 used for internal standard enkephalin. Limits of quantification (LOQs) were 0.16, 0.04, 2.0, 1.0 and 0.10 μg/L for MC-RR, MC-LR, MC-LW, MC-LF and enkephalin, respectively. Intra- and inter-day precisions for the determination of the four variants were better than 5 and 6% in relative standard deviations (R.S.D.s), and recoveries for the four variants were in the range of 95–105%. The developed approach has been applied for the determination of the trace amounts of miccocystins in water samples of Taihu Lake of eastern China. The results showed that MC-RR and MC-LR were major variants detected in the water samples, and their concentrations significantly increased with the season changing from Spring to Summer.