Fast screening of saxitoxin, neosaxitoxin, and decarbamoyl analogues in fresh and brackish surface waters by on-line enrichment coupled to HILIC-HRMS

The proliferation of harmful cyanobacterial algal blooms is of concern due to the associated release of toxins affecting ecosystems and human health. The paralytic shellfish poison saxitoxin (STX) is a small polar alkaloid that can occur in inland and marine aquatic environments. Here, we optimized a fast and sensitive analytical method for the determination of STX, neosaxitoxin (NeoSTX), and their decarbamoyl analogues in surface waters. The method involves a simple filtration, addition of isotope-labelled internal standard (ILIS), and analysis by on-line solid-phase extraction coupled to hydrophilic interaction liquid chromatography high-resolution mass spectrometry (on-line SPE-HILIC-HRMS). Except glass fiber filters, other tested materials (e.g., nylon, nitrocellulose) provided suitable filtration performance. Time-dependent adsorptive losses occurred during the LC-MS batch sequence if glass autosampler vials were used, while no such effect was observed for polypropylene autosampler vials. Matrix effects were evaluated for 4 different quantification scenarios, including external vs. internal curves and neat reagent water vs. matrix-matched curves. Matrix-matched calibration with ILIS correction (NeoSTX-N-15(7)) provided the best performance overall. The analytical method was validated in freshwater lake water and estuarine brackish water (30 parts per thousand salinity), with suitable determination coefficients (R-2 > 0.9975), matrix spike accuracy (90-107%), and intraday/interday precision (RSD of 0.61-16%). Method limits of detection (LOD in lake water: 0.72-3.9 ng/L) are also improved over most of the recent literature. The method was applied to a set of 302 surface water samples collected in Canada, France, and the United Kingdom, and positive detections were reported for STX (max: 98 ng/L), decarbamoyl-STX (max: 15 ng/L), and NeoSTX (max: 87 ng/L).

Automated summary

In this study, a fast and sensitive analytical method was optimized for the determination of paralytic shellfish poison saxitoxin and its analogues in surface waters. The method involved simple filtration, addition of internal standard, and analysis by online solid-phase extraction coupled to hydrophilic interaction liquid chromatography high-resolution mass spectrometry. The developed method showed good accuracy and precision in both laboratory validation and real sample analysis.