Sensitive Analysis of Nodularin and Microcystins of Concern in Drinking Water Using Simplified Sample Preparation

Significance of the Topic

The presence of microcystins and nodularin, toxins produced by cyanobacteria in drinking water, poses a significant public health concern. Regulatory bodies such as the US EPA have established stringent limits and monitoring requirements (e.g., EPA Method 544) to ensure safe water supplies. Traditional approaches rely on solid phase extraction (SPE) and lengthy sample concentration steps to achieve the required sensitivity, leading to high solvent consumption, extended turnaround times, and potential delays in public health advisories.

Study Objectives and Overview

This work aimed to develop a faster, more sensitive analytical method for the quantitation of nodularin and six priority microcystins in drinking water without extensive sample preparation. The goals were:

• To simplify or eliminate SPE-based enrichment.
• To reduce chromatographic run times while maintaining or improving separation.
• To increase analytical confidence through additional MRM transitions.

Methodology and Instrumentation

Sample Preparation:

• Direct analysis of untreated drinking water samples (no SPE or concentration).

Chromatography and Detection:

• UHPLC system fitted with Waters CORTECS C8 column (2.7 µm, 2.1 × 100 mm) and VanGuard guard cartridge.
• Gradient elution with 20 mM ammonium formate (A) and methanol (B) at 0.3 mL/min over a 26 min run time.
• Tandem quadrupole MS (Xevo TQ-S micro) operated in positive electrospray ionization.
• Two MRM transitions optimized per analyte to enhance confirmation and reduce false positives.

Main Results and Discussion

Chromatographic Performance:

• Equivalent or improved separation of nodularin and six microcystins compared to EPA Method 544, despite a shorter total run time.

Sensitivity and Linearity:

• Calibration curves for MC-LR, MC-YR, MC-RR were linear over 0.5–40 µg/L with R2 > 0.99.
• Limit of detection down to 0.5 µg/L without any pre-concentration.

Precision and Reproducibility:

• Five replicate injections of spiked samples at 1 µg/L yielded %RSDs of 5.5–9.1 for different toxins, meeting EPA 544 criteria.

MRM Confirmation:

• Additional transitions provided reliable confirmation through ion ratio checks, reducing potential false positives.

Benefits and Practical Applications

This simplified workflow offers multiple advantages:

• Elimination of SPE reduces solvent use, sample handling time, and potential analyte losses.
• Faster chromatographic runs increase laboratory throughput.
• Enhanced selectivity and confirmation via dual transitions increase data confidence for regulatory compliance.

Future Trends and Potential Applications

Ongoing and future developments may include:

• Integration with high-resolution MS for broader cyanotoxin screening.
• Automation and field-deployable UHPLC-MS systems for on-site water monitoring.
• Application to routine water quality programs and emergency response scenarios.

Conclusion

The optimized UHPLC-MS/MS method using a CORTECS C8 column and Xevo TQ-S micro achieves sensitive, reliable detection of nodularin and key microcystins in drinking water without SPE. This approach streamlines sample preparation, reduces run times, and maintains compliance with regulatory detection limits, offering clear benefits for water quality laboratories.

References

1. The Fourth Unregulated Contaminant Monitoring Rule (UCMR 4). US EPA presentation, April 2017.
2. EPA Method 544: Determination of Microcystins and Nodularin by LC-MS/MS. US Environmental Protection Agency.