1 Direct Quantification of Diquat and Paraquat in Drinking Water Samples Using Ultra-Sensitive UPLC/MS/MS Analysis

Significance of the Topic

Ensuring the safety of drinking water is critical to public health and environmental protection. Diquat and paraquat are widely used bipyridyl herbicides that, despite their effectiveness in weed control, pose toxicity risks to humans and ecosystems. Regulatory bodies such as the U.S. EPA and the European Union have set stringent limits for these compounds in drinking water, necessitating analytical methods capable of direct, sensitive, and rapid quantification at trace levels.

Objectives and Overview of the Study

This application note demonstrates a streamlined workflow for the direct quantification of diquat and paraquat in bottled and tap water samples. The primary goals are to eliminate time-consuming sample preparation steps, achieve detection limits below regulatory thresholds, and maintain robust performance across numerous injections.

Methodology

The study employs reversed-phase ultra-performance liquid chromatography (RP-UPLC) with a volatile ion-pairing reagent (heptafluorobutyric acid, HFBA) and tandem mass spectrometric detection. Calibration standards (50 ppt–100 ppb) were prepared in ultrapure water. A one-minute linear gradient from 2% to 95% methanol (both phases containing 10 mM HFBA) was applied at 0.6 mL/min. Samples (100 µL) were directly injected without extraction or concentration.

Used Instrumentation

• ACQUITY UPLC System with binary solvent manager
• ACQUITY UPLC BEH C18 Column, 2.1×30 mm, 1.7 µm
• Xevo TQ-S Triple Quadrupole Mass Spectrometer with StepWave ion optics
• TrendPlot MS Software for data visualization

Main Results and Discussion

The Xevo TQ-S delivered limits of detection of 50 ppt for both herbicides, surpassing the EU directive (100 ppt). Calibration curves exhibited excellent linearity (r2≥0.995). Recovery studies at 1 ppb spikes yielded 75–107% recovery with RSDs below 8%. Peak shapes remained Gaussian over 250 direct injections, and column backpressure increased minimally (3500 to 3900 psi). High-purity HFBA was essential to suppress background interferences, enabling reliable quantification at sub-ppb levels.

Benefits and Practical Applications

• Elimination of solid-phase extraction and sample concentration steps saves analyst time and resources.
• Fast 3-minute analysis per sample enhances laboratory throughput.
• Robust performance over hundreds of injections supports high-volume monitoring programs.
• Direct injection simplifies workflows for routine water quality testing and regulatory compliance.

Future Trends and Potential Applications

Advances in mass spectrometer sensitivity and miniaturized UPLC systems point toward on-site and real-time monitoring of water contaminants. Emerging ion-pair reagents and novel column chemistries may further improve retention of polar analytes without extensive sample preparation. Integration with automated data platforms and artificial intelligence will enable predictive maintenance, trend analysis, and rapid decision-making in environmental monitoring.

Conclusion

The direct injection UPLC/MS/MS method using volatile ion pairing and the Xevo TQ-S offers a fast, sensitive, and robust approach for trace-level quantification of diquat and paraquat in drinking water. This workflow meets regulatory requirements while reducing labor and consumable costs, making it well suited for routine water quality analysis.

References

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3. U.S. EPA. Pesticides Industry Sales and Usage: 2007 Market Estimates. Washington, DC; February 2011.
4. CDC. Facts about Paraquat. Centers for Disease Control; October 13, 2006.
5. WHO. The WHO Recommended Classification of Pesticides by Hazard and Guidelines to Classification. Geneva; 1996.
6. EPA. Drinking Water Health Advisory: Pesticides. US EPA; 1989.
7. EU. Directive 80/779/EEC on drinking water quality. Brussels; 1980.
8. Van Tran K, Shia JC, Young MS. Waters Application Note No. 720004220en; May 2012.