Improved Resolution for Paraquat and Diquat: Drinking Water Analysis Using the CORTECS UPLC HILIC Column

Significance of the Topic

Paraquat and diquat are highly polar, doubly charged quaternary ammonium herbicides extensively used for crop and aquatic weed control. Their polarity makes them difficult to retain on conventional reversed-phase columns, while regulatory limits in drinking water are stringent (20 µg/L in the US; 0.1 µg/L for individual pesticides in the EU). Reliable analytical methods are therefore essential for routine monitoring and ensuring public safety.

Study Objectives and Overview

This work evaluates a CORTECS UPLC HILIC column for simultaneous separation of paraquat and diquat in drinking water. The goal is to achieve baseline resolution with simplified mobile phases, enabling both UV and MS detection at sub-µg/L levels without the need for ion-pair reagents.

Methodology and Instrumentation

Samples of tap water were dechlorinated, pH-adjusted to 7.0, and enriched by SPE using Oasis WCX cartridges. Chromatography was performed isocratically (50:50 200 mM ammonium formate buffer at pH 3.7 / acetonitrile) at 0.5 mL/min and 30 °C. Detection modes included UV absorbance at 308 nm (diquat) and 257 nm (paraquat), and ESI-MS/MS monitoring MRM transitions 183.1 > 157.1 and 185.1 > 170.1.

Used Instrumentation

• UPLC System: ACQUITY UPLC H-Class with PDA detector
• Column: CORTECS UPLC HILIC, 1.6 µm, 2.1 × 100 mm
• Mass Spectrometer: ACQUITY TQD with positive ESI
• SPE Cartridges: Oasis WCX (33 cc, 60 mg)

Results and Discussion

Compared to a BEH HILIC column, the CORTECS phase delivered increased retention and baseline separation of the two “quat” analytes. UV detection achieved an LOQ of 500 ng/L with recoveries of 74–90% (RSD < 9%), while MS detection reached 50 ng/L LOQ with recoveries of 77–109% (RSD < 7%). Calibration was linear for UV from 100–5000 ng/L (r2 > 0.997) and for MS from 25–2000 ng/L (r2 > 0.998). Calculated MDLs by UV were 0.06 µg/L for diquat and 0.13 µg/L for paraquat, surpassing EPA Method 549.2 requirements.

Benefits and Practical Applications

The method avoids ion-pair reagents, improving MS sensitivity and reducing matrix suppression. Baseline separation enables dual detection modes without altering chromatographic conditions. The protocol meets EU and US regulatory thresholds and is suitable for routine QA/QC in environmental and water laboratories.

Future Trends and Potential Applications

• Application of HILIC separations to a broader range of polar pesticides and metabolites
• Combination with high-resolution mass spectrometry for multi-residue screening
• Automation of SPE workflows for high-throughput monitoring
• Extension to food and environmental matrices requiring trace pesticide analysis

Conclusion

The CORTECS UPLC HILIC column offers superior retention and resolution for paraquat and diquat, enabling robust UV and MS detection at regulatory levels. The streamlined mobile phase and SPE workflow result in a sensitive, reliable method for drinking water monitoring.

Reference

• Council Directive 98/83/EC on the Quality of Water Intended for Human Consumption. Official Journal of the European Communities. 1998.
• Van Tran K., Shia J.C., Young M.S. Fast and Sensitive UPLC/MS(MS) Determination of Diquat and Paraquat in Drinking Water. Waters Application Note 720004220en. 2012.