Analysis of Diquat and Paraquat Using UHPLC Orbitrap MS – Method Development, Matrix Effects and Performance

Significance of the Topic

Paraquat and diquat are widely used quaternary ammonium herbicides with significant toxicity concerns and stringent regulatory limits in water. Sensitive and reliable analytical methods are essential to monitor these contaminants at low concentrations and ensure compliance with environmental standards.

Objectives and Study Overview

The study aimed to develop and optimize a UHPLC–Orbitrap MS method using a mixed-mode Acclaim Trinity Q1 column. Key goals included assessing mobile phase pH and electrospray ionization parameters to achieve clear separation, high signal‐to‐noise ratios, and accurate quantification of paraquat and diquat in environmental water samples.

Methodology and Instrumentation Used

Sample Preparation:

• Direct injection of drinking and filtered surface water without extensive cleanup.
• Silanized glassware and addition of deuterated internal standards to control adsorption and improve accuracy.

UHPLC Conditions:

• Thermo Scientific Dionex UltiMate 3000 with HRG-3400RS pump and Acclaim Trinity Q1 column (2.1×50 mm, 3 µm).
• Isocratic elution using acetonitrile and 100 mM ammonium acetate (75:25 v/v) at pH 5, 0.45 mL/min, 35 °C, total run time 5 min.
• Mobile phases also prepared at pH 3.5, 6.2, and 7.3 to evaluate pH effects.

Mass Spectrometry:

• Thermo Scientific Exactive Plus Orbitrap MS with HESI II probe in positive mode.
• All-ion fragmentation (AIF) with normalized collision energy of 35 eV.
• Spray voltage 2000 V, resolution 140 000 (FWHM), data acquisition >1.5 scans/sec.
• Data processing with Xcalibur, ExactFinder and TraceFinder using narrow mass extraction windows (≤5 ppm).

Main Results and Discussion

Declustering Potential:
Optimization by flow injection showed minimal effect on sensitivity across 700–3200 V; 2000 V was selected.
Mobile Phase pH:
Separation and sensitivity were optimal at pH 5, with lower response at pH 3.5. High‐resolution MS resolved paraquat-diquat isotopic overlap.
Molecular Ions:
[M−H]+ yielded the highest area counts and best precision compared to [M]2+ and [M]+• ions.
Calibration and Sensitivity:
Linear response from 0.5 to 100 µg/L with R2 > 0.999. Method detection limits were 0.05 µg/L for paraquat and 0.15 µg/L for diquat.
Confirmation:
AIF product ions provided additional confirmation via characteristic fragment peaks for each analyte.

Benefits and Practical Applications

• Rapid isocratic analysis in 5 minutes without complex sample cleanup.
• High sensitivity and minimal matrix interference via narrow mass extraction.
• Accurate identification and confirmation using exact mass and fragmentation data.
• Direct compliance with multiple regulatory MCL requirements for drinking water.

Future Trends and Opportunities

Potential extensions include application to other quaternary ammonium compounds, integration with automated sample workflows, further lowering detection limits, and coupling with advanced data‐analysis algorithms for real‐time environmental monitoring.

Conclusion

The developed UHPLC–Orbitrap MS method delivers fast, sensitive, and robust quantification of paraquat and diquat in water, meeting diverse regulatory criteria and offering reliable confirmation through accurate‐mass and fragmentation data.

References

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• Lee XP et al. J Mass Spectrom 39 (2004) 1147.