Determination of cationic polar pesticides in cereals using improved cation-exchange separation technology combined with suppressed conductivity and tandem mass spectrometry (RAFA)

Importance of the topic

Polar pesticides with quaternary amine structures are applied as desiccants in cereal production to accelerate drying and prevent mold growth. However, residual levels of mepiquat, chlormequat, paraquat, and diquat in food may pose health risks. Sensitive and selective analytical methods are therefore critical for monitoring these charged compounds at trace levels in complex cereal matrices.

Study objective and overview

This work presents a validated ion chromatography–tandem mass spectrometry (IC-MS/MS) method for determining four cationic polar pesticides in oat cereal products. The approach combines the European QuPPe extraction protocol with improved cation-exchange separation, suppressed conductivity detection, and selective reaction monitoring for quantitation.

Methodology

Extraction procedure

• Sample matrices: ground oatmeal and toasted oat cereal.
• Extraction: QuPPe version 12 method using formic acid–methanol for mepiquat and chlormequat, and HCl–methanol for paraquat and diquat.
• Sample cleanup: centrifugation and appropriate dilution following EURL guidelines.

Chromatographic separation

• Column: Thermo Scientific Dionex IonPac CS21-Fast-4 µm guard and analytical columns (2 mm i.d.).
• Eluent: methanesulfonic acid gradient from 3 to 25 mM over 15 min, followed by a 1 mM MSA wash for 15 min to remove matrix residues; total run time 34 min.
• Operating conditions: 0.30 mL/min flow rate, column temperature 40 °C, suppressor compartment at 20 °C.

Detection

• Suppressed conductivity detection using a Dionex CDRS 600 suppressor (2 mm, 22 mA).
• Mass spectrometry: Thermo Scientific TSQ Altis Plus triple quadrupole with HESI-II source, operated in positive SRM mode.
• Quantitation: optimized SRM transitions and isotopically labeled internal standards for each analyte.

Instrumentation

• Ion chromatography: Thermo Scientific Dionex ICS-6000 HPIC system.
• Autosampler: Thermo Scientific Dionex AS-AP.
• Suppressor: Dionex CDRS 600 electrolytic suppressor.
• Mass spectrometer: Thermo Scientific TSQ Altis Plus triple quadrupole.
• Data system: Thermo Scientific Chromeleon CDS 7.

Results and discussion

• Chromatographic resolution: baseline separation of quaternary amine pesticides and common matrix cations within 15 min, including closely related paraquat and diquat peaks.
• Sensitivity: limits of detection of 0.07–0.09 µg/L (equivalent to < 0.5 µg/kg in cereal). Calibration curves displayed quadratic behavior over 1–100 µg/L.
• Recoveries: ranged from 85 % to 117 % at 5 µg/L fortification, satisfying typical regulatory performance criteria.
• Matrix effects: a 15-min 1 mM MSA column wash was necessary for HCl-extracted samples to eliminate residual interferences.
• Natural residue levels: 0.4–1.7 µg/kg in oat cereals, well below EU maximum residue limits (0.02–15 mg/kg).

Benefits and practical applications

• High selectivity and sensitivity for quaternary amine pesticides in complex food matrices.
• Efficient removal of matrix components via post-run column wash, preserving column performance.
• Compliance with European QuPPe guidelines and capability to meet stringent regulatory limits.
• Adaptable for routine monitoring in quality control and food safety laboratories.

Future trends and potential applications

• Adaptation of faster gradients and high-throughput column technologies to increase sample throughput.
• Extension of the approach to other charged pesticide classes and diverse agricultural commodities.
• Integration with high-resolution mass spectrometry for broader screening of polar contaminants.
• Automation of sample preparation workflows to enhance reproducibility and reduce labor.

Conclusion

This IC-MS/MS method provides a robust platform for detecting and quantifying quaternary amine polar pesticides in oat cereals with low detection limits, accurate recoveries, and reliable chromatographic separation. It supports regulatory compliance and ensures consumer safety by enabling precise monitoring of residual pesticide levels.

References

1. Kolberg D.I.S.; Mack D.; Anastassiades M.; Hetmanski M.T.; Fussell R.J.; Meijer T.; Mol H.G. Ion chromatography–mass spectrometry analysis of polar pesticides. Anal Bioanal Chem. 2012. DOI:10.1007/s00216-012-6340-9
2. European Commission. QuPPe-PO: Quick Polar Pesticide Method, Version 12. European Research Laboratories. 2017.
3. Madden J.E.; Saini C. Thermo Scientific Technical Note TN73990. 2022.
4. Christison T.; Madden J.E.; Rohrer J. Thermo Scientific Application Note AN000607. 2022.