Determination of cationic polar pesticides in oat cereals by ion chromatography and electrospray ionization mass spectroscopy

Significance of the topic

The increasing concern over pesticide residues in food, particularly ionic and highly polar compounds, drives the need for robust analytical methods. Cationic polar pesticides such as chlormequat, mepiquat, paraquat, and diquat are non-volatile and ionic, making traditional gas or reversed-phase liquid chromatography less suitable. Ion chromatography (IC) coupled with mass spectrometry (MS) offers selective separation and sensitive detection of these quaternary amines in complex matrices like cereals.

Objectives and study overview

This study evaluates a targeted screening method for four cationic polar pesticides in oat cereals. The method employs Thermo Scientific™ Dionex™ IonPac™ CS21-Fast-4 µm cation-exchange chromatography with an electrolytically generated methanesulfonic acid gradient, suppressed conductivity detection, and single-quadrupole electrospray ionization MS (ISQ EC). Recovery, sensitivity, and applicability to commercial oatmeal and toasted oat cereal samples were assessed against EU maximum residue limits.

Methodology and instrumentation

• Chromatography system: Thermo Scientific Dionex ICS-6000 RFIC™ with Dual Pump DP, Eluent Generator EG (500 MSA cartridge), CR-CTC III trap column, DC module with CDRS 600 suppressor, and AS-AP autosampler.
• Columns: IonPac CG21-Fast-4 µm guard (2 × 30 mm) and CS21-Fast-4 µm analytical (2 × 150 mm).
• Eluent: Electrolytically generated methanesulfonic acid gradient (3 → 25 mM) at 0.30 mL/min, 40 °C.
• Detection: Suppressed conductivity (22 mA, recycle mode) and single-quadrupole MS (ISQ EC) in positive SIM mode for target ions.
• Sample preparation: Modified EURL-FV QuPPE extraction of 5 g ground cereal with 1:1 acid-methanol (formic acid for chlormequat/mepiquat; HCl for paraquat/diquat), heated 80 °C for 15 min, centrifuged, filtered, and injected.

Main results and discussion

• Chromatographic separation: Monovalent inorganic cations elute first, followed by resolved peaks for chlormequat, mepiquat, paraquat, and diquat within 15 min, avoiding co-elution issues of previous columns.
• Calibration performance: Quadratic regressions over 10–200 µg/L yielded r² > 0.9994. Estimated LODs in standards ranged from 0.43 to 3.4 µg/L, corresponding to cereal LODs of 0.0043–0.068 mg/kg.
• Recovery and accuracy: Fortified recoveries at 25 µg/L spiking level in oatmeal and toasted oats cereal ranged from 81% to 108% for all four analytes.
• Matrix effects: Suppressed conductivity detection was overwhelmed by the cereal matrix; selective MS detection in SIM mode enabled quantitation of trace residues despite high background conductivity.
• Regulatory compliance: The method meets EU MRLs for chlormequat (15 mg/kg), mepiquat (3 mg/kg), paraquat (0.02 mg/kg), and diquat (2 mg/kg) in oat cereals.

Benefits and practical applications

• Rapid, accurate screening of cationic polar pesticide residues in cereal matrices.
• Combination of conductivity and MS detection balances broad surveillance with selective quantitation.
• Suitable for QA/QC and regulatory monitoring laboratories requiring compliance with EU limits.

Future trends and potential applications

• Integration of tandem MS (IC-MS/MS) or high-resolution accurate mass spectrometry for lower LODs and three-ion identification in line with SANTE guidelines.
• Expansion of IC-MS methods to additional polar pesticide classes and diverse food and environmental matrices.
• Automation and high-throughput workflows leveraging RFIC and autosampler advances for routine monitoring programs.

Conclusion

The developed IC-MS method provides accurate, sensitive determination of four cationic polar pesticides in oat cereal samples, with LODs below regulatory thresholds and recoveries within 81–108%. The approach combines efficient chromatographic resolution using the IonPac CS21-Fast-4 µm column with selective single-quadrupole MS detection, making it an effective screening tool for regulatory and QA/QC applications. For enhanced sensitivity and confirmatory identification, adoption of IC-MS/MS is recommended.

Reference

• English C. ‘Deadly Glyphosate In Your Food? There’s No Reason to Panic.’ American Council on Science and Health, 2021.
• Adams S. Presentation at North American Chemical Residue Workshop, 2016. ‘Analysis of Polar Pesticides by IC-MS’. Fera Science Ltd.
• Adams S. et al. Agric. Food Chem. 2017, 65, 7294–7304. ‘Development and validation of IC-MS/MS for polar ionic pesticides in food.’
• Bousova K. et al. Thermo Fisher Scientific Application Note 661, 2017. ‘Fast routine analysis of polar pesticides by IC-MS.’
• Christison T. et al. Thermo Fisher Scientific Application Note 72908, 2018. ‘Determination of cationic polar pesticides in fruit and vegetables by IC-HRAM MS.’
• European Commission. SANTE/11312/2021. ‘Analytical quality control and method validation for pesticide residues in food and feed.’