Multi-residue Pesticide Analysis in Rice by a Modified QuEChERS Extraction and Ion Trap GC/MSn Analysis

Importance of the Topic

The analysis of multiple pesticide residues in staple foods such as rice is critical for ensuring food safety and regulatory compliance. Modern pesticides often feature low molecular weights and rapid environmental degradation, which complicates their extraction and detection. A robust multi-residue method enables reliable monitoring of hundreds of compounds at trace levels, protecting consumers and meeting international maximum residue limits (MRLs).

Objectives and Overview of the Study

This study evaluates a modified QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) extraction protocol coupled with gas chromatography–ion trap tandem mass spectrometry (GC/MSn) for quantifying 33 commonly used pesticides in rice. Key aims include:

• Optimizing extraction, cleanup, and solvent exchange steps.
• Establishing linear calibration ranges, limits of detection (LODs), and limits of quantitation (LOQs).
• Assessing method precision, accuracy, and matrix effects.
• Comparing sensitivity against international MRLs.

Methodology and Instrumentation

Sample Preparation Workflow:

1. Homogenize 15 g of rice with water and add 15 mL of 1 % acetic acid in acetonitrile.
2. Add buffered salts (MgSO₄ and CH₃COONa), vortex, shake, and centrifuge to partition the extract.
3. Transfer 11 mL of the upper acetonitrile layer to a cleanup tube containing PSA, C18, and MgSO₄, then vortex and centrifuge.
4. Take a 5 mL aliquot, evaporate to dryness under nitrogen at 40 °C, and reconstitute in hexane/acetone (9:1) with an internal standard (d₁₀-parathion).
5. Perform final cleanup, centrifuge, and transfer 200 µL to an autosampler vial for GC/MSn analysis.

Key Notes:

• Consumables such as silanized glassware, PSA sorbent, C18, and high-purity solvents were rigorously tested.
• Solvent exchange to a nonpolar medium optimizes splitless injection efficiency.

Instrumentation Used

• Gas Chromatograph: Thermo Scientific FOCUS GC with TRACE TR-527 column (35 % diphenyl/65 % dimethyl polysiloxane, 0.25 mm×30 m, 0.25 µm film, 5 m guard).
• Injector: Single-channel splitless with surge pressure (250 kPa for 0.5 min) and 5 mm i.d. splitless liner.
• Mass Spectrometer: Thermo Scientific ITQ 700 ion trap operated in MSn mode (EI source, 250 °C, 70 eV, detector gain 3).
• Autosampler: AS 3000 II, 2 µL injection volume, three solvent washes.

Main Results and Discussion

Linearity and Calibration:

• Calibration curves were constructed in matrix from low ng/g to the MRL range per compound.
• Correlation coefficients (R²) exceeded 0.995 for all 33 pesticides (average R² = 0.9993).

Detection Limits:

• LODs ranged from 3 to 25 ng/g and LOQs from 11 to 80 ng/g in rice.
• Average LOD across all analytes was 9 ng/g, well below most international MRLs.

Precision and Accuracy:

• Method validation at three spike levels (160, 320, 480 ng/g) yielded an average recovery of 98.1 % and RSD of 5.9 % (n = 5).
• Low-level spikes (16–120 ng/g) confirmed LOQ calculations, with standard deviations multiplied by typical factors (3× for LOD, 10× for LOQ).

Breakdown Monitoring:

• Daily check with endrin and DDT demonstrated less than 5 % breakdown, indicating minimal active-site induced degradation and prolonged injector life.

Contributions and Practical Applications

This protocol provides a streamlined, cost-effective workflow that integrates extraction, cleanup, and MSn detection for routine high-throughput pesticide surveillance in rice. The use of MSn increases selectivity and sensitivity, reducing matrix interference at trace levels. Surge splitless injection minimizes analyte decomposition and improves quantitation of fragile compounds. The method supports compliance with Codex, EU, Japanese, and US-EPA MRLs in a single run.

Future Trends and Potential Applications

The combination of QuEChERS with advanced ion trap MSn is adaptable to other food matrices and emerging contaminants. Future directions may include:

• Automation of the cleanup step using online SPE or multi-well formats.
• Integration with high-resolution MS for non-targeted screening of pesticide metabolites.
• Expansion to polar and thermally labile compounds using tailored solvent systems and derivatization.

Conclusion

The modified QuEChERS extraction coupled with GC/MSn on the ITQ 700 platform offers accurate, precise, and sensitive determination of 33 pesticide residues in rice. The method demonstrates excellent linearity, low detection limits, and high recoveries, making it suitable for regulatory monitoring and quality control in food safety laboratories.

References

1. Lehotay S.J., et al. AOAC Official Method 2007.01: Pesticide Residues in Foods by Acetonitrile Extraction and Partitioning with Magnesium Sulfate. Journal of AOAC International, 90(2), 485–520 (2007).
2. Okihashi M., et al. Rapid Determination of 180 Pesticide Residues in Foods by GC/MS and Flame Photometric Detection. Journal of Pesticide Science, 30(4), 368–377 (2005).