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

Significance of the Topic

The detection of modern, low-molecular-weight pesticides in food matrices has become critical for ensuring consumer safety and regulatory compliance. Onions, as a widely consumed vegetable, require robust analytical methods to monitor trace pesticide residues accurately and efficiently.

Study Objectives and Overview

This study set out to develop and validate a single-run, multi-residue pesticide analysis in onion using a modified QuEChERS extraction followed by GC/MSn on an ion trap system. The aims included defining linear calibration ranges, establishing detection and quantitation limits, and verifying method precision and accuracy through matrix spike experiments.

Methodology and Instrumentation

Sample preparation combined buffered acetonitrile extraction with salting-out (sodium acetate and MgSO4), dispersive SPE cleanup, and a solvent exchange to hexane/acetone (9:1) to optimize splitless GC injection. Instrumentation

• Thermo Scientific AS 3000 Autosampler
• Thermo Scientific FOCUS GC with surge splitless injection
• Thermo Scientific ITQ 700 GC-ion trap mass spectrometer in MSn mode
• TRACE™ TR-35MS column (35% diphenyl/65% dimethyl polysiloxane, 30 m×0.25 mm, 0.25 µm, 5 m guard)

Main Results and Discussion

Calibration curves in onion matrix achieved R2 > 0.995 over concentration ranges tailored to each pesticide (1–1200 ng/g). Limits of detection varied from 2 to 27 ng/g and LOQs from 6 to 78 ng/g, all compliant with international MRLs. Recoveries averaged 116% (79–159%) with RSDs below 28% at low-level spikes. Daily monitoring of endrin breakdown showed < 5% decomposition, confirming low active-site interactions and extended inlet life. The MSn approach enhanced selectivity by isolating precursor ions and inducing fragmentation, reducing matrix interference compared to full scan and SIM modes.

Benefits and Practical Applications

The validated workflow offers

• Rapid, single-step extraction and cleanup
• Comprehensive coverage of 46 pesticides
• Enhanced sensitivity and selectivity via tandem MS
• Reduced maintenance through low analyte degradation

This method is suitable for routine QA/QC laboratories and regulatory monitoring programs.

Future Trends and Potential Applications

Emerging developments may include

• Automation and micro-scale extraction to increase throughput
• Expanded MSn spectral libraries for new pesticide chemistries
• Integration with data-driven screening and risk assessment tools
• Coupling with high-resolution mass spectrometry for non-targeted analyses

Conclusion

The combination of a modified QuEChERS protocol with ion trap GC/MSn provides a reliable, sensitive, and high-throughput solution for multi-residue pesticide analysis in onions. The method meets stringent regulatory criteria while minimizing instrument downtime and maintenance.

References

• AOAC Official Method 2007.01, Journal of AOAC International, 90(2), 485–520 (2007)
• Okihashi M., Rapid Method for the Determination of 180 Pesticide Residues in Foods by Gas Chromatography/Mass Spectrometry and Flame Photometric Detection, Journal of Pesticide Science, 30(4), 368–377 (2005)
• Commission Decision 2002/657/EC Implementing Council Directive 96/23/EC, Official Journal of the European Communities, 17 Aug 2002
• Codex Alimentarius, Maximum Residue Limits for Lettuce, accessed via www.codexalimentarius.net