Optimizing Recoveries of Planar Pesticides in Spinach Using Toluene and Agilent Bond Elut QuEChERS AOAC Kits with Graphitized Carbon

Importance of the Topic

The analysis of pesticide residues in leafy green vegetables is critical for food safety and regulatory compliance.
High pigment levels in matrices like spinach pose cleanup challenges due to chlorophyll retention and coextracted interferences.
Graphitized carbon black (GCB) is effective for pigment removal but retains planar pesticide molecules, causing low recovery.

Objectives and Study Overview

This study evaluates the impact of adding toluene during the dispersive solid-phase extraction (SPE) cleanup step on the recovery and precision of planar pesticides in spinach using Agilent Bond Elut QuEChERS AOAC kits.
Comparisons are made between the original AOAC method without toluene and the modified approach with various ACN:toluene ratios and SPE tube sizes.

Methodology and Instrumentation

• Extraction: 15 g homogenized spinach extracted with 1% acetic acid in acetonitrile, MgSO4 and buffered salt.
• Cleanup: Dispersive SPE tubes (1 mL or 8 mL) containing PSA, GCB and MgSO4; addition of toluene at an optimized ratio of 8:3 (ACN:toluene).
• Comparison: Original protocol (no toluene) versus modified protocol (with toluene).
• Instrumentation:
  • HPLC-DAD and triple-quadrupole LC-MS/MS (Agilent 1200 Series with 6410 MS/MS).
  • Gas chromatography with backflush and single-quadrupole MS (Agilent 5975C).

Main Results and Discussion

• Recoveries of eight problematic planar pesticides increased from 20–60% to 50–100% with RSD <10% when toluene was added.
• An ACN:toluene ratio of 8:3 provided the highest analyte responses, with more toluene yielding up to 300% response improvement.
• 1 mL dispersive SPE tubes offered slightly higher recoveries than 8 mL tubes and allow concurrent processing of extracts.
• Matrix impurities increased, as shown by UV detection, but had negligible effects on LC/MS/MS and GC/MS pesticide analyses.
• Some non-planar compounds experienced minor recovery losses (<15%), and dichlorvos showed significant signal loss, indicating method limitations for certain analytes.

Benefits and Practical Applications

• Enhanced quantification of planar pesticides in highly pigmented matrices without repeating the initial extraction.
• Maintains the efficiency, simplicity and cost-effectiveness of the QuEChERS workflow.
• Enables simultaneous use of original and modified cleanup to cover a broader pesticide range.

Future Trends and Potential Applications

• Development of tailored sorbents and solvent blends for other challenging food matrices.
• Coupling modified QuEChERS protocols with high-resolution mass spectrometry for comprehensive residue screening.
• Automation of the modified cleanup step for high-throughput testing laboratories.

Conclusion

Adding toluene in the dispersive SPE step significantly improves recovery and precision for planar pesticides in spinach but introduces additional matrix impurities and may reduce performance for some analytes. The modified method serves as a valuable alternative for challenging compounds rather than a full replacement.

Reference

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