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

Significance of the Topic

The rapid, inexpensive and reliable detection of pesticide residues in food is critical for consumer safety, regulatory compliance and public health. The QuEChERS approach has become a standard for multiclass pesticide analysis in fruits and vegetables, but highly pigmented matrices such as spinach can challenge cleanup due to strong retention of planar pesticides by graphitized carbon black (GCB). Optimizing recoveries of these compounds is essential for accurate quantitation.

Objectives and Overview of the Study

This study evaluated the impact of adding toluene to the dispersive SPE clean-up step in Agilent Bond Elut QuEChERS AOAC kits designed for pigmented produce. The primary goal was to improve recoveries and precision for pesticides that interact strongly with GCB, using spinach as a representative highly pigmented matrix.

Methodology and Instrumentation

Spinach samples (15 g) were homogenized, spiked with target analytes and extracted with 1% acetic acid in acetonitrile using standard AOAC QuEChERS extraction salts. After centrifugation, 1 mL of the upper acetonitrile layer was subjected to dispersive SPE containing PSA, GCB and MgSO₄, with or without toluene added in an 8:3 ratio (ACN:toluene). Clean-up extracts were evaporated under nitrogen and reconstituted in 0.1% formic acid in acetonitrile for analysis.

Main Results and Discussion

The addition of toluene dramatically improved recoveries for eight planar pesticides (e.g., carbendazim, thiabendazole, pymetrozine) from 20–60% to 50–100%, with relative standard deviations below 10%. UV–visible profiles indicated increased co-extracted matrix components, but LC–MS/MS and GC–MS detection remained unaffected for most compounds. A ratio study confirmed that an 8:3 ACN:toluene mixture delivered the best enhancement. A 1 mL dispersive SPE format achieved slightly higher recoveries than an 8 mL format, enabling parallel cleanup of original and modified extracts from the same batch of ACN extract.

Benefits and Practical Applications

The modified QuEChERS method offers a simple, cost-effective alternative to re-extract samples when planar pesticides exhibit low recoveries due to GCB retention. By incorporating toluene into the cleanup step, analysts can achieve robust, precise quantitation of a broader range of pesticides in pigmented matrices without substantially altering established workflows.

Future Trends and Potential Applications

Further exploration of alternative dispersive SPE modifiers could refine the balance between matrix cleanup and analyte recovery. The approach may be extended to other challenging matrices or combined with automated sample preparation platforms. Integration with high-resolution mass spectrometry may expand the scope to emerging contaminants and metabolite profiling.

Conclusion

Incorporating toluene into the dispersive SPE step of QuEChERS AOAC kits for pigmented produce significantly enhances recovery and precision for planar pesticides that are typically retained by GCB. While additional matrix impurities are introduced, modern LC–MS/MS and GC–MS detection remains robust, making this a valuable option for laboratories analyzing highly pigmented fruits and vegetables.

Instrumentation Used

• Agilent 1200 Series HPLC with Diode Array Detector
• Agilent 6410 Triple Quadrupole LC/MS/MS with ESI
• Agilent 5975C Gas Chromatograph–Mass Spectrometer
• Agilent Bond Elut QuEChERS AOAC extraction and dispersive SPE kits

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