Optimizing Recoveries of Planar Pesticides in Spinach using Toluene and the AOAC QuEChERS Kits with Graphitized Carbon

Significance of the Topic

Spinach is a heavily pesticide-contaminated produce requiring reliable analytical methods to ensure consumer safety. Planar pesticides with structures prone to retention by graphitized carbon black (GCB) during QuEChERS cleanup often yield low recoveries and poor precision. Improving extraction efficiency for these compounds is critical in routine QA/QC and regulatory monitoring.

Study Objectives and Overview

This study evaluates the impact of adding toluene during the dispersive SPE cleanup step on the recovery and precision of planar pesticides in spinach extracts. The goal was to modify an Agilent SampliQ QuEChERS AOAC method to enhance quantitation via LC/MS/MS for thirteen representative pesticides, focusing on those with planar structures such as pymetrozine, carbendazim, thiabendazole, and cyprodinil.

Methodology

Fresh spinach samples (15 g) were spiked with pesticides and internal standard, then extracted with acetonitrile acidified with acetic acid using QuEChERS salts. After centrifugation, the acetonitrile layer underwent dispersive SPE cleanup in tubes containing GCB and other sorbents. Various ACN:toluene ratios (8:0, 8:1, 8:2, 8:3) and different extract volumes (1 mL vs 8 mL) were tested. Final extracts were diluted and analyzed by LC/MS/MS in MRM mode.

Used Instrumentation

• SampliQ QuEChERS AOAC buffered extraction kits (Agilent)
• Dispersive SPE tubes with Graphitized Carbon Black (GCB)
• Centrifuge (4,000 rpm for extraction, 13,000 rpm for cleanup)
• LC/MS/MS system with MRM detection

Key Results and Discussion

• Toluene addition dramatically improved recoveries of planar pesticides by 50–300%, with average recovery of approximately 91.9%.
• Relative standard deviations decreased from above 15% to an average of approximately 3.3%, meeting precision requirements for routine analysis.
• Optimal ACN:toluene ratio was 8:3; both 1 mL and 8 mL cleanup formats showed significant gains.
• Chromatograms confirmed enhanced peak areas for target analytes without adverse matrix effects.
• Limit of quantitation achieved was 5 ng/g, well below regulatory MRLs.

Practical Benefits and Applications

• The modified QuEChERS protocol is simple, fast, and cost-effective, requiring only addition of toluene to existing workflows.
• Improved data quality supports reliable pesticide compliance testing in spinach and other pigmented produce.
• Method flexibility allows adaptation to different sample sizes and cleanup formats.

Future Trends and Opportunities

Integration of alternative non-polar solvents or sorbents could further tailor cleanup to specific chemical classes. Automation of the modified QuEChERS workflow in high-throughput platforms may increase laboratory efficiency. Expansion of similar strategies to other matrices such as fruits and herbs will broaden application scope. Advances in high-resolution mass spectrometry can complement this optimized cleanup to address emerging pesticide residues.

Conclusion

Introducing toluene into the dispersive SPE step effectively overcomes the loss of planar pesticides to GCB, achieving high recoveries and precision in spinach. The modified Agilent SampliQ QuEChERS AOAC method is a robust solution for pesticide residue analysis, ensuring regulatory compliance and safeguarding consumer health.