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

Importance of the Topic

Spinach ranks among the most pesticide-loaded leafy vegetables, driven by its dark green leaves and widespread consumption. Ensuring accurate quantitation of pesticide residues in spinach is critical for food safety monitoring, compliance with regulatory maximum residue limits, and protection of public health.

Objectives and Overview of the Study

This study aimed to improve recovery and precision for planar pesticides—such as pymetrozine, carbendazim, thiabendazole and cyprodinil—in spinach extracts prepared with AOAC QuEChERS kits. The research evaluated the detrimental impact of graphitized carbon black (GCB) on analyte retention and tested toluene addition during dispersive-SPE cleanup to overcome poor recoveries and high variability.

Methodology and Instrumentation

Sample Preparation and Cleanup:

• 15 g of homogenized spinach spiked with pesticides and internal standard.
• Extraction with 15 mL of 1 % acetic acid in acetonitrile and AOAC buffered QuEChERS salts.
• Partitioning by centrifugation at 4 000 rpm for 5 min.
• Dispersive-SPE using SampliQ AOAC kits containing GCB, with and without addition of toluene in various ACN:toluene ratios (8:1, 8:2, 8:3).
• Final dilution and filtration for LC/MS/MS analysis.

Instrumentation:

• Triple quadrupole LC/MS/MS in MRM mode.
• Graphitized carbon black for pigment removal.
• Agilent SampliQ QuEChERS AOAC buffered extraction and dispersive-SPE kits.
• Standard laboratory centrifuges and nitrogen evaporators.

Main Results and Discussion

The original QuEChERS cleanup with GCB yielded planar pesticide recoveries of 20–40 % and RSDs exceeding acceptable limits (>15 %). Introducing toluene into the dispersive-SPE step substantially enhanced analyte desorption from GCB. At an ACN:toluene ratio of 8:3, recoveries increased by 50–300 % (average 92 %), and precision improved dramatically (average RSD 3.3 %). Chromatograms showed restored peak areas for all targeted planar pesticides without adverse matrix effects or background interference.

Benefits and Practical Applications of the Method

• Strongly improved analytical performance for challenging planar pesticides in pigment-rich matrices.
• Simple modification to existing AOAC QuEChERS protocols without additional instrument investment.
• Lower limits of quantitation (5 ng/g) well under EU and U.S. regulatory maximum residue limits.
• Applicable in routine QA/QC labs, environmental monitoring and research settings.

Future Trends and Potential Applications

Emerging directions include:

• Extension of toluene-based cleanup to other high-pigment fruits and vegetables.
• Integration with high-throughput and automated QuEChERS workflows.
• Exploration of alternative non-chlorinated solvents for greener cleanup options.
• Development of multi-residue methods encompassing a broader range of problematic analytes.

Conclusion

Modifying the AOAC QuEChERS dispersive-SPE step by adding toluene effectively overcomes GCB-induced losses of planar pesticides in spinach. The approach delivers robust recoveries, outstanding precision and low detection limits, enabling reliable pesticide residue analysis in high-pigment produce.

Reference

• Schenck F. Determination of Pesticides in Food of Vegetal Origin. In: Analysis of Pesticides in Food & Environmental Samples. CRC Press; 2008. Chapter 6.