Multi-Residue Method for the Determination of Five Groups of Pesticides in Non-Fatty Food Samples by Dual Stir Bar Sorptive Extraction (Dual SBSE) and Thermal Desorption GC-MS

Importance of the Topic

Monitoring pesticide residues in fruits, vegetables and tea is essential to protect consumer health and ensure regulatory compliance. Traditional multi-residue procedures often involve solvent-intensive extraction, clean-up and concentration steps. The dual stir bar sorptive extraction (SBSE) coupled to thermal desorption GC-MS offers a streamlined, solvent-saving alternative with high sensitivity for a broad range of pesticides.

Objectives and Study Overview

This study aimed to develop, optimize and validate a single-laboratory method for simultaneous determination of 85 pesticides from five chemical classes (organochlorines, carbamates, organophosphorus, pyrethroids and others) in non-fatty food matrices (tomato, cucumber, soybean, spinach, grape and green tea). Key goals were to maximize recovery across a wide polarity range, minimize matrix effects, and achieve limits of detection at low µg/kg levels.

Methodology

Sample extraction was performed by sonicating 25 g of homogenized sample with methanol. The methanol extract was split and diluted with water to twofold and fivefold aqueous mixtures. Each dilution was extracted by a polydimethylsiloxane-coated stir bar (24 µL PDMS) at 24 °C for 60 min with stirring. Both bars were then placed together in a thermal desorption unit (TDU) and thermally desorbed (40 °C→280 °C) into a programmed-temperature vaporization (PTV) inlet for GC-MS analysis.

Instrumentation Used

• GERSTEL Twister® stir bars (24 µL PDMS)
• GERSTEL TDU thermal desorber with MPS 2 autosampler
• GERSTEL CIS 4 PTV inlet (–150 °C to 280 °C)
• Agilent 6890N GC fitted with a 30 m×0.25 mm HP-5MS column
• Agilent 5973N mass selective detector, electron-impact ionization, scan mode (m/z 40–500)

Main Results and Discussion

Out of 100 test compounds, 85 pesticides were successfully extracted and detected with relative standard deviations under 12 %. Recoveries ranged from 0.7 % (very polar analytes) up to 75 % (mid-polarity), correlating with octanol–water partition coefficients (log Kow). A dual-dilution approach balanced adsorption losses (high log Kow compounds favored by 50 % methanol) and partition suppression (low log Kow compounds favored by 10 % methanol). Calibration in fortified spinach extracts yielded linearity (r2>0.99 for 66 pesticides) and detection limits between 0.6 and 26 µg/kg. Application to commercial samples revealed residues in 48 % of fruits, vegetables and tea, with permethrin in spinach approaching the Japanese maximum residue limit.

Benefits and Practical Applications

• Solvent reduction: replaces liquid-liquid and SPE clean-up steps
• High sensitivity in scan mode for multi-residue screening
• Wide polarity coverage via dual dilution extractions
• Direct analysis of solid and tea matrices with minimal preparation
• Potential for high throughput using desorption of two stir bars in one run

Future Trends and Applications

Further developments may include integration of isotope-labeled internal standards for absolute quantification, extension to fatty matrices via matrix simplification or alternative coatings, coupling SBSE desorption directly to LC-MS for thermolabile or highly polar analytes, and full automation of sample handling. Advances in coating materials could expand analyte scope and lower detection limits further.

Conclusion

The dual SBSE-TD-RTL-GC-MS method provides a robust, sensitive and eco-friendly platform for comprehensive multi-residue pesticide analysis in non-fatty foods. Balancing dilution factors enables effective coverage of compounds across a wide polarity range, achieving µg/kg detection limits without extensive solvent use or laborious clean-up.

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