Determination of thiamethoxam residue in bean leach waters using liquid-liquid extraction with low-temperature partitioning

ABSTRACT – Thiamethoxam, a systemic insecticide of the neonicotinoid class, is one of the main pesticides used in bean crops, presenting high solubility in water and low retention in the soil. These properties favor leaching through the soil and contaminating surface and groundwater with thiamethoxam residues. In this study, we developed a method based on liquid-liquid extraction with low-temperature partitioning (LLE-LTP) since it is a simple technique with low analyst intervention and is efficient for extracting pesticide residues from complex matrices.^[1] The extracts were analyzed by high-performance liquid chromatography with diode array detection (HPLC-DAD). The development of the method consisted of the univariate optimization of the extracting solvent, solvent volume, water sample volume, and freezing time in the LLE-LTP. As a result, acetonitrile was identified as the best solvent for extracting 2.0 mL of solvent and 2.0 mL of sample, with a freezing time of 2 h. Under these conditions, an extraction rate of 110 ± 1% was achieved, demonstrating the influence of soil organic components during the leaching process. The method's performance was evaluated according to the criteria established by the SANTE/11312/2021.V2 validation guide, evidencing its selectivity, accuracy, and precision, with an average recovery of 103 ± 10%. The quantification limit obtained was 10 µg L^-1, below the maximum allowed limit of thiamethoxam in water of 36 µg L^-1.^[2] The matrix effect recorded was 20.3%, and the method's robustness was demonstrated by applying the Youden test. The methodology was used to water samples leached from a bean crop after spraying, seed treatment, and bioactive organomineral fertilizers (with thiamethoxam in the composition). The insecticide residue was detected in water samples collected 15 and 30 days after spraying. The results confirm that the proposed method for monitoring thiamethoxam residues in leached water is an alternative to the official methods that require many steps and employ high volumes of organic solvents.