Analytical method validation to evaluate dithiocarbamates degradation in biobeds in South of Brazil

In order to evaluate the efficiency of biobeds on DTC degradation, the aim of this study was to apply, optimize and validate a method to determine dithiocarbamate (mancozeb) in biobeds using gas chromatography-tandem mass spectrometry (GC-MS). The DTC pesticide mancozeb was hydrolysed in a tin (II) chloride solution at 1.5% in HCl (4 mol L^-1), during 1 h in a water bath at 80 °C, and the CS₂ formed was extracted in isooctane. After cooling, 1 mL of the organic layer was transferred to an auto sampler vial and analyzed by GC-MS. A complete validation study was performed and the following parameters were assessed: linearity of the analytical curve (r²), estimated method and instrument limits of detection and limits of quantification (LODm, LODi, LOQm and LOQi, respectively), accuracy (recovery%), precision (RSD%) and matrix effects. Recovery experiments were carried out with a standard spiking solution of the DTC pesticide thiram. Blank biobed (biomixture) samples were spiked at the three levels corresponding to the CS₂ concentrations of 1, 3 and 5 mg kg^-1, with seven replicates each (n = 7). The method presented satisfactory accuracy, with recoveries within the range of 89-96% and RSD ≤ 11%. The analytical curves were linear in the concentration range of 0.05-10 µg CS₂ mL^-1 (r² > 0.9946). LODm and LOQm were 0.1 and 0.5 mg CS₂ kg^-1, respectively, and the calculated matrix effects were not significant (≤ 20%). The validated method was applied to 80 samples (biomixture), from sixteen different biobeds (collected at five sampling times) during fourteen months. Ten percent of samples presented CS₂ concentration below the LOD (0.1 mg CS₂ kg^-1) and 49% of them showed results below the LOQ (0.5 mg CS₂ kg^-1), which demonstrates the biobeds capability to degrade DTC.