Pre-concentration of 218 multiclass pesticide in groundwater samples using MSU-1 mesoporous sorbent

Abstract

A mesoporous material (MSU-1), prepared in the laboratory, was applied as a solid phase sorbent to pre-concentrate 218 multiclass pesticides in environmental waters. The pesticides present in the sample extracts were determined by ultra-high-performance liquid chromatography coupled with tandem mass spectrometry. The most intense ion was selected as precursor ion. After fragmentation in the collision chamber, the most intense transition (SRM1) was used for quantification and three points were used for identification and confirmation, as established by Directive 96/23/EC. A comprehensive study of the main variables affecting the sorption/desorption of pesticides in the MSU-1 was performed using step-by-step (elution solvent, sample pH and salt content) and multi-response surface (sorbent amount, water volume and solvent elution volume) methodologies, with the optimal values being no addition of NaCl, 100 mg MSU-1 sorbent, 50 mL water sample at pH = 6 and elution of pesticides with 5 mL of acetonitrile. The matrix effect was tested by comparing the slopes of solvent-based and matrix-matched calibration graphs. 80 % of pesticides showed a low matrix effect, and 20 % showed a medium or high matrix effect. Two calibration strategies were tested to correct the matrix effect by adding the pesticide standards before or after the pre-concentration step. The first one corrected for systematic errors due to the low adsorption of pesticides in the MSU-1 and corrected for the matrix effect. The method was validated using blank groundwater samples spiked with 0.1, 0.25 and 0.5 μg/L of all pesticides. Recoveries between 54 and 130 % (RSD ≤ 20 %) were obtained for most pesticides (90 %), while recoveries ≥ 130 % were obtained for 22, 10 and 5 pesticides at 0.1, 0.25 μg/L and 0.5 μg/L, respectively. A monitoring study of thirteen groundwater samples collected in Almería (South of Spain) was performed applying the proposed methodology. Thirty-eight different pesticides were found in the groundwater samples analyzed. However, most of them were found in concentrations below 0.1 µg/L, with imidacloprid, hexaflumuron and oxadixyl being the most frequently detected in 7, 6 and 5 samples, respectively. Only in 5 samples, methiocarb (three samples), pyrethrin I and pytethrin II (one sample) and hezaflumuron (one sample) were found at levels higher than 0.1 μg/L. The method green profile was evaluated using National Environmental Methods Index and analytical Eco-scale assessment tools.