Investigation of preconcentration strategies for the trace analysis of multi-residue pesticides in real samples by capillary electrophoresis

In this work, on-line preconcentration strategies were investigated for the multi-residue analysis of pesticides in drinking water and vegetables using micellar electrokinetic chromatography. Among the on-line strategies, sweeping and stacking with reverse migration of micelles (SRMM), with and without the insertion of a plug of water before sample injection, were contrasted. A new version of SRMM was also introduced. The modification consisted of momentarily applying a positive voltage at the inlet vial right after sample has been injected, increasing the efficiency by which the analytes are captured. Nine pesticides from different classes, carbendazim (benzimidazole), simazine, atrazine, propazine and ametryn (triazine), diuron and linuron (urea), carbaryl and propoxur (carbamate), were baseline separated in less than 6 min with a electrolyte composed of 20 mmol l−1 phosphate buffer at pH 2.5, containing 25 mmol l−1 sodium dodecyl sulfate and 10% methanol. Limits of detection (LODs) in the order of 2–46 μg l−1 for the pesticides under investigation were obtained solely using the on-line strategies. Enrichment factors of 3–18-fold were obtained. These factors were computed as the improvement of the concentration LODs with respect to the reference condition (injection of 10 s at 2.5 kPa pressure). The proposed methodologies were applied to the analysis of pesticides in complex matrices such as carrot extracts where the detection of 2.5 μg l−1 was illustrated. By combining off-line solid-phase extraction and the proposed on-line strategies, the detection of pesticides in drinking water at the 0.1 μg l−1 level was conceived.