A method for the quantification of low concentration sulfamethazine residues in milk based on molecularly imprinted clean-up and surface preconcentration at a Nafion-modified glassy carbon electrode

An electrochemical method for the determination of sulfamethazine at a low concentration level (25 μg l−1) in milk is reported. The method involves sample clean-up and selective preconcentration of sulfamethazine with a molecularly imprinted polymer (MIP), and a further electrode surface preconcentration of the analyte at a Nafion-coated glassy carbon electrode (GCE). Square wave (SW) oxidative voltammetry of accumulated sulfamethazine was employed for its quantification. Sulfamethazine electrode preconcentration was carried out in 0.1 mol l−1 Britton–Robinson buffer of pH 1.5, and by applying 5 min of accumulation at open circuit. A linear calibration graph was obtained for sulfamethazine at the Nafion-modified GCE over the 1.0 × 10−8 to 1.0 × 10−6 mol l−1 concentration range, with a detection limit of 6.8 × 10−9 mol l−1 (1.9 μg l−1). This detection limit is remarkably better than those reported previously in the literature using electroanalytical techniques. Although the detection limit achieved was sufficient to allow the direct determination of sulfamethazine at the concentration level required in milk, a sample clean-up was shown to be necessary to obtain analytically useful SW voltammograms. This was accomplished by processing the deproteinized milk through a cartridge containing a molecularly imprinted polymer for sulfamethazine, also allowing a selective preconcentration of the analyte. Elution of the analyte from the MIP cartridges was carried out with 2 ml of a (9:1) MeOH:acetic acid mixture. Determination of sulfamethazine in milk samples was accomplished by interpolation into a calibration graph constructed with sulfamethazine standard solutions which were subjected to the same procedure than the deproteinized milk samples. Results obtained for five samples, spiked at the 25 μg l−1 level, showed a mean recovery of (100 ± 3)%.