Solid-phase molecularly imprinted on-line preconcentration and voltammetric determination of sulfamethazine in milk

The synthesis and evaluation of a molecularly imprinted polymer (MIP) as a selective solid-phase extraction sorbent, coupled to voltammetric detection, for the efficient preconcentration and determination of sulfamethazine in milk is reported. The polymer was prepared using sulfamethazine as the template molecule, methacrylic acid as the functional monomer and ethylene glycol dimethacrylate as the cross-linking monomer in the presence of acetonitrile as the solvent. The detection of sulfamethazine was carried out by square wave voltammetry (SVW) at a glassy carbon electrode in a MeOH:acetic acid (9:1) medium. This solvent mixture was also used for the elution of the analyte from the MIP microcolumns. Optimisation of the variables involved in the sulfamethazine binding/extraction process was carried out (solvent used for rebinding, pH of the rebinding solution, eluent volume, analyte and eluent flow rates). Using solid-phase extraction cartridges, containing 0.16-0.18 g of MIP, an eluent volume of 2 ml and a sample volume of 90 ml, resulted in a nominal enrichment factor of 45, which was sufficient to analyse sulfamethazine at the maximum level permitted by the Codex Alimentarius Commission in milk (25 mg 1−1). The selectivity of the MIP was evaluated by considering different substances with molecular structures similar to sulfamethazine. Sulfamethazine was determined in milk samples spiked at two concentration levels: 696 mg 1−1 in the buttermilk obtained after milk deproteinisation, and 25 mg 1−1 both in the buttermilk and in the milk before deproteinisation. Recoveries of practically 100% were achieved in all cases.