Flow injection determination of choline in milk hydrolysates by an immobilized enzyme reactor coupled to a selective hydrogen peroxide amperometric sensor

A choline oxidase (ChO) immobilized enzyme reactor (IMER) prepared by glutaraldehyde coupling of the enzyme on aminopropyl modified controlled pore glass beads is described. The ChO–IMER was coupled, in a flow injection configuration system, to an interference free hydrogen peroxide amperometric sensor based on a Pt surface modified by an overoxidized polypyrrole film. The resulting analytical device responds selectively to choline and displays a sensitivity of 46.9 ± 0.2 μC mM−1 and a limit of detection, calculated at a signal-to-noise ratio equal to 3, of 7 μM. Sensitivity remains constant for about 20 days and then starts to slowly deteriorate and after 2 months a 70% of the initial sensitivity was still retained. The application to choline determination in milk hydrolysates is demonstrated. Short- and long-term drift observed in the analytical response can be corrected by a bracketing technique.