A microelectrochemical detector for use at low linear velocities in capillary column systems

Abstract

An end-column microelectrochemical detector, based on a 25 μm platinum electrode, was used in miniaturised flow and separation systems, at linear mobile phase velocities below 0.1 cm s −1 (i.e. a volumetric flow of a few μl min −1 for a 200 μm i.d. capillary). The detector response was found to be independent of the linear velocity between 0.053 and 1.1 cm s −1, indicating that the detector, which is based on a wall-tube design, is truly concentration sensitive. This and the velocity within the liquid jet emerging from the column ensures that the electrodes can be placed up to 200 μm (i.e. one column i.d.) from the column outlet without a loss of sensitivity or chromatographic efficiency. Beyond this distance, band broadening by diffusion becomes significant. These findings are supported by simulations of the axial variation of the centerline velocity within the jet emerging from the column end. Since the detector is truly concentration sensitive, the positioning of the electrodes in the y and z direction was not found to be critical. The latter facilitates a rapid positioning of the electrodes significantly. The detector may therefore be an attractive alternative to detection at carbon fibres inserted in the column end, especially as the electrodes of the present design are easier to position, less fragile and as the electrode surfaces can be regenerated by polishing. The detector is shown to be compatible with reversed phase packed capillary column liquid chromatography (LC), as well as, normal-phase applications in the absence of an added electrolyte.