A MICROELECTROCHEMICAL DETECTOR FOR USE AT LOW LINEAR VELOCITIES IN CAPILLARY COLUMN SYSTEMS

An end-column microelectrochemical detector, based on a 25 mu m platin um 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 mu l min(-1) for a 200 mu m i.d. capillary). The detecto r 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 ve locity within the liquid jet emerging from the column ensures that the electrodes can be placed up to 200 mu m (i.e. one column i.d.) from t he column outlet without a loss of sensitivity or chromatographic effi ciency. Beyond this distance, band broadening by diffusion becomes sig nificant. These findings are supported by simulations of the axial var iation of the centerline velocity within the jet emerging from the col umn end. Since the detector is truly concentration sensitive, the posi tioning of the electrodes in the y and z direction was not found to be critical. The latter facilitates a rapid positioning of the electrode s significantly. The detector may therefore be an attractive alternati ve to detection at carbon fibres inserted in the column end, especiall y 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 ca pillary column liquid chromatography (LC), as well as, normal-phase ap plications in the absence of an added electrolyte.