A glass capillary ultramicroelectrode with an electrokinetic sampling ability

A glass capillary ultramicroelectrode (tip diameter approximate to1.2 mum) having an electrokinetic sampling ability is described. It is composed of a pulled glass capillary filled with an inner solution and three internal el ectrodes (Pt working and counter electrodes and an Ag/AgCl reference electr ode). The voltammetric response of the capillary electrode is based on elec trokinetic transport of analyte ions from the sample solution into the inne r solution across the conical tip. It was found that the electrophoretic mi gration of analytes at the conical tip is faster than electroosmotic flow, enabling electrokinetic transport of analyte ions into the inner solution o f the electrode. By using [Fe(CN)(6)](4-) and (ferrocenylmethyl)trimethylam monium (FcTMA(+)) ions as model analytes, differential pulse voltammetric r esponses of the capillary electrode were investigated in terms of tip diame ter of the capillary, sampling voltage, sampling time, detection limit and selectivity. The magnitude of the response depends on the size and charge o f analyte ions. With a capillary electrode having a approximate to1.2-mum t ip diameter, which minimizes non-selective diffusional entry of analytes, t he response after 1 h sampling at + 1.7 V is linearly related to [Fe(CN)(6) ](4-) concentration in the range of 0.50 - 5.0 mM with the detection limit of 30 muM. Application of a potential of the same sign as that of the analy te ion forces the analyte to move out from the electrode to the solution, e nabling reuse of the same capillary electrode. The charge-selective detecti on of analytes with the capillary electrode is demonstrated for [Fe(CN)(6)] (4+) in the presence of FcTMA(+).