Integrated Capillary Electrophoresis/Electrochemical Detection with Metal Film Electrodes Directly Deposited onto the Capillary Tip.
The practical application of electrochemical detection in capillary electrophoresis has been hampered by irreproducibility and inconvenience related to capillary/electrode alignment. In order to eliminate these problems, a simple, flexible method by which the capillary and the working electrode were integrated into a single operational unit was devised and evaluated. The electrodes were formed by sputtering a thin conductive layer of Au or Pt onto the exit tip of the capillary. Depending on the size of the capillary used (i.e., both inner and outer diameters), Au on-capillary electrodes (OCEs) gave detection limits at the micromolar level and slightly below for the test analytes dopamine and catechol. More important, operation of the OCEs required no alignment procedures beyond immersion in the CE buffer reservoir/detector cell. OCEs used in this manner exhibited relative standard deviations of 2-4% for repeated injections even if removed from solution between runs. Finally, the Au and Pt OCEs could themselves be modified further by conventional electrochemical procedures. Here, Cu OCEs, formed by electrodeposition onto Au, were used to detect carbohydrate compounds; and an enzyme OCE, formed by adsorption of glucose oxidase onto Pt, was used to detect glucose.