A new fabrication method for a Nafion joint for capillary electrophoresis with on-column electrochemical detection (CEEC) is described. The Nafion joint was cast directly on the capillary column. A tungsten wire was used to define the flow channel during fabrication and removed for operation. This method allowed the construction of Nafion joints over 1 mm in length, which did not significantly contribute to band broadening. The increased length of these joints resulted in a significant decrease in the noise at the electrochemical detector. The relationship between the noise, the magnitude of the electro-

An end-column electrical decoupler contructed with perfluorosulfonated ionomer (Nafion) is described. This decoupler was fabricated at the cathodic end of the separation capillary by casting liquid Nafion ion exchange powder over a copper-plated tungsten wire. The internal diameter of the flow channel was controlled by adjusting the thickness of the copper plating. This design overcomes problems of conventional end-column detection such as low sensitivity due to low collection efficiency of analytes at the detection electrode, difficulty in precise placement of the detection electrode, and the need to use small-bore capillaries (<-25 micron). The loss of cationic analytes observed with long-cast Nafion on-column decouplers was significantly reduced. The high current shunting capability of the long Nafion decoupler was maintained in this configuration. Elimination of the detection capillary required for on-column electrical decouplers provided higher separation efficiency by maintaining plug-type flow throughout the system. Four model catecholamines were well separated within 5 min with separation efficiency of up to 230 000 plates and migration time reproducibilities of <0.6% RSD. With the optimized experimental conditions, detection limits of 3 nM for the catecholamines were achieved in a Ringer's solution matrix (to model a microdialysis sample).

High-ionic-strength samples result in poor electrophoretic separation efficiencies due to ''destacking'' of the sample. A simple method is described to stack high-ionic-strength samples without matrix modification prior to injection. Using a weak-acid buffer such as acetate as the background electrolyte, the electrokinetic injection of the sample is immediately followed by electrokinetic injection of an acid solution. The resulting titration of the background electrolyte produces a low-conductivity zone across which sample cations are focused. A sevenfold sensitivity enhancement was achieved with improved separation efficiency relative to normal electrokinetic injection.

Several approaches to implementing amperometric detection for capillary electrophoresis have been reported. This report describes the development of a voltammetric detector for CE. The detector is designed to minimize distortion of the voltammetry due to ohmic potential drop. This was accomplished by using a cast Nafion detection cell at the end of the separation capillary. The cast Nafion detection cell provided a low-dead-volume, low-resistance cell that minimized ohmic potential drop and peak band broadening. The ability to detect the current due to oxidation of analytes superimposed on a large background current was also improved. A dynamic background subtraction scheme was used in which a second working electrode, positioned in the electrochemical cell but outside of the detection cell, was used to compensate for the background current in real time. The output of the compensating working electrode was subtracted from the output of the detecting working electrode prior to analog-to-digital conversion. Postexperimental digital background subtraction was also implemented. This approach provided optimal elimination of the background current with maximal detection of the analytical signal. The voltammetric detector developed produced high-quality voltammetric response of analytes with injected concentrations as low as 0.20 microM. The system was evaluated by obtaining CE voltammograms of a mixture of eight test phenolic acids.