MEMBRANE-BASED ON-COLUMN MIXER FOR CAPILLARY ELECTROPHORESIS/ELECTROCHEMISTRY

One of the drawbacks of chemically modified electrodes for electrochemical detection in conjunction with capillary electrophoresis (CE) is that the optimal ran buffer for the CE separation is often not compatible with the electrolyte conditions necessary for maximum detector response. This paper reports the development of a membrane-based on-column mixer for CE which makes it possible to perform the separation and detection using different buffer conditions. The mixer consists of a cellulose acetate tube ca. 2-mm length and 50-mm i.d. The pH and/or ionic strength of the CE run buffer can be altered at the detector end by placing the mixer in a cathodic buffer reservoir containing a modifying solution of either HCl or NaOH. Ions from the modifying solution diffuse through the membrane into the detector end of the capillary and cause the desired change in pH. The use of this system was demonstrated for the detection of several biologically important compounds by capillary electrophoresis- eletrochemistry (CEEC) using chemically modified elec trodes. This included the use of a CoPC electrode for the detection of ribonucleosides, a D-amino acid oxidase electrode for the detection of amino acids, and a mixed-valent RuCN electrode for the detection of disulfides. Detection limits and detector stability were improved over those reported previously for unoptimized systems.