Carbon Composite Electrodes for Liquid Chromatography/Electrochemistry: Optimizing Detector Performance by Tailoring the Electrode Composition

Abstract

Results obtained in this laboratory and elsewhere suggest that carbon composite electrodes may possess a signal-to-noise (S/N) advantage compared to continuous electrodes such as glassy carbon when used for detection of analytes in flowing streams. One succomposite electrode which appears partic- ularly attractive in this regard is the Kel-F-graphite (Kelgraf) electrode, compression molded from Kel-F and powdered graphite and containing 5 to 30% graphite by weight. Studies of the electrode surface by scanning electron microscopy and X-ray photoelectron spectroscopy in conjunction with electrochemical investigations employing chronoamperometry, cyclic voltammetry, and capacitance measurements have led us to view the electrode surface as an ensemble of rnicroelectrodes, the dimensions of which can be varied by changes in particle size and/or ratio of Kel-F to graphite in the composite. The S/N advantage of the composite electrode apparently arises from a signal (current) enhanced by radial diffusion of analyte to the individual microelectrodes, resulting in a response greater than that obtained from a continuous electrode of equal active area. Since detector noise is generally assumed proportional to the active area of the electrode, S/N enhancement results.

For composite electrodes employed in a thin-layer channel design LC detector, the observed variations in the S/N ratio with changes in (1) composite composition (%C), (2) particle size of Kel-F used in fabrication of the composite, and (3) area of composite exposed in the flow channel are discussed within the context of the microelectrode ensemble model. It is further demonstrated that the ability of the electrode to resist fouling can be modified by variation in composite composition.