Liquid Chromatography Detectors - The Electrochemical Detector > Electrode Construction > Page 92
The choice of electrode construction material is restricted due to the need for mechanically ruggeness and long term stability. The most common material is carbon paste made from a mixture of graphite and some suitable dielectric substance. This material has the disadvantage that it is soluble in some solvents, although, using special waxes or polymers as dielectric binders to contain the graphite, helps reduce the solubility problem. Vitreous or 'glassy' carbon is an excellent electrode material particularly if organic solvents are to be used and is probably the most popular contemporary electrode material. Glassy carbon is produced by slowly baking a suitable resin at elevated temperatures until it is carbonized and then heating it to a very high temperature to cause vitriation. Vitreous carbon is relatively pure, mechanically strong, has good electrical properties and can be readily cleaned mechanically. It also performs particularly well when operated at a negative potential. Glassy carbon electrodes are preferable to carbon paste electrodes due to their inherent resistance to solvents.
Electrochemical detection imposes certain restrictions on the type of chromatography that is employed and the mobile phase that is used. The detecting system requires a conducting mobile phase and thus must contain water. Thus, the majority of 'normal phase' systems are not usable. In addtion, very high solvent concentrations may render it insufficiently conducting. Reversed phase chromatography, however, is ideally suited to electrochemical detection. Nevertheless, certain precautions must be taken for its effective use. The mobile phase must be completely oxygen free which can be removed by bubbling helium through the solvent reservoir. It is also important to remove oxygen from the sample before making an injection. The solvents must also be free of metal ions or serious base line instability will result. Non-aqueous solvents such as pure acetonitrile can be employed but certain salts like tetrabutyl-ammoniumhexafluorophosphate must be added to render the solvent conducting.