The Electrochemical Detector The electrochemical detector responds to substances that are either oxidizable or reducible and the electrical output is an electron flow generated by a reaction that takes place at the surface of the electrodes. If the reaction proceeds to completion (exhausting all the reactant) the current becomes zero and the total charge generated will be proportional to the total mass of material that has been reacted. This process is called coulometric detection. If, however, the mobile phase is flowing past the electrodes, the reacting solute will be continuously replaced as the peak passes through the detector. All the time there is solute present between the electrodes, a current will be maintained, albeit varying in magnitude. Until relatively recently, this procedure was that most common employed in electrochemical detection and is called amperometric detection. The electrochemical detector requires three electrodes, the working

impression of detector sensitivity when a high efficiency column is used. The Multi–Electrode Array Detector The success of the electrode array as an LC detector is probably due to the development of the porous carbon electrode. This electrode is made of porous graphitic carbon, which has a very high surface area, is mechanically robust and, more important, is permeable to the mobile phase. As a consequence, flow through electrodes can be constructed. The material ideal for electrochemicaldetection in a number of ways. As the surface area is greatly in excess of that required for efficient electrochemical reaction, it can be severely contaminated before it fails to function. In fact, as much as 95% of the surface can be contaminated before it requires cleaning. When the electrode becomes sufficiently contaminated to require cleaning (which, according to the manufacturers, may occur between one and three years of continual use), the contamination can be rapidly removed by flushing

. 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

A comparison between amperometric and coulometric detection is shown in figure 61. In amperometric detection only a small part of the solute is reacted and so the remainder can proceed to the next electrode system and be detected again. In this way each of the units will detect all the solutes and the graph shown in the upper part of the figure is produced. It is seen that there is no discrimination by the different electrode voltages. Coulometric detection results in the total solute being reacted and thus it will be detected by that unit that

14. tyramine  1 ng 6. 3-methoxy,4-hydroxyphenylglycol     400 pg 15. 5-hydroxyindole-3-acetic acid        200 pg 7. epinephrine 200 pg 16. 3-methoxytyramine 400 pg 8. 3,4-dihydroxybenzylamine 200 pg 17. 5-hydroxytryptamine 200 pg 9. normetanephrin 400 pg 18. homovanillic acid 400 pg   Figure 59. The Separation of Some Catacholamines Monitored by an Electrochemical Detector. A more serious problem arises from the adsorption of the oxidation or reduction products on the working electrode surface. As a result, the electrode system must be frequently calibrated to ensure accurate quantitative analysis. In addition, the detector must be regularly dissembled and cleaned (usually by a mechanical abrasion). Many attempts have been made to avoid this contamination problem but, although it has been reduced, it has not been completely eliminated (