Liquid Chromatography Detectors - The Electrochemical Detector > Page 89

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 electrode (where the oxidation or reduction takes place), the auxiliary electrode and the reference electrode (which compensates for any changes in the background conductivity of the mobile phase).

The processes taking place at the electrode surface can be very complex; nevertheless, the dominant reaction can be broadly described as follows.

At the actual electrode surface the reaction is extremely rapid and proceeds almost to completion. This results in the layer close to the electrode being virtually depleted of reactant. As a consequence, a concentration gradient is established between the electrode surface and the bulk of the solution.

This concentration gradient causes solute to diffuse into the depleted zone at a rate proportional to the solute concentration in the bulk of the mobile phase. Thus, the current generated at the electrode surface will be determined by the rate at which the solute reaches the electrode and consequently, as the process is diffusion controlled, will depend on solute concentration and the magnitude of solute diffusivity.