Understanding Working Electrodes The Function of a Working Electrode A fixed potential difference is applied between the working electrode and the reference electrode. This potential drives the electrochemical reaction at the working electrode's surface. The current produced from the electrochemical reaction at the working electrode is balanced by a current flowing in the opposite direction at the counter electrode. The reference electrode acts as a reference point for the redox couple. The current resulting from the electrochemical reaction is amplified and, when plotted as a function of time, appears as a peak on the recording device. Current measured at the working electrode surface results from, not only the redox reaction of interest (faradaic current), but also unwanted redox reactions coming from the mobile phase (faradaic noise) and other sources of noise, such as the working electrode material itself, the solvent delivery system and the potentiostat (non-faradaic noise). Some of the faradaic and non-faradaic noise can be minimized by careful mobile phase prepration, HPLC system cleanliness, a quiet, pulse-free pump and the correct choice of working electrode material.
Types of Working Electrodes A wide variety of working electrodes are available for use with HPLC-ECD. The most common working electrode materials utilize carbon. Originally the carbon paste electrode was developed but this was soon replaced by more "convenient" and stable carbon-based working electrodes including those made from glassy carbon, pyrolytic carbon and porous graphite. Metals such as platinum, gold, silver, nickel, mercury, gold-amalgam and a variety of alloys are now also commonly used as working electrode materials. The optimal working electrode choice is dependent upon many factors, including the usable applied potential range, involvement of the electrode in the redox reaction, and kinetics of the electron transfer reaction. Other factors, such as compatibility with and the composition1 of the mobile phase, will also play a role. For example, carbon paste electrodes cannot be used with mobile phases containing high amounts of organic modifier because the electrode will dissolve unless a polymeric binder is used. 1 For an electrochemical detector to function, the mobile phase MUST contain an electrolyte to permit the flow of current. Too little electrolyte may prevent electrolysis from occurring at the working electrode resulting in diminished response. Furthermore, in some electrochemical detectors the inability to monitor current may cause the potentiostat to apply a considerable potential to the working electrode which may destroy it. Too much electrolyte can result in considerable background current (noise) limiting the sensitivity of the system and potentially damaging the working electrode. |