Working Electrode Efficiency

Amperometric

Amperometric "thin-layer" working electrodes typically oxidize (or reduce) <10% of an electrochemically reactive species passing over them.  This level of efficiency is a direct consequence of two factors.  First, there is limited diffusion to the working electrode's surface, where reactions occur.  This means that a molecule has to be in "the right place at the right time" if it is going to react and be measured.  Second, at the typical flow rates used in HPLC most molecules will be swept through the detector without the possibility of reacting at the working electrode's surface. Consequently, the signal obtained from amperometric sensors is notoriously dependent upon the flow rate (Figure 3).

Coulometric

Coulometric electrodes typically oxidize (or reduce) up to 100% of an electrochemically reactive species passing through them. This extreme efficiency is a direct consequence of minimizing the diffusion distance so that regardless of the direction in which a molecule diffuse, once in the electrode matrix, the molecule is bound to encounter the working electrode's surface and react. Consequently, the signal obtained from a coulometric sensor is independent of the flow rates typically used for HPLC.

The efficiency of amperometric electrodes is also highly dependent upon surface area contamination due to electro-deposition and adsorption.  The coulometric electrode, however, due to its enlarged surface area, remains unaffected by such poisoning until more than 90% of the electrode is fouled.

Figure 3. The graph shows electrode efficiency as a function of flow rate.  Conversion efficiency decreases for the thin-layer amperometric electrode but remains at 100% for the coulometric electrode over the flow range used with HPLC.