Efficiency of Coulometric Detection

100 % Coulometric Efficiency

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.

A practical advantage of the coulometric electrode is that a fixed concentration of standard will always produce the same signal day in and day out, assuming neither standard decomposition nor column failure. This translates to more reliable data and a saving in time as fewer standards have to be analyzed in order to validate the system. The amperometric sensor, however, is not as reproducible due to its dependence on flow rate and severity of surface contamination. In some applications surface contamination of the thin-layer working electrode can occur so rapidly that there is a noticeable reduction in the signal from the same standard when analyzed repetitively. The only recourse is to dismantle the sensor for cleaning and repolishing. This and the necessary wait for restabilization of the sensor can lead to significant down-time. The coulometric sensor never needs to be dismantled or polished.

Resolving Co-eluting Peaks

In Figure 4. the differences between dual amperometric and dual coulometric electrodes are shown for the measurement of compound A (oxidizing at 200mV) and compound B (oxidizing at 600mV)

Summary

The enlarged surface area of the coulometric sensor, not only means improved signal, but also leads to increased noise.  However, the twenty-fold improvement in the signal over the thin-layer electrode far outweighs the slight increase in noise seen with the coulometric sensor.

Figure 4A. shows that with the inefficient amperometric electrodes, signal from compound A "spills over" onto the downstream electrode. If these compounds were poorly resolved chromatographically, then measurement of compound B would be compromised.

Figure 4B. shows that with coulometric electrodes all of compound A is measured on the upstream electrode, thereby effectively removing it from interfering at the downstream electrode. By such screening, the coulometric approach overcomes poor chromatographic resolution. Notice that with the coulometric measurement, the signal is twenty-fold that of the amperometric sensor.