How electrochemical detection for HPLC works
The electrochemical sensors for Electrochemical detectors (ECD) for HPLC usually contains 3 separate electrode elements; the working, the counter (auxiliary) and reference electrode.
A fixed potential difference is applied between the working electrode and the reference electrode. This potential drives an 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 current resulting from the electrochemical reaction at the working electrode is amplified and, when plotted as a function of time, appears as a peak on the recording device. The potential applied to the working electrode is measured within the context of a known potential, which is in turn obtained from the reference electrode.
Electrode elements can be aligned in several different geometries. Most ECDs can be divided into one of three basic designs based upon how the eluent from the chromatographic column impinges upon the working electrode's surface. The eluent can flow through (Coulometric "flow-thru"), flow at (amperometric "wall jet"), or passed over (amperometic "thin-layer") the working electrode.
The design of the sensor has several important analytical ramifications including sensitivity, selectivity and stability. The amperometric thin-layer sensor was the first to be developed but is inefficient and lacks selectivity and stability. The flow-through coulometric sensor is a highly efficient amperometric sensor with high selectivity.
Ameperomtric Type
The thin-layer type or amperometric sensor exhibits a low conversion efficiency due to diffusion limitations and is only capable of measuring 5-10% of an electrochemically reactive species passing by. Figure 1. shows the special relationship between working, counter (or auxiliary) and reference electrodes. The "wall jet" is a specialized amperometric sensing device, where the eluent impinges perpendicularly to the working electrode.
Figure 1.
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Dual electrode Coulometric cell
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In a flow-through cell, Compound A is completely reacted at Sensor 1, thus eliminating it from detection at Sensor 2. Compound B passes Sensor 1 unchanged and is detected at Sensor 2. The result is that A and B are resolved electrochemically. |
Coulometric Type
Figure 2. A flow-through "frit" type porous graphite working electrode, or coulometric sensor, is extremely efficient and capable of measuring signal from 100% of an electrochemically reactive species passing through it. The lower figure shows the special relationship between the working, counter and reference electrodes for the dual coulometric electrode cell.
Figure 2.
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