Understanding Coulometric Technology

The current-voltage relationship (CV)

The electrochemical detector (ECD) is regarded as the most sensitive detector for HPLC analysis. Furthermore, it is extremely selective as only a fraction of molecules in a sample are usually electrochemically active. Like many other HPLC-based techniques, the identity and authenticity of an analyte is usually based upon matching its retention time to that of an external standard. But herein lies a problem, as many compounds are capable of eluting from the analytical column at the same time. This lack of selectivity may result in an overestimate of a compound's concentration due to unseen coelutions. For example, this problem was first seen with absorbance-based detection techniques. It was not until the advent of the photodiode array (PDA) detector and liquid chromatography with mass spectrometry (LC/MS) that peaks previously thought to be pure were shown to be due to coelutions. Furthermore, some compounds were even misidentified. The photo diode array detector (PDA) gives a "third dimension" to HPLC-absorbance techniques by examining an analyte's spectrum. Now compounds can be identified both by retention time and by spectral behavior. Unfortunately, the PDA detector often lacks adequate sensitivity.

Some researchers have tried to make single-channel ECDs more qualitative by examining the voltammetric behavior of the analyte. Here, they construct hydrodynamic voltammograms (HDV) of the analyte once it has been chromatographically resolved from other components of the matrix using HPLC (Figure 1). Although this can generate qualitative data there are two major problems. First, to construct an HDV, the analyte's response is measured as a function of applied potential. When using a thin-layer amperometric electrode this can be incredibly time consuming due to the time needed for the electrode to stabilize following a change in the applied potential to the working electrode. Second, this approach requires that sufficient analyte (sample) is available for repeated analysis.

Figure 1. A theoretical plot of the current-voltage relationship (CV) or hydrodynamic voltammogram of a substance showing both oxidative and reductive behavior. By convention the lower left quadrant is the oxidation quadrant. This is equivalent to the construction of a spectrum when using an absorbance detector.