The current is plotted against the working potential to give a voltammogram (I/E curve), as shown
in the scanning voltammetry potential waveform figure.
In scanning voltammetry, no HPLC separation is involved. The signal is the sum of all EC active
substances. It takes only a few minutes to construct a scanning voltammogram. This is an
advantage, especially when a number of analytes must be characterized. However, it is a
prerequisite to have the pure analyte dissolved in buffer. A scan of the buffer (blank) should be
used to distinguish between solvent peaks and analyte peaks.
Note:
Any contamination in the buffer may lead to artifacts.
As seen in the previous and following figures, when the working potential is increased the
electrochemical reaction is enhanced and the signal increases. At a certain potential, the I/E
curve flattens. All analyte molecules that reach the working electrode are converted at such a
high rate that the analyte supply becomes the limiting factor. At the working electrode surface, a
stagnant double layer exists where molecular transport takes place by diffusion only. Therefore,
the current at (and beyond) this potential is called the
diffusion limited current
.
Figure 9–4: Scanning voltammetry of 1.0 µmol/L norepinephrine (A) at a glassy carbon
working electrode at a scan speed of 10 mV/s. Scan B is the blank solvent.
With respect to
sensitivity
, a high working potential is important. However, at higher working
potentials, more analytes are detectable. With respect to
selectivity
, a low working potential is
favorable.
Working at a potential on the slope of the I/E curve results in less reproducibility in HPLC. A small
fluctuation in the applied potential, or any change in the system (a pH change for instance) may
result in differences in measured peak height. In practice, the choice of working potential is a
compromise among sensitivity, selectivity, and reproducibility. In the example of
scanning
voltammetry potential waveform
, a working potential (E1) of 0.8 V was chosen.
9.4 Optimization using a voltammogram
When interfering peaks appear in the chromatogram, it is sometimes possible to optimize the
method with regard to selectivity. If the interfering compound has a higher oxidation potential, a
working potential is chosen that gives the best selectivity (the largest difference in peak height).
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