CHAPTER 7 Optimisation of working potential
61
Fig. 24. Scanning voltammetry of 1.0 µmol/l nor epinephrine (A) at a glassy
carbon working electrode, at a scan speed of 10 mV/s. Scan (B) is the blank
solvent.
As peak heights are used, the signal in Fig. 23, line A is only due to the
analyte. The signal in Fig. 24, line A is the sum of the analyte signal and the
background signal. Subtracting both lines in Fig. 24 results in a similar I/E
relationship as in Fig. 23, line A. It takes only a few minutes to construct a
scanning
voltammogram. This is an advantage, especially when a number of
analytes have to be characterised. However, as the scan is obtained in flow
injection analysis (FIA, without analytical column), it is a prerequisite to have
the
pure
analyte dissolved in buffer.
Any contamination may lead to
artefacts
. A blank scan of the buffer should be used to distinguish between
solvent peaks and analyte peaks.
As can be seen in both Fig. 23 and Fig. 24, when the working potential is
increased the electrochemical reaction is enhanced hence 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
.
With respect to
sensitivity
, a high working potential is important. However, at
higher working potentials, more analytes are detectable. So, as to
selectivity
,
a low working potential will be favourable.
Working at a potential on the slope of the I/E curve will result in less
reproducibility. Not only a small fluctuation in the applied potential, but any
change in the system may result in a large difference in current.
In practice the choice of the working potential is a compromise between
sensitivity, selectivity and reproducibility. In the example of Fig. 23 a working
potential (E
1
) of 0.8 V is chosen.
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