Antec Scientific DECADE II 171, User Manual, Page: 65 / 87

CHAPTER 8 Optimisation of working potential 65
As can be seen in both Fig. 36 and Fig. 37, 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 .
Fig. 37. Scanning voltammetry of 1.0

mol/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. 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. 36 a working
potential (E
1
) of 0.8 V is chosen.
Optimisation using a voltammogram
Sometimes, when interfering peaks appear in the chromatogram, it is
possible to optimise 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, i.e. the largest difference in peak height. In the
example of Fig. 38 the selectivity for compound X is improved considerably
by decreasing the potential to E
2
or E
1
. Obviously, if compound Y is the