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Chapter 7 Optimization of working potential 73
As can be seen in both Fig. 27 and Fig . 29 , when the working potential is
increased the electrochemical reaction is enhanced hence the signal in-
creases. 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 sur-
face 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. 29. 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.
In practice the choice of the working potential is a compromise between
sensitivity, selectivity and reproducibility or the yield in desired metabolite
formation (in ROXY Ec applications). In the example of Fig. 27 a working
potential (E
1
) of 0.8 V is chosen.
0.0 0.4 0.8 1.2
0
5
10
B
Ic
ell
(nA)
Ecell (V)
A