In the example, the selectivity for compound X is improved considerably by decreasing the
potential to E
2
or E
1
. If compound Y is the compound of interest, optimization of selectivity in this
way is not possible and the chromatography must be optimized.
Electrochemical detection differs from most other LC detection methods in that a reaction takes
place in the detection cell. Due to reaction kinetics, an increased temperature speeds up the
oxidation/reduction reaction. However, this holds not only for the analyte but also for the
background current and possible interferences. An elevated temperature will therefore not
automatically lead to a better detection. A constant temperature is essential for a stable baseline
and reproducible detection conditions.
Figure 9–5: Selectivity in LC-EC of compounds X and Y is optimized by choosing the
working potential with the largest difference in peak height.
Electrochemical reactions are pH-sensitive. For norepinephrine, the I/E curve shifts to a lower
potential at higher pH. When the working potential is high (E
2
) and the signal is diffusion limited,
an increase in pH results only in a small increase of the peak height. When the working potential
is lower (E
1
) and the signal is not diffusion limited, the signal strongly increases at higher pH. In
both cases the background current increases at a higher pH.
Figure 9–6: At a higher pH, the I/E curve of norepinephrine shifts to the left.
Reaction kinetics predict that electrochemical detection is mass flow dependent. When the LC
flow is stopped in LC-EC, the analyte is oxidized completely and the signal decreases rapidly.
This means that the flow rate affects not only temporal peak width and analysis time but also
peak height. Also, the background signal is sensitive to fluctuations in the flow rate. It is important
to use a pulse-free solvent delivery system.
9.5 Construction of a hydrodynamic voltammogram
Before a hydrodynamic voltammogram can be obtained, the chromatographic conditions should
be optimized. Then the following steps are taken:
December 16, 2021, 715007395 Ver. 00
Page 79
