62 DECADE user manual, edition 9
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. 25 the selectivity for compound X is improved considerably by
decreasing the potential to E
2
or E
1
. Obviously, if compound Y is the
compound of interest, optimisation of selectivity in this way is not possible and
the chromatography has to be optimised.
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 not only holds 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 of
paramount importance for a stable baseline and reproducible detection
conditions.
Fig. 25. Selectivity in LC-EC of compound X and Y is optimised by choosing
the working potential with the largest difference in peak height.
Electrochemical reactions are pH sensitive (Fig. 26). For nor epinephrine the
I/E curve is shifted 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 will
result 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 will strongly
increase at higher pH. In both cases the background current increases at a
higher pH.
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