Knauer Azura ECD 2.1, Инструкция по эксплуатации

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57 Optimization of the working potential
AZURA® Detector ECD 2.1 Instructions, V6685
Fig. 43 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.
With respect to sensitivity, a high working potential is important. Howe-
ver, at higher working potentials, more analytes are detectable. So, as to
selectivity, a low working potential will be favorable.
Working at a potential on the slope of the I/E curve will result in less re-
producibility in HPLC. A small fluctuation in the applied potential, or any
change in the system (like for instance a pH change) may result in diffe-
rences in measured peak height. In practice the choice of the working
potential is a compromise between sensitivity, selectivity and reproducibi-
lity. In the example of Fig. 41 a working potential (E1) of 0.8 V is chosen.
9.4 Optimization using a voltammogram
Sometimes, when interfering peaks appear in the chromatogram, it is
possible to optimize the method with regard to selectivity. If the interfe-
ring 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. 44 the selectivity for compound X is im-
proved considerably by decreasing the potential to E2 or E1. Obviously,
if compound Y is the compound of interest, optimization of selectivity in
this way is not possible and the chromatography has to 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 not only holds for the analyte but also for the background
current and possible interferences. An elevated temperature will therefo-
re not automatically lead to a better detection. A constant temperature is
of paramount importance for a stable baseline and reproducible detec-
tion conditions.
0.0 0.4 0.8 1.2
0
5
10
B
Ic
el
l (
nA
)
Ecell (V)
A