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AZURA® Detector ECD 2.1 Instructions, V6685
9. Optimization of the working potential
9.1 Introduction
A current - voltage (I/E) relationship, or voltammogram, characterizes an
analyte. It gives information on the optimum working potential, which can
be used to improve detection sensitivity and selectivity.
There are several ways to obtain a voltammogram:
A hydrodynamic voltammogram is obtained in the DC mode by run-
ning several chromatograms at different working potentials. Both peak
height and background current are plotted against the working poten-
tial. A hydrodynamic voltammogram has as an advantage that the I/E
relationship of all analytes of interest can be obtained simultaneously
in one set of experiments (boundary condition: all analytes should be
sufficiently separated under the applied LC conditions). Furthermore,
under real chromatographic conditions reliable information about the
S/N ratio is obtained.
A scanning voltammogram is obtained in the so-called scan mode of
the ECD 2.1: the voltage runs between two preset potential values (E1
and E2) and scan speed (in mV/s) and the current is measured.
Optimization of the working potential and the construction of a voltam-
mogram is described.
9.2 Electrochemical reactions
In electrochemical detection (ECD) a reaction of the analyte at an electro-
de surface is monitored. This distinguishes ECD from most other detec-
tion techniques where detection is based on the physical properties of an
analyte (i.e. mass spectrometry: molecular mass, absorbance detection:
molar absorptivity). For electrochemically active compounds, the poten-
tial between reference electrode (REF) and working electrode (WE) de-
termines the reactivity of the analyte at the WE. The potential difference
supplies the energy level needed to initiate or enhance the electrochemi-
cal reaction. Different analytes may have different oxidation or reduction
potentials, which determines the selectivity of ECD.
Fig. 40
Oxidation/reduction reaction of norepinephrine.
An example of an electrochemical reaction is shown in Fig. 40, norepine-
phrine is converted into a quinone by oxidation at the WE. Two electrons
are transferred at the WE resulting in an electrical current that is amplified
by the controller.
Summary of Contents for Azura ECD 2.1
Page 1: ...HPLC Detector ECD 2 1 Instructions Document No V6685 ...
Page 32: ...28 ECD 2 1 controller AZURA Detector ECD 2 1 Instructions V6685 Pulse mode ...
Page 33: ...29 ECD 2 1 controller AZURA Detector ECD 2 1 Instructions V6685 Scan mode CONFIC menu ...
Page 34: ...30 ECD 2 1 controller AZURA Detector ECD 2 1 Instructions V6685 DIAG menu ...
