Knauer Azura ECD 2.1, Instructions Manual

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37 Detection and Parameters
AZURA® Detector ECD 2.1 Instructions, V6685
Essentially, for the oxidation or reduction reaction it would be sufficient to
use only two electrodes. However, the three-electrode configuration has
several advantages over a two-electrode configuration. If the working po-
tential would be applied only over an AUX versus the WE (without REF),
the working potential would continuously change due to polarization
effects at the electrodes, resulting in highly unstable working conditions.
If the working potential would be applied only over the REF versus the
WE (without AUX), the working potential would be very well defined.
However, the potential of a REF is only well defined if the current drawn is
extremely low (pico-amperes) resulting in a very limited dynamic range.
A three-electrode configuration, combines the best of both electrodes.
The REF stabilizes the working potential and the AUX can supply high
currents. This results in the tremendous dynamic range of a three-electro-
de system.
6.3 Internal organization
At the working electrode (WE) in the electrochemical flow cell the elec-
tron transfer takes place due to an oxidation or reduction reaction. The
resulting electrical current is amplified by the current-potential (I/E) con-
verter (Fig. 19).
Fig. 19 ECD 2.1 signal processing from electrochemical flow cell to output.
The signal from the I/E converter can be compensated with auto zero or
offset, and is digitized using a 24-bits ADC. In the CPU the signal is pro-
cessed, for example noise filtering, or more complex data processing in
PAD. Finally after the 16-bits DAC the signal is set to a 1 V full scale analog
data output (by default Output = ADC). Also the true analog signal from
the I/E converter (before AD conversion) is available via the ‘Analog data
output’ connector. This output can be selected in the CONFIG menu by
setting the parameter Output = I/E.
6.4 Dual flow cell control
The ECD 2.1 electronics are located on 2 different PCB‘s (printed circuit
boards). The control board and the sensor board. The control board is
dedicated to communication with PC (LAN) and keyboard & display. It
has a processor with a so called ‚event handler‘ that takes care of all user
commands and hardware interrupts. The sensor board is fully dedicated
to data acquisition and flow cell control. By using this architecture it is
possible to extend the functionality of the detector to more than one flow
cell by simply adding a sensor board. The control board and other hard-
ware is prepared for more than one sensor board. Typically, a two flow
cell configuration can be used in serial or parallel mode detection.