Knauer Azura ECD 2.1, Instructions Manual

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AZURA® Detector ECD 2.1 Instructions, V6685
6. Detection and Parameters
6.1 Introduction
One of the characteristics of electrochemical detection is its tremendous
dynamic range. In amperometric detection peak heights may vary from
micro-amperes down to the pico-ampere range. The ECD 2.1 covers such
a wide range from 200 µA down to 10 pA full scale, without being limited
by electronic noise. For this reason the ECD 2.1 is equipped with a 24-bit
ADC and 16-bit DAC for analogue data output. One of the key features is
that data can be sampled with data collection rates up to 100 Hz (100 pts/
sec) in DC mode which assures that fast peak responses typical for UHPLC
can be detected with sufficient resolution.
6.2 Three-electrode configuration
The circuitry of the ECD 2.1 detector is designed for operation with elec-
trochemical flow cells with a three-electrode configuration (see Fig. 18).
The working potential is set between the working electrode (WE) and the
auxiliary electrode (AUX). The AUX is kept at a precisely defined reference
electrode (REF) potential by means of the so-called voltage clamp. This is
an electronic feedback circuit that compensates for polarization effects at
the electrodes.
At the WE, which is kept at virtual ground, the electrochemical reaction
takes place, i.e. electrons are transferred at the WE. This results in an
electrical current to the I/E converter, which is a special type of operatio-
nal amplifier. The output voltage of the I/E converter is digitized in the
instrument by means of a 24-bit A/D converter and processed, and the
resulting output current Ic can be acquired digitally by PC control soft-
ware (Elite Dialogue or ClarityChrom®) or analog using the ‘Analog Data
output’ on the rear panel connected to a recorder or an external A/D
converter.
Fig. 18 Schematic representation of an electrochemical cell with a three-elec-
trode configuration.