Knauer Azura ECD 2.1, Instructions Manual

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AZURA® Detector ECD 2.1 Instructions, V6685
8. Pulsed Amperometric Detection (PAD)
8.1 Introduction
The ECD 2.1 can operate in PAD mode. In pulsed amperometric detecti-
on (PAD) the working electrode (WE) is regenerated at a frequency of 0.5
- 3 Hz by the application of a series of potential changes. This is particu-
larly useful for certain applications where the working electrode is rapidly
fouled due to adsorption of insoluble reaction products. A well-known
application area of PAD is the analysis of carbohydrates (Fig. 37) [1].
Fig. 37
Carbohydrate analysis in the pulse mode.
Pulse vs. DC mode
The pulse mode is quite different from the DC mode. Instead of a cons-
tant potential, a series of potential steps is applied in a cyclic manner.
The signal is sampled during a fraction of the total pulse cycle. During
the sampling time (ts) the signal is collected and this value is sent to the
detector output. The frequency of data output is determined by the pulse
duration: t1 + t2 + t3 + t4 + t5. Usually the duration is between 0.5 and 2s
(data rate between 2 – 0.5 Hz).
The background or cell current is usually considerably higher
(100 - 1000 nA) than in the DC mode. Therefore, only nano- and micro-
ampere ranges are available in the pulse mode. Typically, the background
current is between 0.1 – 2 µA.
When the frequency of the data acquisition system (integrator) is higher
than the pulse frequency a typical stepwise pattern may appear in the
chromatogram. This is called ‘oversampling’ and these steps are usually
only visible after considerable magnification of the chromatogram. It is
recommended to keep the data acquisition at 1 Hz.