54 DECADE user manual, edition 9
Optimisation of wave forms
LaCourse and Johnson [2-4] have published several papers on optimisation
of wave forms in PAD. Several considerations are important for the choice of
the pulse duration. Optimisation is depending on the working electrode
material, the sample constituents and the required detection frequency. The
impression may arise that the number of variables, 3 potential steps and 4
time settings, may lead to a time-consuming optimisation procedure. In
practice, the pulse mode is more straightforward.
The potential for the cleaning steps, E2 and E3, are determined by the WE
material. At alkaline pH gold oxide is already formed at E2 > +200 mV (
vs.
Ag/AgCl). At a higher potential the formation of a metal oxide layer is
accelerated and a shorter time setting may be chosen. In practise an E2
value of +750 mV during 200 ms (t2) gives good results.
The choice of t3 is depending on the potential E3 and the t2 and E2 setting. It
is essential that the duration of t3 and the magnitude of E3 is such that a
complete removal of the metal oxide is achieved. Reductive dissolution
already occurs at E3 < 0 mV, but a more negative voltage speeds up this
process. An E3 value of -800 mV during 200 ms (Table X), or -300 mV during
360 ms [4] can be used.
The measuring potential is compound dependent, usually literature data can
be used as a starting point for further optimisation. A sampling time ts can be
chosen between 20 and 100 ms in 20 ms steps. These are multiples of the 50
Hz, to prevent noise due to oscillations of the AC power supply. Until a certain
limit, increasing ts will result in an increase of signal. A limiting factor is the
accumulation of adsorbed species at the working electrode that attenuate the
signal. Another consideration, not only for ts but for all time settings, is that
increasing the time will decrease the detection frequency.
Before sampling a stabilisation time is applied, set by the duration of t1. In
practice the stabilisation time determines the level of the background current.
When, for example, t1 = 100 ms and ts = 100 ms, there is no stabilisation of
the current before sampling (t1 - ts = 0 ms). Depending on the potential
setting of E2 and E3, a large positive or negative background current (micro
amperes) may be detected which is seriously limiting the detection. In
practice, often a 100 - 400 ms stabilisation time is used.
Recorder output frequency
An important difference between the DC and the pulse mode is the frequency
of the output signal on the recorder output. In the DC mode the signal has a 9
Hz frequency, in the pulse mode the frequency is determined by the duration
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