Antec Scientific DECADE II 171, User Manual, Page: 58 / 87

58 DECADE II user manual, ed. 11
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 VII), 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.
Fig. 32. A magnified view of a chromatogram obtained with PAD. The
integrator frequency is 5 times higher than the detector output frequency
resulting in a typical stepwise signal.