56 DECADE user manual, edition 9
Fig. 20. A detailed part of a chromatogram acquired at different integrator
frequencies. The integrator frequency is (A) 5x, (B) 2.5x, (C) 1.2x, (D) 0.6x
and (E) 0.3x the frequency of the pulse.
In case of PAD the duration of the pulse should also be taken in account.
When the frequency of the pulse is 2 Hz, it makes no sense to acquire data
on an integrator with a significantly higher frequency. This would result in
acquisition of multiple data points containing the same output value (Fig. 20A
and B). Matching the frequencies keeps the peak shape unchanged (Fig.
20C). Decreasing the integrator frequency to less than half the pulse
frequency, changes the peak shape (Fig. 20E).
When the peak width is too small there are two options: either less data
points are collected for such a peak, or the pulse and integrator frequency are
both increased. In the latter case the pulse duration is decreased which will
change other detection parameters as well. In practise, the pulse frequency
almost never interferes with the HPLC analysis.
Working electrode material
Gold and platinum are used as working electrodes for PAD. Glassy carbon
appears to be unsuitable due to the high electric capacitance of this material.
Furthermore, resurfacing of the noble metal working electrode is based upon
formation and removal of a (metal-) oxide layer. This is impossible with glassy
carbon.
The change in cell current during the pulse mode is illustrated in Fig. 21.
When the potential is changed, a large charging current is detected (Fig. 21,
peak 1, 3 and 5), followed by a stabilisation of the current (Fig. 21, part 2, 4
and 6). The output signal is sampled during a fraction of part 2, depending on
the pulse settings. The response of the glassy carbon material is considerably
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