CHAPTER 6 Pulsed amperometric detection
55
of the pulse. Once every cycle, the ts signal is sent to the output. This can be
visualised by magnification of a peak in the chromatogram (Fig. 19).
Fig. 19. 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.
A stepwise pattern in the chromatogram is only seen on an (analog) chart
recorder, or on an integrator that has a higher sampling frequency than the
output frequency of the detector. In fact, when this pattern is seen this means
that the integrator has an unnecessarily high sampling frequency. This leads
to large data files, but certainly not to a better chromatogram.
Peak width and integrator frequency
There are two important considerations with respect to integrator frequencies
applied in HPLC. If the frequency is too low, data will be lost and artefacts
may be introduced. If the frequency is too high, large data files are generated
which take up an unnecessary large amount of disk space. As a rule of
thumb, the sampling frequency of the integrator is set such that a
chromatographic peak is build up of at least 10 data points. For a peak width
of 10s this means that a sampling frequency of 1 Hz should be sufficient.
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