60 DECADE II user manual, ed. 11
and B). Matching the frequencies keeps the peak shape unchanged (Fig.
33C). Decreasing the integrator frequency to less than half the pulse
frequency, changes the peak shape (Fig. 33E).
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. 34.
When the potential is changed, a large charging current is detected (Fig. 34,
peak 1, 3 and 5), followed by a stabilisation of the current (Fig. 34, 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 different from the noble metals. The capacitance of the
electrode material is very high, such that the charging current is not stabilised
before start of the next potential step. This pattern makes detection
impossible.
Examples of carbohydrate analyses are given in Fig. 30. Typical PAD pulse
settings are given in Table VII and Table VIII, these settings may be used as
starting point for further optimisation. Carbohydrates are oxidised at a pH of
12 or higher, which puts specific demands on the HPLC system used (see
above).
Table VII. Potential settings for PAD of carbohydrates at a gold working
electrode.
s
1
2
3
t (ms)
100
400
200
200
E (mV)
+150
+750
-800
Table VIII. Potential settings for PAD of glycols, alcohols, aldehydes at a
platinum working electrode.
s
1
2
3
t (ms)
20
300
100
100
E (mV)
+200
+1300
-100
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