CHAPTER 6 Pulsed amperometric detection
53
chromatography. The CO
2
-free sodium hydroxide is available from several
suppliers as a 50% solution (19.2 mol/l). NaOH pellets are not
recommended because of their high CO
2
content.
•
The accuracy of certain pH-electrodes is poor at high pH. For applications
at high pH it is sometimes better to
calculate
the pH from the OH
-
concentration.
•
Organic modifiers (acetonitrile) strongly attenuate the signal of most
carbohydrates in PAD.
Some of these aspects will be discussed in detail.
Pulse settings
In PAD of carbohydrates the working potential is applied as a series of 3
potentials. During time interval t1 the detection potential is applied. The data
collection occurs within t1, during time interval ts (sampling time). The time
difference t1 - ts is the stabilisation time.
Fig. 18. Potential steps in pulsed amperometric detection. A part of t1 is used
for detection (ts). The metal oxide layer that is formed during t2, is removed
during t3, resulting in a renewal of the electrode surface.
During the next time interval (t2) a monolayer of metal oxide is formed at the
working electrode due to the high positive potential. This monolayer is
electrochemically removed from the electrode surface during time interval t3,
by applying a negative potential.
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