Antec Scientific DECADE II 171, User Manual, Page: 57 / 87

CHAPTER 7 Pulsed amperometric detection 57
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. 31. 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.
Optimisation of wave forms
LaCourse and Johnson [2-4] have published several papers on optimisation
of wave forms in PAD. Several considerations are important for the choice of
the pulse duration. Optimisation is depending on the working electrode
material, the sample constituents and the required detection frequency. The
impression may arise that the number of variables, 3 potential steps and 4
time settings, may lead to a time-consuming optimisation procedure. In
practice, the pulse mode is more straightforward.
The potential for the cleaning steps, E2 and E3, are determined by the WE
material. At alkaline pH gold oxide is already formed at E2 > +200 mV (
vs.
Ag/AgCl). At a higher potential the formation of a metal oxide layer is