Frans_M_Everaerts_Isotachophoresis_378342.pdf

DETECTION LIMITS 193
electric driving current, a series of experiments was performed with the test mixture of
anions as described in Fig.6.15.
Also in this series of experiments, a non-linear a.c. conductimeter was used. The
operational system at pH 6 (Table 12.1) was chosen and the direct driving current was
stabilized at 80 PA, unless mentioned otherwise in the figure captions.
In Fig.6.46, a series of isotachopherograms are shown, which indicate that the ax.
method gives unusual results for the test mixture of anions before the coating. The
sensitivity (selectivity) of the combination of the a.c. method with coated electrodes
for the doubly charged sulphate ion was such that the following zone of chlorate was
measured with a negative step, which suggests that this zone has a higher conductivity
than the preceding zone. This is in contradiction to the isotachophoretic principle, if
these ions are involved. When the thickness of the coating layer was increased, this effect
also increased, as shown in Fig.6.46 (3 and 4). An increase in frequency of the measuring
current also demonstrates this effect. Even the acetate-adipate transition shown in
Fig.6.46 (5) is recorded with a negative step. In Fig.6.46 (3 and 4), the acetate-adipate
transition was recorded with a smaller difference than under normal conditions (without
a coating).
The difference between the simultaneous detection by the a.c. and d.c. methods of
determination of the conductivity, as already found in some instances when passivated
gold electrodes were applied, must be ascribed to the change in capacity of the conductivity
cell. In all experiments, the simultaneously performed d.c. method of conductivity
detection showed a normal isotachophoretic pattern, as does UV detection. Similar
behaviour was found if cations were separated, as can be seen in Fig.6.47. Hence the
coating layer is only selective for the difference between singly and doubly charged ions.
If, during an isotachophoretic run, a coating is deposited on the micro-sensing electrodes
by an electrode reaction due to a leak current or a change in the nature of the sensing
electrode from polarized to charge transfer due to the driving potential, similar effects
can be expected. The effect occurs especially when this coating layer is formed very slowly
after a series of experiments, even if the most stringent precautions are taken. Cleaning
must therefore be carried out from time to time.
That a coating is formed more quickly if a high current density of the driving current is
applied is shown in Fig.6.48.
Owing to the higher potential gradient, the electrode reactions typical of the chromate
zone are a function of the driving current. Also, the ratio of step heights is changed more
quickly than under normal conditions, for which a change in the ratio of step heights
could not be observed.
6.8. DETECTION LIMITS
6.8.1. Introduction
Although it is somewhat premature at the present stage of isotachophoretic develop-
ment, brief information will be given on detection limits in isotachophoretic experiments.
More research aimed at optimizing detectors, equipment and Operational systems will