Frans_M_Everaerts_Isotachophoresis_378342.pdf

186 DETECTION SYSTEMS
ions, the pH of the leading electrolyte was about 6, containing 0.01 Nhydrochloric acid
(pro analysi grade). The pKa values for pyridine and P-picoline are 5.25 and 5.69,
respectively. Analyses of the test mixture of anions (Fig.6.15) were carried out and sharp
zones were observed. Unfortunately, W detection cannot be used, because the W
absorption of these counter ions is strong between 250 and 300 nm.
It must be remembered that the effective mobilities of pyridine and 0-picoline are
greater than that of hstidine. This results in the need for longer narrow-bore tubes for
the separation of similar mixtures, because mixed zones are formed much more easily as
components with a higher effective mobility transport a higher proportion of the elec-
tricity. Nevertheless, here also the disturbance in the detection of the chromate zone
(electrode reaction) was found to be a function of the driving current, which illustrates
further that an electrode reaction indeed occurs, as shown in Fig.6.42 (1-3). The
electrode reaction is shown to be a part of the profile finally recorded if chromate is
used as the terminating ion. The reaction time for the electrode is therefore of the order
of seconds. All analyses, carried out with the 0.05-mm platinum electrodes and the coil,
for galvanic separation of the high potential of the micro-sensing electrodes and the low
potential of the conductivity-measuring electronics, which are not well insulated, show
this typical reaction, but at low pH (e.g., 4) it is less pronounced.
If the chromate zone has passed the measuring electrodes, these electrodes are
(partially) passivated. All other zones following the chromate zone are measured correctly
if the chromate zone instead of the leading electrolyte is taken as a reference. Thus a
shift is obtained: all zones before chromate (of the test mixture of anions) are of correct
height and all zones after chromate are of correct height. An extra impedance, reversible
and stable, arises during the analysis, but if the narrow-bore tube is rinsed after the
experiment has been completed, this impedance ‘disappears’ again.
If malonic acid is injected as a sample or is used as the terminating ion, while no
chromate zone is created before the malonate, a similar behaviour to that found with the
chromate is found, as shown in Fig.6.42 (4 and 5). If both chromate and malonate are
present, and the chromate zone may be very small, the typical behaviour of the electrodes
can be recognized only during the passage of the chromate zone, as shown in Fig.6.42(6).
If we look more closely at the isotachopherograms in Fig.6.42, in which chromate and
malonate are used as terminating ions, a typical shape can be seen in both traces. Even
sulphate, when used as a terminating ion, shows this behaviour if the recording of the
sulphate step is scaled up. The shape has three different and clearly distinguishable parts:
an overshoot, a slow decrease and an increase towards a constant value.
The following explanation can be put forward. The anionic constituent present in a
zone, at the concentration and pH determined by the operating conditions chosen, may be
the cause of a change in behaviour from a polarized micro-sensing electrode partially to
a charge-transfer electrode, (during the passivation of the electrode by the chromate ion).
This always causes an overshoot, because the concentration must decrease if a current is
applied to give an electrode reaction in addition to the electrophoretic transport, in order
to fulfil the isotachophoretic condition and the mass balance of the buffer. Especially if
oxygen is generated, for instance as a result of the electrode reaction (which means
passivation of the electrode), the gas diffuses into the metallic structure. We found that
passivated electrodes record the conductivity of a particular electrolyte with an apparently