Frans_M_Everaerts_Isotachophoresis_378342.pdf

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DETECTION SYSTEMS
described in the legend to Fig.6.15 and all of the separations were carried out in the
operational system at pH 6 (operational system prepared with histidine, see Table 12.1).
The amount of additive differs enormously from compound to compound, and the
optimal amounts were therefore established in separate series of experiments. The
electric direct driving current was stabilized at 80 PA.
In Fig.6.39 and later in this chapter, unless otherwise stated, the isotachopherograms
were unfortunately obtained from an a.c. measuring circuit that was not as good as
those now available; the linearity is shown in Fig.6.20. The a.c. measuring circuit was
poor not only with respect to linearity, but also with respect to the insulation towards
earth and the RC time.
The measuring probe was constructed of 0.05-mm platinum foil (nowadays Pt-Ir of
0.01 mm thickness is used). Later experiments showed that the typical reaction during
the passage of chromate and malonate, components of the test mixture, could be
suppressed by addition of 0.05% of Mowiol8-88 (polyvinyl alcohol).
In order to check separately the influence of additives on electrode processes, current-
voltage characteristics were obtained. The equipment used is shown in Fig.6.40.
An improvement in the detection was found when either an a.c. or d.c. method was
used for the determination of the conductivity, although clearly a difference in behaviour
between these two modes was observed. In order to accentuate this difference, gold was
used as the electrode material, because it is known that it can easily be passivated and
the influence of possible electrode reactions is greater. Unusual effects were obtained
when gold was used. In order to give an impression of the recording of isotachophero-
grams with a conductivity detector with the sensing electrodes made of gold, the test
mixture of anions (Fig.6.1 5), in the operational system with histidine hydrochloride as
the leading electrolyte at pH 6 (Table 12.1), was again examined (Fig.6.41). The
isotachopherogams in Figs.6.39 and 6.41 show that additives need to be used.
In the experiments shown in Fig.6.41, the direct driving current was stabilized at
80 PA: It should be noted that in the trace shown in Fig.6.41 (l), where the impression
is given that the conductivity of the zones decreases towards the terminator zone, this
isotachopherogram was obtained by using the ax. method of conductivity determination.
The simultaneous detection of the conductivity with aid of the d.c. method, as in
Fig.6.41(2), shows the normal isotachophoretic tendency, viz., a stepwise increase in the
resistance. This difference must be ascribed to the dominating influence on the capacity
of a passivated oxide layer of the gold surface of the micro-sensing electrodes. These
effects are discussed in more detail in section 6.7. Many other coatings were also tested
with these gold electrodes because they easily adhered to gold. Sometimes the coatings
were formed automatically by the driving current during the electrophoretic process.
Some other unusual effects occurred when surface-active compounds were added. For
instance, the amount of a surface-active agent added does not influence the analyses
substantially; The concentration of many of them can be varied from 2 to 0.5% without
any recognizable difference in the recording of the zone boundaries. Of course, the
material added to the electrolytes must not contain ionic impurities. Another phenom-
enon that occurs is the ‘memory’ effect. When an analysis was carried out with Nonic 2 18
and the narrow-bore tube was then rinsed carefully, subsequent experiments without the
addition of Nonic 218 gave a low resolution. However, when experiments with Mowiol