Frans_M_Everaerts_Isotachophoresis_378342.pdf

COUNTER FLOW OF ELECTROLYTE 231
instances this factor may be zero or even that the separation may be positively influenced.
Also, when the difference in effective mobility is minimal, one cannot expect always a
complete separation because in some cases the ions have a mutual adverse influence on
the pH of the mixed zone and give a poorer separation. Once the ions are separated they
will form discrete zones owing to the difference in the pH values in the two zones.
We found the counter flow of electrolyte to be successful especially when samples need
to be separated with large concentration differences between the various zones. The
use of a counter flow of electrolyte has also proved of value in elucidating whether a
separation is completed or not (i.e., mixed zones are present or not).
Although the use of a counter flow of electrolyte in isotachophoretic experiments
can be seen to be a valuable tool, it also has disadvantages. If a counter flow of electrolyte
is to be considered, the chemicals must be of the highest purity available, and even then
they often are not pure enough. The impurities may sometimes be collected betwetn the
leading and terminating zones and influence the analysis. Sometimes the zone still
undergoes a small migration and cannot be stopped owing to impurities present. The
impurities in the leading electrolyte and/or in the terminating electrolyte must be
removed by recrystallization, zone refining or electrophoretic procedures, etc., if
Eqn. 7.2 relates to the leading electrolyte and eqn. 7.3 to the terminating electrolyte,
where meff,T, meff.,I and are the effective mobilities of the terminating ion,
impurity and leading ion, respectively.
7.5.2. Counter flow with level regulation
Fig.7.17 shows schematically the equipment with which a counter flow of electrolyte
can be applied, and the circuit for the regulation of the counter flow of electrolyte is
shown in Fig.7.18.
The moment at which the counter flow is to be started can be selected with the
lO-kf2 potentiometer. The switch A is provided in order to have the possibility of
selecting a high potential on the side of the injection block of chosen polarity. It has to
be borne in mind that for optimal functioning of the conductimeter, the probe must be
at a ‘low’ potential, Le., less than 10 kV. As soon as the voltage selected by the lO-kS2
potentiometer has been reached, the level is controlled by the plunger (Fig.7.17) by means
of a coil. Before the experiment, this level is adjusted approximately to the level in the
compartment of the terminating electrolyte, such that the sample zones still migrate in the
appropriate direction by means of the electric field strength (possibly a small flow in the
direction of the movement of the zones is permitted). Owing to the construction of the
counter electrode compartment, the pH jump across the membrane is of minor importance.
Experiments with a counter flow of electrolyte showed that it is important that the
compartment in which the driving electrode is mounted should contain electrolyte also.
The electrolyte, containing buffer ions, decreases the potential at the measuring electrodes
of the conductivity probe and diminishes the pH jump across the membrane.