Frans_M_Everaerts_Isotachophoresis_378342.pdf

316 COUNTER FLOW OF ELECTROLYTE
detector is not needed. This regulation, however, places demands on the electrolytes of
the operational system, which must be of high purity and also of carefully selected
concentration. If the terminating electrolyte is impure, an impurity may influence the
final recording if its effective mobility is greater than that of the terminating ion (see
eqn. 7.4.). Both the pH and the concentration of the terminating electrolyte may have
an influence on the regulation of the counter flow of electrolyte. At the position
where the terminator is present in the narrow bore (1 mm) of the injection system (see
Fig.7.5.), a well defined potential gradient is obtained as a result of the electrophoretic
driving current chosen and the concentration of the electrolyte present (Ohm’s law). In
isotachophoretic experiments without a counter flow of electrolyte, the concentration
of the terminator has hardly an influence, assuming that it is present in such a concentra-
tion that it is able to adapt to the isotachophoretic concentration, which is determined
by the concentration of the leading electrolyte chosen (ratio of the anion and cation
concentrations). If a counter flow of electrolyte is now applied that is regulated by the
total potential gradient between the electrodes, the total resistance over the bore (1 mm)
filled with terminating electrolyte changes, owing to the counter flow of electrolyte applied,
if the composition of the terminating electrolyte is not already that which it would
finally attain as a result of the isotachophoretic conditions.
If the concentration of the terminating electrolyte is too low, then in a long run the
potential gradient will decrease, because the final conductivity* is higher. This results in a
movement of the sample zones in the direction of the detector. If the Concentration of
the terminating electrolyte is too high, then the sample zones will be pushed back,
although this effect is much smaller and generally has a less important effect on the final
results.
If a later experiment with a counter flow of electrolyte is to be carried out, it is
preferable to rinse the reservoir fdled with terminating electrolyte and to re-fill it with
a fresh solution, although this is not always necessary in experiments without a counter
flow of electrolyte**. Moreover, it was found experimentally to be preferable to fill the
reservoir at least three-quarters full with the terminating electrolyte, otherwise impurities
can penetrate into the narrow-bore tube more easily. If impurities are present in the
leading electrolyte, they may influence the final recording (see eqn. 7.3).
Particular attention should be paid to the electrolyte present in the counter electrode
compartment at the side of the semi-permeable membrane where the electrode is
mounted. In experiments without a counter flow of electrolyte, double-distilled water is
suitable because the membrane is not polluted by the counter ion chosen. Especially if
the equipment is used for various operational systems, the mutual effect of the various
counter ions is minimal. If experiments with a counter flow of electrolyte are considered,
the pH shift due to the membrane and the electrode reaction may influence the final
result more quickly. If double-distilled water is still preferred, this compartment must
be rinsed continuously during the analysis. If leading electrolyte is present, this compart-
ment must still be rinsed and re-filled after each experiment, assuming that the time of
analysis is not excessive, otherwise it must also be supplied continuously with fresh
electrolyte.
*The buffer ions are flushed into the canals filled with terminating ions.
**If experiments are carried out at a low concentration (0.01 M), it is preferable to start each experiment
again with a fresh solution of the terminating electrolyte.