Frans_M_Everaerts_Isotachophoresis_378342.pdf

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7.5. COUNTER FLOW OF ELECTROLYTE
7.5.1. Introduction
INSTRUMENTATION
Many of the papers describing electrophoretic techniques have dealt with the elec-
trolytic counter flow of electrolyte, and a large number of other papers could be cited,
especially relating to equipment filled with various stabilizing media. In ths field,
experiments are carried out to increase the length of the separation, especially for
improving the separation of isotopes, and it has been found that mainly enrichment
could be obtained. The electrophoretic techniques applied usually involved a moving-
boundary system, in whch a complete separation cannot be expected. However, if an
isotachophoretic system is chosen for the separation of isotopes, the separation procedure
is also a moving-boundary system that may result in a complete separation, ie., the
steady state.
In this section, some possible methods for regulated and non-regulated counter flows
are given, and a newly developed pumping system is described in which the gas produc-
tion is used for pumping hydrodynamically the liquid needed for the counterflow of
electrolyte. The driving current can be regulated by signals from the isotachophoretic
equipment, by means of which the zones can be stopped in the separation chamber (the
narrow-bore tube) if counter flow is applied. Of course, it is beyond the scope of this book
to discuss all possible systems for electrolytic counter flows.
We can consider a regulated counter flow in terms of the main basic principles, as
follows. (a) The electric current is constant during the analysis, and the hydrodynamic
counter flow of electrolyte is regulated and controlled by signals derived from the electro-
phoretic apparatus. (b) The hydrodynamic counter flow of electrolyte is constant during
the time the counter flow of electrolyte is required, and the electric current is adjusted
to this counter flow by means of signals derived from the electrophoretic equipment.
During the detection, the electric current is stabilized again. (c) The electric current is
constant in the initial phase and the hydrodynamic counter flow of electrolyte is started
as soon as a pre-set value of the voltage of the current-stabilized power supply has been
reached. The counter flow of electrolyte is then adjusted until no further increase in
voltage is obtained. If for any reason a lower pre-set value is reached, the counter flow of
electrolyte is stopped.
It should be pointed out that although the length of separation is generally increased,
the counter flow of electrolyte disturb the electrophoretic separation (Chapter 17).
The method of producing the counter flow can vary widely, and syringe pumps,
peristaltic pumps, level differences or ‘gas pumps’ can be applied.
Two main reasons can be given for wanting a counter flow of electrolyte, both
originating from the fact that the narrow-bore tube is not long enough for a particular
separation: (1) the concentration differences between the ions to be separated are too
laIge; and (2) the differences in (effective) mobility between the ions of interest are small.
Of course, these two factors may be combined in a specific instance.
In those instances when the difference in (effective) mobility is minimal, the use of a
counter flow of electrolyte will generally fail. More research needs to be carried out in
order to determine the effect of the ‘disturbance factor’. It is not unlikely that in specific