Frans_M_Everaerts_Isotachophoresis_378342.pdf

COUNTER FLOW OF ELECTROLYTE
Fig.7.20. ElecQonic circuit for stabilizing the electric current by signals derived from a thermocouple
mounted around the narrow-bore tube in which the electric current flows. This circuit can also be
applied for regulation of the various zones moving isotachophoretically in the narrow-bore tube in
experiments with a counter flow of electrolyte. 1,2 = connections for the thermocouple (copper-
constantan); 3 = + 15 V; 4 = common terminal; 5 = - 15 V,
Therefore, during the counter flow of electrolyte, a thermocouple mounted around the
narrow-bore tube is applied and during the detection of the zones the current is stabilized
by an extra resistor mounted in series with both the LDR and the narrow-bore tube.
The counter flow of electrolyte can be produced in various ways, although only the
syringe pump is shown in Fig.7.19. Particularly if the counter flow is produced by a
difference in levels, a complication can arise because the level is not controlled, and the
counter flow will thus change with time. As will be discussed later, there are two limits
for the counter flow and if at a certain moment the lower limit is exceeded the counter
flow of electrolyte is no longer able to stop the zones. For a counter flow over a long
period of time, the electrode compartment that contains the counter flow electrolyte
must be very large and it is preferable to use a pump, especially that discussed in
section 7.5.5.
It wdl be noticed immediately that the adjustment of the electric current as described
here will automatically result in an oscillation of the zones around the regulating thermo-
couple. For thermometric recording, the zone must have passed the thermometric
detector by about 1-2 cm for complete qualitative and quantitative determination, but
for the regulation a much lower signal is needed. We found that this method of regulation
gives a negligible oscillation; the experimental conditions were checked with coloured
ions for which the sharpness of the boundaries was studied.
Fig.7.21 shows two isotachopherograms for the separation of formate and acetate
with and without a counter flow of electrolyte in order to demonstrate LDR regulation
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