Frans_M_Everaerts_Isotachophoresis_378342.pdf

COUNTER FLOW OF ELECTROLYTE 231
around which the narrow-bore tube was mounted (section 7.4.3) was thermostated at
18°C. A 2-pl injection was made, containing 0.02 mole of sodium acetate and 0.02 mole
of sodium formate. The current was 92 pA in the initial phase and the temperature of the
zone of the leading ion was used as reference for the current stabilization (about 25°C).
In the initial phase, the current is not yet stabilized, possibly owing to the movement of
ions ahead of the zones of formate and acetate, which increase the conductivity. Subse-
quently the real zones of formate and acetate reach the regulation thermocouple. Because
these zones are considerably hotter than the leading zone (see Fig.6.7), even if the
aluminium block is applied as a thermostat, the electrophoretic driving current will
decrease. Fig.7.21 shows clearly a drop in electric current from ima. = 92pA towards
imh. = 52 pA, because the temperature of the glutamate is used for stabilization of the
electric current. In Fig.7.21 b, from the initial phase a counter flow of electrolyte is
produced in such an amount that the zones still have a movement in the appropriate
direction. Fig.7.21 shows that the temperature of the formate zone does not give such a
low electric current that the zones are stopped by the counter flow chosen. Between the
formate and acetate zones, however, a temperature is attained such that the zones are
stopped. The counter flow produced was 200 pllh. After the counter flow of electrolyte
has stopped, the current decreases further to the in,h. value and the experiment is
completed, as shown in Fig.7.21a.
It need not be explained that the thermocouple used for the detection of the various
zones must be mounted as far as possible from the regulating thermocouple, otherwise
some material may pass the recording thermocouple too soon, especially if components
are present in the sample that normally have a temperature in the zone lower than that
at which equilibrium is obtained.
7.5.4. Counter flow with direct control on the pumping mechanism via the power supply
A counter flow of electrolyte can be obtained in this way if the initial and end voltage
over the narrow-bore tube are known. If both of these values are known, the position of
the zones as a function of the potential gradient and the approximate counter flow
required in order to stop the zones can be calculated (Fig.7.22). A circuit such as that
shown in Fig.7.23 can be applied, with which it is possible to select a voltage of the
current-stabilized power supply at which the pump is started. Because the counter flow
to be produced is calculated only roughly, a counter flow must be selected such that the
zones are stopped and slowly pushed back. If the counter flow is insufficient, the zones
are not stopped by the pump and finally reach the detector, while if the counter flow
matches the movement of the zones the pump will be in action continuously.
If the zones are pushed back, the voltage across the narrow-bore tube will decrease. As
soon as a chosen lower limit has been reached, the counter flow of'electrolyte is stopped
and the zones will again move in the required direction. It needs no further explanation
that the range of voltage in which the operation of the pump is planned must be very
small. Experiments with coloured ions showed that the zone boundaries are less sharp
during the period when they are being pushed back, but as soon as the pumping was
stopped sharp boundaries were recorded very rapidly.