Frans_M_Everaerts_Isotachophoresis_378342.pdf

THERMOMETRIC RECORDING
Potential gradient Temperature PH
Determined by the direct driving
current chosen and the conductivity
of the L.E.
Determined by the direct driving
current chosen and the concen-
tration of the anion of the L.E.
(anion separation) or the cation of
the L.E. (cation separation) and the
effective mobility of the ionic
constituent present.
Determined by the direct driving
current chosen and the concentration
of the L.E. initially chosen
and the effective mobility of the
terminating ion.
Determined by the direct driving Determined by the ratio of the
current and the conductivity
of the L.E.
121
concentration of the ionic
species of the leading electro-
lyte and their pKa values.
Determined by the direct driving Determined by the ratio of the
current and the concentration concentration of the ionic
of the anion of the L.E. (anion species present in each zone
separation) or cation of the and their pKa values.
L.E. (cation separation).
Determined by the direct driving Determined by the ratio of the
current and the concen-
concentration of the ionic
tration of the L.E. initially species present in this zone
chosen. and their pKa values.
The potential gradient is a constant The temperature is constant for The pH tends to increase in
for each zone and increases
each zone and increases from anion separation and to
from the L.E. towards the T.E. the L.E. towards the T.E. decrease in cation separation
(except for ‘enforced’ isotacho- (except for ‘enforced’ iso- from the L.E. towards the
phoretic systems). tachophoretic systems). T.E.
If this thermocouple is mounted around the narrow-bore tube, a linear stepwise
recording of the isotachophoretic zones is obtained as soon as they have passed this fixed
thermocouple. In order to make the zone boundaries more pronounced electronically,
the differential of this signal can be obtained. An example of a possible circuit is given in
Fig.6.2.
Another possibility is to make a differential thermocouple with two copper-constantan
junctions, about 2 mm apart. This thermocouple can be mounted around the narrow-bore
tube in a similar way by means of a suitable elastic adhesive. The two twisted (and silver
soldered) ends must be bent until they are approximately at right-angles to the surface
of the narrow-bore tube. While the values from the passage of a zone boundary recorded
by a normal thermocouple can be expected to be of the order of millivolts, a differential
thermocouple indicates the passage of a zone boundary with a signal of the order of
microvolts, and amplification is therefore necessary.
The differential thermocouple has to be balanced. Normally, there is a difference in
temperature between the two junctions of the differential thermocouple, because both