Frans_M_Everaerts_Isotachophoresis_378342.pdf

124 'DETECTION SYSTEMS
balanced by cutting a piece from one of the junctions, by setting one of the junctions
at a different angle to the wall or by putting extra adhesive on one of the junctions (or
alternatively by removing some of the adhesive from the other junction with a suitable
solvent). In order to check if the differential thermocouple is well balanced, the electric
current is switched on and off, when the signal derived from the differential thermocouple
should not vary. However, an effect can still be obtained, even if the differential
thermocouple is well balanced, if difference in heat capacity between the two junctions
is large. The switching on and off of the electric current may then result in a peak or a
dip, the height of which and the time required to reach the balanced position (zero)
again may be such that they cannot be considered to be negligible. This may disturb the
electrophoretic pattern recorded by the differential thermocouple, especially if large
temperature differences need to be recorded. The impression can be given that a zone is
preceded by a small zone of lower temperature (enforced isotachophoretic system) or
that an extra component (impurity) is present. If the electronic differential is taken, of
course, no balancing procedure needs to be carried out.
The great advantage of a differential thermocouple, however, is that the direction of
movement of the temperature step is recorded. Mistakes can always be made in the
interpretation if a zone of high conductivity (e.g. , H'ions that originate from the pH
jump at the membrane [16] moving through the narrow-bore tube) migrate in a direction
opposite to that of the sample ions. This zone causes a lower temperature, which migrates
and is therefore detected by the differential thermocouple as a hot zone coming from the
opposite direction. Because the differential thermocouple never has a position on the
narrow-bore tube similar to that of the linear thermocouple, which has only a single
junction on the wall, the time of recording clearly shows this effect. This effect, however,
may cause severe problems, especially because in practice a lower temperature zone may
migrate in front of zone of higher temperature in exceptional cases (e.g., enforced isotachophoretic
systems). If electronic circuits are used in order to differentiate the signal derived
from the linear thermocouple, this problem can be solved if at least two thermocouples
are mounted axially on the wall of the narrow-bore tube. The simultaneous recording of
these thermocouples indicates both the rate of separation (are mixed zones still present?)
and the direction of movement of various zones. Although a thermometric detector is
very cheap, its resolving power is, compared with that of other detectors, not very high,
although for many applications it is sufficient. This will be shown in later chapters, where
some applications are discussed.
The fronts, as finally detected by the thermometric detector, lack sharpness with
respect to the concentration profiles inside the narrow-bore tube, because the heat
generated inside has to pass through the wall*. The longitudinal conduction of heat, both
in the liquid and the insulator, spreads the temperature change along the tube. Also,
considerable time is required to heat the part immediately behind the front in order to
obtain dynamic equilibrium of the temperature step. The wall must not be too thin,
however, because the thermocouple and the instrumentation connected to it must be
insulated from the high potential inside the narrowbore tube.
Even if another precaution is taken, e.g., by using an insulated amplifier, and the
thermocouple is mounted closer to the centre of the narrow-bore tube, the final recording
is not improved very much (Fig.6.3), because the time required for dynamic equilibrium
*See also Appendix B.