Frans_M_Everaerts_Isotachophoresis_378342.pdf

EQUIF'MENT 223
No effect on the resolution of the fact that the narrow-bore tube is no longer straight
could be observed on the thermometric detector. Later experiments with high-resolution
detectors showed that the narrow-bore tube must be mounted as straight as possible,
especially the last 2 cm before the detector. If even a small kink was present just before
the detector, the resolution was decreased.
Gaps between the narrow-bore tube and the aluminium block are carefully filled
with a heat-sink compound (zinc oxide powder in silicone oil). Because the heat produced
in the narrow-bore tube is transferred so quickly to the aluminium block, a special
compartment (see Fig.7.12) is created where the thermocouples are mounted in order to
ensure that there will still be a signal to detect. If this compartment is too small, a noisy
baseline results because a very high amplification has to be used. Of course, a com-
promise must be sought, because if this compartment is too big a situation similar to that
described in section 7.4.2 results, and the narrow-bore tube is cooled only by thermo-
stated air surrounding it. The temperature of the reference junctions of the thermocouples
is always the same as that of the aluminium block, because a certain amount of heat-
sink compound is smeared on the junction that is insulated with a PTFE spray, which is
glued to the aluminium block in order to guarantee good thermal contact with the
aluminium block. The narrow-bore tube and the heat-sink compound are fixed by a thin
layer of shellac spray (Krylon). For thermostating the aluminium block, thermostated
water (accurate to +O.0loC) is used. A temperature sensor (100-L2 Pt resistor) is mounted
in the neighbourhood of the detector compartment. Also here gaps are filled with the
heat-sink compound. In the centre of the aluminium block a load of 60 W is mounted. The
Pt resistor and the load are both connected to the temperature control unit, as shown in
Fig.7.13. Rubber O-rings are employed to prevent contact of water, circulating inside
the aluminium block, with the electrical circuits.
The narrow-bore tube protruding from the thermostat is connected at one side with
a type of injection block as shown in Fig.7.7. The narrow-bore tube is connected to the
injection block, without adhesive, by means of the clamping device discussed in section
7.2.3. As the counter electrode compartment, the cylindrical construction shown in
Fig.7.8 was used.
The proportional temperature controller used in the thermostat is based on the
relatively high temperature coefficient of a Pt resistor, a Pt resistor of 100 52 at 0°C being
used. This Pt resistor forms an a.c. bridge (R5) together with the resistors R1, R2, R3 and
%. The temperature coefficients of all resistors in the a.c. bridge, apart from the resistor
R5, must be chosen to be as small as possible. The smaller these values are, the more
accurate will be the thermostatic control. Of course, one of the resistors of the a.c. bridge
is variable so as to permit the a.c. bridge to be balanced. If the bridge is unbalanced, a
signal, the result of the unbalanced position, will be fed to a pre-amplifier, the phase of
this signal being dependent on the polarity of the imbalance of the bridge. The pre-
amplifier generates a sinusoidal voltage and this will be transformed by a voltage limiter
into a symmetrical square wave. By means of the very high amplification of the pre-
amplifier and the voltage limiter, the amplitude of the bridge voltage is transformed into
a phase-shifted square wave. The leading edge of this square wave is amplified and triggers
two antiparallel-connected thyristors that control the amount of heat dissipated in the
load, which is mounted in the direct neighbourhood of the Pt resistor (Fig.7.12).