Frans_M_Everaerts_Isotachophoresis_378342.pdf

224 INSTRUMENTATION
If the bridge approaches its balanced condition, the output voltage of the bridge
decreases and the phase shift of the thyristor trigger pulse will thus be reduced. The
thryistor will trigger later and the heat produced in the load will also decrease, and a
steady state will result.
The temperature (T, "C) can be selected by the variable resistor & of the a.c.
bridge, according to the equation
T= 2.59 (R4 - 0.5835) (7.1)
(temperature coefficient Rs = 0.003916). Some possibilities are shown in the Table 7.1.
An RC filter in front of the input of the pre-amplifier corrects the phase of the trigger
pulse, and the proportional band (system gain) can be changed by varying the a.c. voltage
over the bridge. Incorrect temperature regulation may result if the temperature coef-
ficients of the other resistors of the a.c. bridge are poor, leading to instabilities if the
thermal resistance between the load and the Pt resistor is large or if the heat capacity of
the object to be thermostated is too high.
A photograph of the equipment with indirect thermostating via the aluminium block
is shown in Fig.7.14, which also shows the power supply. A pressure system has been
developed for rinsing and re-filling the different compartments of the equipment with the
chosen electrolytes. In order to prevent the dissolution of air in these electrolytes, the
surfaces of the electrolytes were covered with a layer of kerosene. An additional
advantage is that negligible amounts of carbon dioxide dissolve in the electrolytes if a
'high' pH is chosen in performing the analysis (even pH 7). If experiments are to be
carried out in parallel-mounted narrow-bore tubes, this method of mounting the narrow-
bore tubes on a thermostated aluminium block is recommended.
7.4.4. Equipment with high-resolution detectors
Fig.7.15 shows a schematic diagram of isotachophoretic equipment for use with a highresolution
W absorption detector and a conductimeter, and a photograph is shown in
Fig.7.16.
The equipment consists of a PTFE narrow-bore tube in which the analysis is performed,
the length being about 25 cm, although a longer tube can easily be mounted. It is
produced by Habia (Breda, The Netherlands), with O.D. c.a 0.7-0.75 mm and I.D. ca
TABLE 7.1
SOME VALUES FOR THE RESISTANCES (a) TO BE MOUNTED IN THE BRIDGE (FIG.7.13) OF
THE TEMPERATURE-REGULATING UNIT TO SELECT A TEMPERATURE RANGE
Resistors R, , R, and R, : 1%, 50 ppm/"C.
Resistance Temperature range ('0
0-50 0-125 0-300
R, =R, 120 150 180
R3 100 100 100
R, 50 50 100