Frans_M_Everaerts_Isotachophoresis_378342.pdf

228 INSTRUMENTATION
The W light is generated by a microwave-powered low-pressure mercury lamp. This
W light is transported by an optical quartz rod and fed into a cylindrical slit of width
0.05-0.3 mm. As already mentioned, the PTFE narrow-bore tube is not interrupted and is
clamped by the slit, which is made of brass, and the W light passes through the narrow-
bore tube and is again transported via an optical quartz rod to a set of filters (an
interference filter in combination with an end filter). The UV quanta then illuminate a
UV light-sensitive photodiode (S330; Hamamatsu, Hamamatsu City, Japan). The quality
of the PTFE narrow-bore tube is not sufficiently constant for the UV detector, because
the PTFE material itself has a high W absorption. On mounting a particular narrow-bore
tube, the amount of light that passes through it and finally reaches the detector may
vary by a factor of up to three compared with a previously used narrow-bore tube owing
to the difference in the thickness of the two tubes, if they are filled with a non-UV-
absorbing liquid. On one hand the high absorptivity of the PTFE material in the UV
range is a disadvantage, while on the other hand the dark current, i.e., the current that is
transported by the PTFE wall and reaches the detector without passing through the
narrow-bore tube, is reduced to a minimum.
The signals are handled electronically and result in a trace on a potentiometric recorder.
The trace does not have a continuous stepwise character if the isotachophoretic zones pass
the detector.
At the position where the conductimeter is mounted, the narrow-bore tube is inter-
rupted by a piece of insulating material (Perspex or TPX) in which the micro-sensing
electrodes are mounted (Chapter 6). As a result, there is always a slight difference in cell
volume between the conductimeter and the UV detector. The sequence of mounting
these two detectors was tested and it proved to be of no importance; experiments were
carried out to prove this only with components that were stable in the UV region, and
possible deleterious effects due to W light were not studied.
The conductivity detector can be applied for measurements of the conductivity
(a.c. method) or for measurements of the potential gradient (d.c. method) via two micro-
sensing electrodes (10-pm Pt-Ir foil) mounted axially and in direct contact with the
electrolytes inside the narrow-bore tube. In Fig. 7.16 can be seen the position where
the coil is mounted in order to give good galvanic separation of the high potential on
the micro-sensing electrodes from the circuit for measuring the conductivity (potential
gradient) at low potential. As discussed in Chapter 6, a leak current towards earth (even
lo-” A) must be prevented. For this reason the conductivity probe is surrounded by
PTFE insulation. Even the wires connecting the micro-sensing electrodes with the
electronic measuring circuit are provided with extra insulation by means of a PTFE
narrow-bore tube.
The signals derived from the conductivity probe are fed to a field effect transistor
(potential gradient measurement) or directly to a well insulated transformer for good
galvanic insulation. If the measuring electrodes are mounted equiplanar, only the
conductivity can be determined of course.
For the measurement of the conductivity (a.c. method) with axially mounted elec-
trodes, these electrodes must be separated from each other via a capacitor, otherwise an
electric current will flow, due to the potential gradient, and electrode reactions will
result, e.g. , coating or gas production. Even when the micro-sensing electrodes were