Frans_M_Everaerts_Isotachophoresis_378342.pdf

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DETECTION SYSTEMS
addition of an external source, in spite of its high stability, gave poor results with respect
to the drift. There appears to be no other explanation that no compensation can be
made on the input of the 272 J amplifier. The drift obtained if an external source is used
for zero adjustment originates mainly from the variations in the current of the currentstabilized
power supply. Experimentally, we found that for simple compensation, the
driving current itself could be taken, as shown in Fig.6.11.
The value of the compensation is chosen such that the resistance halfway between the
resistances of the leading and terminating electrolytes is optimally compensated. This
method of compensation works better if the relative change in conductivity of the various
zones is smaller or if the distance between the micro-sensing electrodes is reduced as
much as possible. The signals finally obtained were of such a value that an attenuator
had to be applied for recording on a 100-mV recorder. Because the driving current is
used for compensation, changes in the electric current have less influence on the detec-
tion, as mentioned before. Because the measuring current must be low, if the d.c. method
is chosen for resistance determination, the micro-sensing electrodes may be made
extremely thin. In order to demonstrate this, experiments were carried out in which
electrodes were made by sputtering Pt on both sides of a foil of an insulator (0.05 mm
thick). An isotachopherogram is shown later in Fig.6.50. If these thin electrodes are
mounted in the conductivity cell, no simultaneous a.c. measurements can be made
because this destroys the electrode surface.
The disadvantage (a.c. method) of the axial construction of the sensing electrodes is
that the measuring current will flow mainly directly along the wall between the sensing
electrodes. Wall effects, e.g., electroendosmosis, will influence the detection more than
when the measuring current can flow through the centre of the narrow-bore tube. The
addition of surface-active substances, which directly influence these wall effects, improved
the detection by the a.c. method more than that by the d.c. method, in which much
less current has to pass the electrode-electrolyte interface. Nevertheless, surface-active
substances need to be added in order to achieve high resolution (as was found to be neces-
sary, too, if W detection was applied).
In order to make a comparison of a detector with the sensing electrodes in direct
contact with the electrolytes inside the narrow-bore tube with a thermometric detector
possible, a thermocouple was mounted around the same narrow-bore tube. The micro-
sensing electrodes and the thermocouple were mounted as close to each other as possible.
The result is shown in Fig.6.12.
The measured isotachopherogram was compared with a theoretical curve calculated
from a rough model of the current distribution. Fig.6.13(2) shows how, in this model,
the current used for the detection is distributed over the electrolytes in the neighbourhood
of the micro-sensing electrodes. Two cross-sections of the narrow-bore tube are shown;
one is perpendicular to the axis (Q), located between the two electrodes, and the other
coincides with the axis (P). When the narrow-bore tube is homogeneously filled, the
detection current is perpendicular to the cross-section Q. The simplified current pattern
is represented by parallel currents through different resistances a and b in Fig.6.13(2).
These two resistances are assumed to be proportional to the length of the lines a and b
and to the resistivity of the electrolyte.
In this model, the passage of a zone boundary can be dealt with by dividing each