Frans_M_Everaerts_Isotachophoresis_378342.pdf

CONDUCTIMETRIC MEASUREMENTS
factor of 10. Other factors, e.g. , electroendosmosis and temperature profiles, place
limits on the dilution of the leading electrolyte. Moreover, the pH range in which the
analysis can be carried out will be considerably smaller if dilute solutions are applied.
Elution effects due to OH and H’ soon appear. Also impurities, especially those
present in the terminating electrolyte, may play a dominant role. For correct sampling,
especially with a micro-syringe, the concentration of the terminating electrolyte must
be adjusted to the low concentration of the leading electrolyte. More attention must
be paid to the pH*.
The detection limit can be decreased by injecting a larger sample, and a sample tap is
particularly suitable for ths. Of course the availability of a counter flow of electrolyte
can also decrease the detection limit.
The time required for the analyses depends on the length of the narrow-bore tube
needed for separation, the electric current used, the type of leading and counter io;r;s
present, the pH, the differences in effective mobility of the most difficult pair of ions
that need to be separated, the volume injected, the concentrations of the various sample
ions, etc. The time required for the analyses discussed above was cu. 45-60 min.
10.4. CONDUCTIMETRIC MEASUREMENTS
10.4.1. Reproducibility
Experiments are often carried out at low concentration regions in which it is impossible
to use a thermometric detector. Experiments with thermometric detectors have shown,
however, that the greatest reproducibility and linearity are obtained if low concentra-
tions and small amounts of sample are used.
For the experiments with the conductivity detector, the improved injection block
(section 7.2.4) can be used and the injection of the sample made in the leading electrolyte,
the terminating electrolyte or at the boundary between them. The effects of the
terminators applied can be studied more precisely. The experiments showed that the
best terminator is a component with a suitable pK value and that its concentration must
be made as similar as possible to the adjusted concentration inside the narrow-bore tube.
Also, the pH must be adjusted to an appropriate value. However, all of these precautions
with respect to the terminating electrolyte need not be taken in all experiments.
Not only the injection block, but also the use of a high-resolution detector improves
the reproducibility. The experiments here were also carried out in the operational system
at pH 6 (Table 12.1.).
Again formic acid was injected ten times with glutamic acid and acetic acid as the
terminators. The terminator solution was carefully prepared, the pH being adjusted to
that of the leading electrolyte by addition of recrystallized Tris. The average zone length
was found to be L* = 65 sec for both series of experiments, with an average deviation of
0.4 sec.
*An isotachopherogram of a separation at a low concentration of the leading electrolyte is shown in
Chapter 6, where the thermometric detector is discussed (section 6.2.4, Fig.6.6).
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