Frans_M_Everaerts_Isotachophoresis_378342.pdf

INTRODUCTION 25 1
the various ionic species in the operational systems and they indicate which ions can be
separated in a given length of narrow-bore tube, assuming that the concentration differences
are not too great. If the concentration differences are too great, a longer narrowbore
tube or a counter flow of electrolyte must be used (see Chapter 17). It should be
noted that no corrections for the temperatures of consecutive zones have been made to
the results presented from either the thermometric or conductivity detector.
If the influence of the counter ion chosen is great (high effective mobility in the opera-
tional system chosen), greater differences in effective mobility between the sample ions
are needed for a complete separation. If the pH of the consecutive zones increases
regularly (in anionic separations) or decreases regularly (in cationic separations), small
differences in effective mobility are often sufficient because once the ions diffuse into
the zone where the pH is higher (anionic separations) or lower (cationic separations), they
may attain a greater effective mobility owing to dissociation. More attention is paid to
this phenomenon in Chapter 9.
It is sometimes easier and quicker to apply two or more operational systems, as the
equipment can be rinsed in a few minutes and is then ready for another operational
system, than to try to carry out a complete separation in one operational system. More
attention is paid to this aspect in Chapter 11. Sometimes only the concentration of the
operational system needs to be changed. The sulphate ion, for instance, in the operational
system at pH 6 (Table 12.1) moves behind the chloride ion (concentration of the leading
ion, C1- = 0.01 N), while it has a greater mobility than the chloride ion if the concentra-
tion of the ‘leading’ chloride ion is changed to 0.001 N. Another example is given in
Fig.12.7. In principle, we do not recommend the use of too long a narrow-bore tube,
because the voltages needed are too high and electroendosmosis may dominate the
separation.
If two types of detectors are available, in many instances the analysis can be carried
out, in spite of stable mixed zones (see Fig.6.33), in one operational system. From time
to time the equipment used in isotachophoretic analyses must be washed well with a non-
ionic detergent followed by thorough rinsing with double-distilled water. Adsorption of
many types of compounds may influence the detection (see Chapter 6), because amongst
other effects the {-potential may be changed. Because the resolution of both the W
absorption detector and the conductivity detector (ax. method) decreases in such
instances, an electrode reaction only must be rejected. We prefer the a.c. method to the
d.c. method because the detector indicates more quickly if something is going wrong and
measures can be taken directly (see Chapter 6). We recommend that, before a series of
analyses, a test mixture of anions or cations should be examined, because it can be seen
very quickly if the resolution and reproducibility are adequate. If the test mixture indi-
cates that non-reproducible data can be expected, appropriate measures must be taken,
as shown in Fig.8.1.
In our analyses, we pay special attention to the pH and the concentration of the
terminating electrolyte, although from various papers one may obtain the impression that
this is unnecessary. If too high a concentration of the terminating electrolyte is chosen,
eluting effects due to the impurities can be expected, the sample zones may still migrate
if a 100% counter flow of electrolyte is present (while the regulation is made via the