Frans_M_Everaerts_Isotachophoresis_378342.pdf

Chapter 8
Introduction
SUMMARY
In ths chapter some practical information is given on the Section Applications, and a
scheme is given for ‘trouble-shooting’.
8. INTRODUCTION
The Section Applications contains almost all of the practical information about
isotachophoretic separations in narrow-bore tubes. In this section, applications and results
are given for separations classified according to chemical compounds that belong to clearly
distinguishable classes.
The separations were carried out in so-called operational systems in which the electro-
lytes were shown to give optimal results. The operational systems are listed in tables, in
order to make a comparison between them possible. The systems listed were chosen
somewhat arbitrarily; many more possibilities could be given. Also, the separations con-
sidered were mainly chosen arbitrarily: many real problems from industry or hospitals
proved to be much simpler. The separations are shown in order to indicate their possi-
bilities and to make patterns recognizable.
When a specific operational system is chosen, one always has to bear in mind that the
pH in anionic separations by isotachophoresis tends to increase, while in cationic separa-
tions it tends to decrease, going from the leading zone towards the terminating zone.
Therefore, the pH must always be chosen such that the optimal effect of the buffering
counter ion is used. In some instances a buffering counter ion is not necessary, while in
other instances two or more counter ions with overlapping buffer regions are needed.
A difference of 0.5 pH unit can give an operational system that has completely
different characteristics for a specific analytical problem, as shown in the following
example.
If a separation of anions is sought and the operational system at pH 6 (Table 12.1) is
found to be suitable, one can adjust the pH of the leading electrolyte from its initial
value of 6; the buffering capacity of histidine is sufficient until a pH of ca. 7. If, however,
an anion is present with a very low effective mobility, which needs a terminator with an
even lower effective mobility than the anion to be separated, it may be preferable to
adjust the pH of the leading electrolyte to 5.5. If the pH of the leading’electrolyte is
decreased too much, sometimes difficulties can arise because too few counter ions are
present and the buffering capacity may not be sufficient (see Fig.9.5). Experimentally, we
found it best to adjust the pH of the leading electrolyte, with the chosen counter ion, to
the selected value as accurately as possible and to check the pH of the solution again the
following day. In most instances the pH is shifted (kO.1-0.2 pH unit).
Step heights listed in the various tables are proportional to the effective mobilities of
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