Frans_M_Everaerts_Isotachophoresis_378342.pdf

84 CHOICE OF ELECTROLYTE SYSTEMS
(4) Further, other factors such as complex formation, precipitation [ 1 ] and other
specific interactions can be used. We shall not look into these possibilities too deeply,
although they automatically affect the effective mobility. In the Section Applications,
where practical information is given, this subject is discussed in more detail.
A general method of findmg a suitable electrolyte system cannot be given, but in this
chapter we shall discuss several important factors that play a role in the choice of
electrolyte systems (operational systems) and must therefore be taken into account.
At the end of the chapter, we present examples of some problems and separations
that have been solved in order to illustrate the principles of choosing electrolyte systems.
5.1.1. General remarks
In choosing a suitable electrolyte system, several factors play an important role. In
general, the most important requirement is that the system must be chosen in such a
way that the ionic species to be separated have maximal differences in effective mobilities,
in order to achieve a rapid and complete separation. Sometimes, however, it is not
possible to follow this general rule as particular conditions may be necessary for the
sample and there may be limitations to the apparatus available. The choice of a suitable
electrolyte system wil often be a matter of experience by which, sometimes intuitively,
the advantages and disadvantages of the different possibilities must be weighed against
each other. In some instances, a suitable electrolyte system can only be determined
experimentally (Section Applications). In this chapter a number of factors that can
play a role in choosing the electrolyte system are discussed, and some practical examples
are given of electrolyte systems for which data and separations are considered elsewhere
in this book.
The most important factors in choosing electrolyte systems that are discussed are:
the choice of the solvent;
the choice of the buffering counter ionic species;
the choice of the pH of the leading electrolyte;
the choice of the leading ionic species;
the choice of the terminating ionic species;
additions such as spacers (and carriers), stabilizers, surfaceactive compounds and
reference compounds for identification and concentration calibration (internal
standards).
5.2. CHOICE OF THE SOLVENT
The first problem in choosing an electrolyte system is to decide which solvent can be used.
In most instances, water is used as a solvent in electrophoresis, not only because of its
price, availability, etc. but particularly because of its superior solubility properties and its
ionization power. However, other solvents can be used that are more suitable for particular
applications, such as for the separations of substances that are insoluble or only slightly
soluble in water (fatty acids, amino acids, proteins and complexes). When a non-aqueous
solvent has to be used, the choice depends mainly on the properties of the sample.