Page 56 - Tandem Techniques

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Eventually each sample component is separated from its neighbor in order of their increasing mobilities,
the one with the least mobility being situated next to the terminating electrolyte and that with the
greatest mobility next to the leading electrolyte. The separation is depicted in Figure 1.10. The bands in
isotachophoresis are not dispersed by diffusion in the normal way, as any dispersion that does occur
must result in the solute entering its neighboring bands. However, the solute will immediately be driven
back as a result of their differential mobilities and thus the solute bands are self-sharpening. This
system, in its present form, would be difficult to use in tandem with another instrument, but with some
development, tandems systems employing isotachophoresis as the separating technique may well
become a practical and useful combination in the future.

Isoelectric Focusing

Isoelectric focusing is used to separate amphoteric substances and, as a consequence, the separation is
based not on their differential mobilities but on their different isoelectric points (pI). The isoelectric
point of an ampholyte is that (pH) at which it has no net charge.

Figure 1.11
A Separation by Isoelectric Focusing