The separation of macromolecules presents major problems and techniques such as dialysis permit only fairly gross separations to be achieved. However, the development of gel permeation chromatography introduced an extremely valuable method by which molecules may be separated from each other on the basis of their size (Figure 3.34). A wide range of molecular sizes can be separated by this method but for any particular gel the range is relatively narrow and there are a large number of different gels available commercially.
Ultracentrifugal techniques strictly speaking separate on the basis of the mass and not the size of particle. A variety of ultracentrifugal techniques are available and appropriate to different analytical problems. Preparative techniques generally involve iso-density or maximum velocity ultracentrifugation, whereas it is possible to investigate the relative molecular mass, diffusion characteristics and shape of a molecule using an analytical ultracentrifuge and equilibrium or maximum velocity techniques.
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