Isopycnic zone ultracentrifugation

Isopycnic zone ultracentrifugation, on the other hand, deals with the preparative analog to equilibrium sedimentation in a density gradient. Here, large density gradients are used and one waits until equilibrium is attained. Both band and isopycnic zone ultracentrifugation can be carried out with very small solute concentrations. These methods have become, therefore, particularly important in the separation of biological macromolecules (biopolymers). Both methods are also described in the literature under a variety of other names. 

Figure 9-15. Kinds of preparative ultracentrifugation methods. (S) Normal ultracentrifugation, (B) band ultracentrifugation in a stabilized gradient, (I) isopycnic zone centrifugation p is the density of the gradient-forming substance (O) high molar mass, low density ( ) low molar mass, high density.

The differences in buoyant density as well as molecular size and shape provide a means for separating the nucleic acids by isopycnic ultracentrifugation, a centrifugation technique in which particles sediment through a gradient medium (CsCl or sucrose) until they reach a zone of equal densities and band at that position. 

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