Moving-boundary methods sedimentation

The sedimentation constant s has been determined by applying the moving boundary method (46)... 

Differential ultracentrifugation methods may also be applied to analysis of the purity of macromolecular samples. If one sharp moving boundary is observed in a rotating centrifuge cell, it indicates that the sample has one component and therefore is pure. In an impure sample, each com ponent would be expected to form a separate moving boundary upon sedimentation. 

By measuring the rate at which the boundary moves, an average sedimentation coefficient for the sample may be obtained. If multiple components are present, the boundary will broaden and may even resolve into discrete steps. However, diffusion will also cause the boundary to spread. Numerical or graphical methods are usually required to distinguish... 

The diffusion coefficient is related to the rate at which molecules migrate down a concentration gradient (it is treated in detail in Section 8.5) and can be measured by observing the rate at which a concentration boundary moves or the rate at which a more concentrated solution diffuses into a less concentrated one. The diffusion coefficient can also be measured by using laser light-scattering methods (Section 11.3). It follows that we can find the molar mass by combining measurements of sedimentation and diffusion rates (to obtain S and D, respectively). 

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