Band sedimentation

Fig. 52. Coagulation of mixed colloidal solutions of iron and silica. Environment A—slightly acid B—slightly alkaline. Concentration of recent waters indicated at right. / = total precipitation of iron and silica with formation of banded sediments // = same, with formation of mixed sediments /// = total precipitation of iron and partial of silica / F= total precipitation of iron, silica in solution V = incomplete precipitation of iron, silica in solution VI— no precipitation observed.

Interaction of relatively concentrated alkaline mixed sols with sea water of normal salinity is necessary for the formation of banded sediments of jaspilite type. 

Mimk (144) has recently reviewed the use of sedimentation velocity and sedimentation equilibriiun to characterize molecular weight. A new absolute method for the determination of molecular weight distribution based on band sedimentation and LALLS has been apphed to water-soluble polymers (145). 

Another problem area in band sedimentation is the determination of sedimentation coefficient. Since there is a density gradient, the net force acting on the molecule is never a simple function of distance. Secondly, if the determination is being done in a preparative... 

While these problems do exist for band sedimentation, there are its uses which make it an indispensable technique. If the mixture consists of particles with discrete sedimentation coefficients, this technique is the best to either separate them or study their behaviour. Boundary sedimentation may fall here for the simple reason that if there is a minor component of a high molecular mass among other components of smaller masses, the former may just become indistinguishable. On the other hand, if the mixture Is heterogeneous, boundary sedimentation is the best method to use. 

Band sedimentation has been used extensively in the study of nucleic acid structure and function. We give below a few examples. 

This method, which is less used today, is nevertheless a great example of the use of band sedimentation technique for the simple reason that it takes advantage of the shape of the macromolecules in order to separate them. As such it is hugely instructive of the range of the technique and the manipulation that a researcher can resort to in order to exploit the technique fully. Also notable is the fact that both boundary and band centrifugation are resorted to. 

Zonierung, Stufung zone electrophoresis Zonenelektrophorese zone melting/ zone refining Zonenschmelze(n) zone sedimentation/ zonal sedimentation/ band sedimentation Zonensedimentation zwitterion Zwitterion... 

Figure 2.12. Normalised molecular weight distribution (MWD) of an HPAM sample using band sedimentation and LALLS (from Holzwarth et a/., 1988).

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