Molecular weight distribution methods ultracentrifugation

Svedberg proves unambiguously the existence of macromolecules by means of the ultracentrifuge he also develops the first precise method for obtaining the molecular weight distribution... 

Number-average molecular weights naturally can also be calculated from methods that provide a detennination of molecular weight distribution. These include fractional precipitation, ultracentrifugation, and gel permeation chromatography. From such determinations, M is nearly gratis information to be accepted with reservation. 

Other methods of molecular weight distribution can also be influenced by compositional variations. The birefringence detection method in ultracentrifugation, for example, is sensitive to polymer type. The polymer separation itself, based on density differences, is also influenced by composition. 

Figure 2.13. Molecular weight distribution (MWD) of a series of HPAM samples using an ultracentrifugation method (from Argabright et ai, 1982).

An alternative method of using the ultracentrifuge to measure molecular weights is to allow the distribution of particles to reach equilibrium. As sedimentation occurs in the ultracentrifuge a concentration gradient is established, and this will cause the molecules to diffuse in the opposite direction. Eventually the system reaches a state of equilibrium at which the rate with which the solute is driven outwards by the centrifugal force just equals the rate with which it diffuses inwards under the influence of the concentration gradient. 

Fractional precipitation is of very great practical value and its range of applicability is still increasing rapidly. In many instances it represents the only method by which the distribution of molecular weights in a sample may be determined. In principle this could also be achieved by ultracentrifugal means. We have seen, however, in section 5, p. 137 that this method is often hampered by serious practical obstacles. 

---