Molecular weight: light scattering method ultracentrifugation

Weight average molecular weight may be experimentally determined by any method in which molecular size or molar mass is the parameter being measured (e.g., light scattering or ultracentrifugation). 

Ultracentrifugation experiments yield Mz values. They are used primarily for biological polymers. The random coil conformation of most synthetic polymers in solutions makes it difficult to interpret these data and this method is little used with such materials. Among the absolute methods of molecular weight determination, only the osmotic method and the light scattering method are given primary consideration in this chapter. 

In Table 5.1, membrane osmometry, cryoscopy, ebulliometry, end-group analysis, light-scattering and ultracentrifugation belong to the category of so-called absolute methods. Vapour pressure osmometry, viscometry and GPC can be classified as relative methods, which require samples of known molecular weight for calibration. GPC is discussed in chapter 6, and will not be dealt with here. 

As noted earlier, certain techniques such as colligative methods, light-scattering photometry, special mass spectrometry (MS) techniques, and ultracentrifugation allow the calculation of specific or absolute molecular weights. Under certain conditions some of these also allow the calculation of the MWD. 

To calculate the molecular formula of the peptide—the actual number of each kind of residue in each peptide molecule—one needs to know the molecular weight. Molecular weights can be determined by chemical methods and by various physical methods osmotic pressure or light>scattering measurements, behavior in an ultracentrifuge, x-ray diffraction. 

Methods for the determination of the molecular weight of a polysaccharide include gel-filtration (Andrews, 1970), ultracentrifugation, light scattering, and osmotic pressure determinations (Banks and Greenwood,... 

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