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Absolute Molecular Weight

Absolute Molecular Weight

Analysis (GPC-LS) Theory

Light scattering is an absolute technique for the determination of the molecular weight of macromolecules (e.g., polymers, copolymers, proteins, etc). This technique is independent of the column calibration procedure and determines the absolute molecular mass directly. Light scattering is used when the true value of molecular weight is of great importance. The molecular weight value obtained by light scattering can be matched with the values obtained by spectroscopy techniques. Light scattering detector theory is based on Rayleigh’s equation relating the intensity of scattered light, R(θ), to the molecular mass of macromolecules, Mw, and the concentration of macromolecules in solution, c:

This technique can be employed in batch mode or in line with size exclusion. In GPC/SEC, light scattering is employed to determine the absolute molecular weight for each fraction of distribution. For example, high molecular weight polystyrene: