Light Scattering is an absolute technique for the determination of molecular weight of macromolecules. This technique is independent of the column calibration procedure and determines the ABSOLUTE or the TRUE molecular mass DIRECTLY. The weight-average molecular weight value obtained by light scattering can be matched with the values obtained by spectroscopy techniques. Light scattering detector theory is based on Raleigh’s equation relating the intensity of scattered light, R(θ), to the molecular mass of macromolecules, Mw, and the concentration of sample solution, c:
The relationship between LS and MW is described by the Zimm equation where the optical constant, K, is dependent on (dn/dc)2 and is related to the intensity of scattered light (Rϴ):
_Optical Constant Equation _
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 the polymer’s distribution. Light scattering also provides Radius of Gyration, Rg, by measuring the angular dependency of the scattered light.
Combining light scattering, dilution solution viscometry and SEC/GPC is also known as Triple Detection SEC/GPC. It is the most comprehensive chromatography technique for polymer characterization. It employs Refractive Index Detector (with or without UV Detector), Dilute Solution Viscometry and Light Scattering all in series to determine molecular weights, distribution and related solution parameters. The triple detection GPC method is independent of calibration standards. Refractive index detector is employed to calculate concentration, refractive index increment (dn/dc), and injection recovery. Light scattering measures absolute molecular weight (M), molecular weight distribution (Mw/Mn), and radius of gyration (Rg) and viscometer reveals intrinsic viscosity ([η]), hydrodynamic radius (Rh) as well as Mark-Houwink parameters (α and logK). UV-vis detector is employed to calculate concentration and in the copolymer composition analysis.
In Triple Detection SEC/GPC, all three detectors are complementary to each other. In combination they can be used for various complex macromolecular studies such as structure and branching analysis, aggregation and crosslinking analysis, conformation and folding analysis, sample purity analysis, copolymer composition analysis and others.