Static Light Scattering: Zimm Plot for Mw Choose a low angle if the sample is very clean and you want the effect of extrapolating to zero angle as shown below. Choose a middle angle if you want to verify such molecules exist, or not, in the sample. Thus, choose a back scatter device if you are sure you have no need to know about any larger molecules. Small angles maximize the effect of these larger scatters. Back angles minimize the effect of any residual and unwanted larger particles (dust, large aggregates or impurities). Therefore, any experimentally convenient angle is sufficient. A point has no dimensions and size cannot be determined this way. As long as the radius of gyration, R g (a z-average if there is a distribution of chain lengths), is less than about 12 nm, the polymer or protein acts like a point dipole when it comes to scattering the electric field of the light source. That was the year Debye made the first measurements of the weight-average molecular weight, MW, of small polymers. Static Light Scattering: Peter Debye 1944 The author will take an historic approach starting with SLS, DLS and finally the ELS/PALS techniques. The purpose of this introduction is to answer these questions and reduce the amount of time it takes to decide on what technique to use in what situation. When should ELS be used and when should PALS be used in characterizing the stability of a suspension with electrostatic repulsion? And if a single angle is used, what are the advantages and disadvantages of back scatter versus ninety degrees or a forward angle. Questions arise about what is meant by “absolute” molecular weight and should one, two or multiple angle measurements be made. While the number of useful properties is large, so are the various configurations. Then there are the indirect, relative or empirical relationships that allow, for example, a molecular weight calculation from a DLS determination of R h (hydrodynamic radius), the technique at the heart of ASEC (absolute size exclusion chromatography). With an appropriate sample and the right instrument configuration, SLS can be used to determine the fractal dimension, DLS can be used to determine rotational diffusion, and combinations of the properties determined by DLS and SLS can be used to determine simple shapes. And these are just the routine measurements. The large number of properties that can be determined using various forms of light scattering is impressive: molecular weight, radius of gyration, and second virial coefficient using static light scattering (SLS) on synthetic polymers and natural biopolymers like proteins diffusion coefficient, hydrodynamic radius, and size distribution using dynamic light scattering (DLS) on polymers, proteins, and other nanoparticles electrophoretic mobility and zeta potential using electrophoretic light scattering (ELS) or phase analysis light scattering (PALS) on nanoparticles and other colloids.
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