Size
Both Multi-Angle static Light Scattering (MALS) and Dynamic Light Scattering (DLS) provide non-perturbative, solution-based size measurements based on absolute analyses, independently of calibration standards. These techniques are complementary and orthogonal, analyzing different properties of the scattered light:
I. Size by MALS
MALS examines the angular dependence of the time-averaged scattering intensity to determine the mass-averaged root mean square radius rg (a.k.a. 'radius of gyration') from 10 nm to several hundred nanometers, independent of shape, as described in Classical Light Scattering Theory.
If the sample conforms to specific shape models such as sphere, rod, random coil or coated sphere then the radius or rod half-length can be determined to as much as 1000 nm by a DAWN® HELEOS® II 18-angle MALS detector. The miniDAWN® TREOS® II 3-angle detector characterizes sizes up to 50 nm, independent of shape, and up to 150 nm if the sample conforms to shape-specific models.
MALS determines size via the angular dependence of the scattered intensity.
II. Size by DLS
DLS utilizes the time-dependent fluctuations of scattered intensity, which arise from Brownian motion, in order to determine the diffusion constant. The hydrodynamic radius Rh is then calculated directly as described in DLS Theory. Unlike MALS, DLS does not usually require accurate knowledge of sample concentration but does need accurate values of solvent viscosity and temperature.
DLS determines size via the rate of fluctuation of the scattering signal.
II. Size by Viscometry
For polymers, size determination by viscometry is often more appropriate than DLS. Size is determined from the Einstein-Simha equation relating intrinsic viscosity to size and molar mass. Viscometry is more sensitive than DLS to small macromolecules and hence is the go-to technique for polymers and peptides below 10 nm in rg.
