The molecular size or hydrodynamic radius RH can be determined in two ways. The first method is by Dynamic Light Scattering (DLS), which is generally used as a batch technique to measure the average size in the whole sample, but can be used as a detector for Size Exclusion Chromatography (GPC/SEC).
When measuring molecular size a very accurate and precise method of obtaining RH of all components in a sample is by triple detection after separation by SEC.
By the use of online light scattering and viscometer detectors, the MW and intrinsic viscosity of the polymer or protein can be determined and hence the RH calculated at any point on the chromatogram.
The benefit of both these techniques is that there is no practical upper or lower size limit for the materials being investigated.
The measurement of molecular size is a useful parameter for a number of applications:
Comparison with molecular weight gives information about structure
DLS can use molecular size to give a quick estimate of molecular weight