There are many different analytical methods that can be classed under the topic of light scattering and there are a variety of parameters that can be measured.
Further to this, some systems offer more than one technique in a single instrument. Hence, selecting an appropriate light scattering technology suitable for a specific application can be a challenging task.
Static light scattering, also known as classical light scattering, is a method employed for measuring molecular weight and molecular radius of gyration. There are many different technologies available to implement this technique, including SLS, RALS, LALS, and MALS. Each of these technologies is slightly different and has their own advantages and disadvantages.
Light Scattering
During the collision of a photon with a molecule, some amount of the photon energy is utilized for initiating what is called an oscillating dipole within the molecule.
The molecule subsequently re-emits this energy in all directions as light. A number of properties associated with the molecule can be measured by using the principles behind this light scattering.
Molecular Weight
An individual molecule’s mass is described as its molecular weight, which is defined as the mass of a material needed to create one mole of the material. The molecular weight of a material is expressed in grams per mole (g/mol). A pure protein sample is expected to have a fixed molecular weight, with all of its molecules required to have the same molecular weight.
However, the molecular weight of the molecules of a natural or synthetic polymer will have a distribution with many different forms. The overall width of the distribution is called the ‘polydispersity.’ Most samples consist of a range of molecular weights, especially natural and synthetic polymers. These molecular weight distributions can be expressed in many ways, and one common method is expressing in terms of the molecular weight moments Mn (number-averaged molecular weight), Mw (weight-averaged molecular weight) and Mz (z-averaged molecular weight).
Properties such as osmotic pressure, freezing point depression, and vapor pressure lowering are based on the count of molecules rather than their size. The average molecular weight is the product of the mass of the molecules, so the average is bias towards the larger molecules in the distribution. Properties such as light scattering, sedimentation, and diffusion rely on both the mass and size of the molecules.
Using Mn, Mw and Mz, the entire molecular weight distribution of a sample can be predicted. Polydispersity is lower if these values are closer to each other, and is higher if they are further apart. It is defined as follows:
Pd=Mw/Mn
From the equation, there is no theoretical upper limit for polydisperisty, but the lowest possible value is 1.
Molecular Size
Molecular size is a molecule’s physical size, typically expressed as a single value in terms of the radius of a sphere, having a size equivalent to the molecule under study. Hydrodynamic radius (Rh) and radius of gyration (Rg) are the two most commonly used values of molecular size.
Rh is the radius of an equivalent sphere that diffuses at the same rate as the molecule being measured. It is computed either from intrinsic viscosity or from dynamic light scattering. Rg is the root-mean-square of the radii from the centre of mass to the various mass cores within the molecule, and is measured by means of static light scattering.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
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