Static light scattering (SLS) is a technique to measure absolute molecular weight using the relationship between the intensity of light scattered by a molecule and its molecular weight and size, as described by the Rayleigh theory. In the simplest terms, Rayleigh theory says that larger molecules scatter more light than smaller molecules from a given light source and that the intensity of the scattered light is proportional to the molecule’s molecular weight.

There are two ways to measure absolute molecular weight by static light scattering:

  1. Batch measurement using a cuvette
  2. In combination with a chromatography instrument.

Batch measurement with cuvette based instruments, such as the Zetasizer series, is an ensemble technique. Therefore the result calculated is the weight average molecular weight of the entire sample measured.

However, the most common way of measuring absolute molecular weight is to add an SLS detector e.g. Low Angle Light Scattering LALS, Right Angle Light Scattering RALS or Multi Angle Light Scattering MALS to a GPC/SEC system. By combining SLS with the separation technique you can calculate the absolute molecular weight at any point in the eluting chromatogram and determining the molecular weight of any population in a mixed sample becomes possible.