Why Use Multi-Angle Light Scattering (MALS)?



The addition of a MALS detector coupled downstream to a chromatographic system allows the utility of SEC or similar separation combined with the advantage of an absolute detection method. The light scattering data is purely dependent on the light scattering signal times the concentration; the elution time is irrelevant and the separation can be changed for different samples without recalibration. In addition, a non-size separation method such as HPLC or IC can also be used. As the light scattering detector is mass dependent, it becomes more sensitive as the molar mass increases. Thus it is an excellent tool for detecting aggregation. The higher the aggregation number, the more sensitive the detector becomes.

 

As mentioned above, the MALS detector can also provide information about the size of the molecule. This information is the Root Mean Square radius of the molecule (RMS or Rg). This is different from the Hydrodynamic Radius (Rh), in that it is not affected by the hydration layer and is purely the root mean square of all the radii making up the molecule multiplied by the mass at that radius. This would seem like a strange parameter to measure but in fact this is very useful as it is sensitive to changes in shape of the molecule. If you consider adding a side group to a large molecule (branching), this would hardly affect the hydrodynamic radius (Rh) but would have a significant impact on the Radius of Gyration (Rg). In addition, if you plot the Log Rg vs. Log M then the conformation of the molecule can be derived. If there are any shape changes which are associated with changes in molar mass, they will be identified.

 

Multi-angle light scattering coupled to SEC or other fractionation methods can provide an absolute means for measuring the molar mass, size, and distribution of polymers of all sorts. Wyatt Technology’s MALS also enables the elucidation of additional data such as branching, conformation, and eluent behaviour. The same instrumentation also allows observation of molecular interactions in a real-time environment. As MALS is an absolute mass and size measuring technique it is insensitive to the separation mechanism and as such is ideal for both highly variable research applications as well as routine quality assurance of previously characterised samples.