Multi-angle light scattering (MALS) is a technique for determining, independently, the absolute molar mass and the average size of particles in solution, by detecting how they scatter light. Collimated light from a laser source is most often used, in which case the technique was referred to as multiangle laser light scattering (MALLS). The insertion of the word “laser” was intended to reassure those used to making light scattering measurements with conventional light sources such as Hg-arc lamps that low angle measurements could now be made. Until the advent of lasers and their associated fine beams of very narrow width, the width of conventional light beams used to make such measurements prevented data collection at smaller scattering angles. In recent years, since all commercial light scattering instrumentation use laser sources, this need to mention the light source has been dropped and the term MALS used throughout.
The "multi-angle" term refers to the detection of scattered light at different discrete angles as measured, for example, by a single detector moved over a range that includes the particular angles selected or an array of detectors fixed at specific angular locations. A discussion of the physical phenomenon related to this static light scattering, including some applications, data analysis methods and graphical representations associated therewith are presented.
Associated with a MALS measurement are a variety of ancillary elements. Most important among them is a collimated or focused light source (nowadays, usually a laser) producing a fine beam of monochromatic light to illuminate a region of the sample. In modern instruments, the beam is generally polarized, though other polarizations may be used especially when studying anisotropic particles. Early work in the field was performed with unpolarized sources such as Hg-arc lamps. Essential is an optical cell containing the sample being measured. Alternatively manifold elements can be used for the case of a flowing system. If single-particles scattering properties in air are to be measured, a means to introduce such particles one-at-a-time through the light beam at a point generally equidistant from the surrounding detectors must be provided.
Although most MALS-based measurements are performed in a plane containing a set of detectors usually equidistantly placed from a centrally located sample through which the illuminating beam passes, three-dimensional versions also have been developed wherein the detectors lie on the surface of a sphere with the sample controlled to pass through its center where it intersects the path of the incident light beam passing along a diameter of the sphere. The former framework is used for measuring aerosol particles while the latter was used to examine marine organisms such as phytoplankton.