Low-angle laser light scattering
Encyclopedia
Low-angle laser light scattering or LALLS is an application of light scattering
that is particularly useful in conjunction with the technique of Size exclusion chromatography
, one of the most powerful and widely-used techniques to study the molecular weight distribution of a polymer.
Typically the eluent of the SEC column is allowed to pass through both a refractive index
detector (that gives measures for the concentration in the solution as a function time) and through a laser scattering cell. The scattered intensity is measured as a function of time under a small angle with respect to the laser beam. The low-angle light scattering data can be analyzed if one assumes that the low-angle data is the same as the scattering at zero angle. For the relevant equations, see the article on static light scattering
. Under these conditions the laser signal together with the concentration data can be translated into a curve that yields both the Mn and the Mw, the molar mass weighted by number and by weight respectively. The combination of those two data gives information on the linearity of the polymer.
The technique is sometimes complemented or combined with viscometry and polystyrene standards are available for validation of the results.
Light scattering
Light scattering is a form of scattering in which light is the form of propagating energy which is scattered. Light scattering can be thought of as the deflection of a ray from a straight path, for example by irregularities in the propagation medium, particles, or in the interface between two media...
that is particularly useful in conjunction with the technique of Size exclusion chromatography
Size exclusion chromatography
Size-exclusion chromatography is a chromatographic method in which molecules in solution are separated by their size, and in some cases molecular weight . It is usually applied to large molecules or macromolecular complexes such as proteins and industrial polymers...
, one of the most powerful and widely-used techniques to study the molecular weight distribution of a polymer.
Typically the eluent of the SEC column is allowed to pass through both a refractive index
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
detector (that gives measures for the concentration in the solution as a function time) and through a laser scattering cell. The scattered intensity is measured as a function of time under a small angle with respect to the laser beam. The low-angle light scattering data can be analyzed if one assumes that the low-angle data is the same as the scattering at zero angle. For the relevant equations, see the article on static light scattering
Static light scattering
Static light scattering is a technique in physical chemistry that measures the intensity of the scattered light to obtain the average molecular weight Mw of a macromolecule like a polymer or a protein. Measurement of the scattering intensity at many angles allows calculation of the root mean square...
. Under these conditions the laser signal together with the concentration data can be translated into a curve that yields both the Mn and the Mw, the molar mass weighted by number and by weight respectively. The combination of those two data gives information on the linearity of the polymer.
The technique is sometimes complemented or combined with viscometry and polystyrene standards are available for validation of the results.