LASER LIGHT SCATTERING

Laser Light Scattering is one of the most important techniques for the characterization of polymers and particles in solution. Based on different physical principles, a variety of Laser Light Scattering techniques has been developed. Some of these Laser Light Scattering techniques are presented in the following listing.

1) Static Laser Light Scattering Techniques
 
Typical molar mass range: ca. 10³ g/mol to ca. 10⁹ g/mol
Typical particle size range: ca. 10 nm to ca. 500 nm

• Low Angle Static Laser Light Scattering – LALS:
  Theoretically the molar mass of macromolecules can be directly calculated using Low Angle Static Laser Light Scattering at 0° scattering angle. But in reality it is not possible to detect the scattered light at a 0° angle. As a compromise a single low angle very close to 0°, mostly 7° or 15°, is selected for the measurement. The inherent problem with Low Angle Static Laser Light Scattering is that the complete calculation is dependent on quality of the data from one scattering angle only. This can be risky, as Low Angle Static Laser Light Scattering typically is more sensitive to impurities and dust causing a noisy detector signal, because it is located in very close proximity to the main laser beam. Also, only the molar mass but no radius can be calculated from Low Angle Static Laser Light Scattering.
   

• Right Angle Static Laser Light Scattering – RALS
  The molar mass of macromolecules also can be directly calculated using Right Angle Laser Light Scattering at 90° scattering angle. But by using Right Angle Static Laser Light Scattering the correct calculation of the molar mass of a macromolecule is only possible in a limited molar mass range up to ca. 100.000 g/mol (depending on polymer). Above this value a significant and increasing angular dependence (=change of scattering intensity with angle) is observed, clearly causing wrong molar mass values when only the 90° angle is used. Another drawback is that no size information can be calculated from Right Angle Static Laser Light Scattering data.
   

• Dual Angle Static Laser Light Scattering – DALS
  To overcome some of the limitations of Low and Right Angle Static Laser Light Scattering, the technique of Dual Angle Static Laser Light Scattering can be used. Dual Angle Static Laser Light Scattering utilizes a low and a right scattering angle. The data of the two angles can be plotted into a so-called “Zimm-Plot” with two data points and a straight line. The slope of this straight line gives the size and the interception of this line with the y-axis gives the molar mass. The limitation by using Dual Angle Static Laser Light Scattering is that for larger macromolecules the Zimm-Plot function is far from being linear. Thus a simple two angle Zimm-Plot using a straight line is becoming meaningless and results in completely wrong molar mass and size values.
   

• Multi Angle Static Laser Light Scattering - MALS
  The only way to overcome the inherent limitations of the Low, Right and Dual Angle Static Laser Light Scattering technique is the use of Multi Angle Static Laser Light Scattering. Molar mass and size can be calculated not only for smaller but also for bigger macromolecules and even nanoparticles. In Multi Angle Static Laser Light Scattering the data of a series of different scattering angles is plotted into the Zimm Plot and the resulting function allows the determination of molar mass (slope of curve at angle 0° angle) and size (intercept value of graph with y-axis). Thus Multi Angle Static Laser Light Scattering is the only light scattering technique capable of providing precise and correct molar mass and size values for macromolecules and nanoparticles.

2) Dynamic Laser Light Scattering Techniques

Typical particle size range: ca. 1 nm to ca. 5000 nm

• Dynamic Laser Light Scattering - DLS: Dynamic Laser Light Scattering is also called Photon Correlation Spectroscopy (PCS) or Quasi Elastic Laser Light Scattering (QELS). Dynamic Laser Light Scattering is primarily used for the determination of particle size of macromolecules and particulates in solution. In Dynamic Laser Light Scattering the fluctuation of the scattered light caused by the Brownian Motion of the molecules, is detected by a photon detector. The collected data is used to establish a correlation function from which the Hydrodynamic Radius can be calculated. Molar mass can only be calculated from the hydrodynamic size when certain assumptions concerning the conformation (sphere, rod, etc.) are made.
   

• Multi Angle Dynamic Laser Light Scattering - DLS: In most cases Dynamic Laser Light Scattering is done at a 90° scattering angle. In some complex mixtures and when high or low sample concentrations are present, also other scattering angles (e.g. back scattering angles) can be used. The most advanced Dynamic Laser Light Scattering technologies work with multiple angles to provide a maximum of information concerning the sample.

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