There are various well-established methods of carrying out particle analysis including dynamic image analysis (DIS), static light scattering (SLS), and sieve analysis—sometimes called gradation testing. These methodologies can often be complementary, each providing information on particles on different size ranges. Image analysis is suitable for larger particles while laser light scattering is ideal for particles of sub-micrometer (μm) dimensions. SLS, for example, can reliably size particles as small as 0.01 μm.
Dynamic light scattering (DLS) is the preferred analytical method for screening samples with extremely small particles, covering everything from a few micrometers down to an individual nanometer (nm).
Here we will discuss some of the basics of particle analysis via light scattering, and why DLS excels at providing a full particle size distribution.
Particle Analysis via Light Scattering: What’s the Angle?
Light scattering is often referred to as laser diffraction, which can be broken down into Fraunhofer diffraction or Mie scattering. These formulae are used to model the size of particles based on the extent to which they diffract an incident laser beam. When carrying out particle analysis via SLS, the particulate media is dispersed in a mobile phase and a laser beam is passed through the solution. Particles within the dispersion scatter the light at angles relative to their size (i.e. large particles have small scattering angles relative to the beam and vice versa).
Particle size analysis is subsequently carried out by measuring the variation in scattered light intensity as a function of the scattering angle. However, SLS focuses almost exclusively on intensity versus angle and does not account for particle motion which may cause variations in intensity.
Why Use DLS for Particle Analysis?
SLS is an excellent tool for working out key particle analysis parameters such as average particle size and particle size distribution (PSD) on the sub-micrometer to millimeter range. However, DLS is often preferred for dispersions where tiny Van der Waals forces can affect the intensity of scattered light, or where additional particle analysis parameters like sample structure must be considered.
DLS is useful for characterizing the size of a wide range of different particles, including carbohydrates, micelles, nanoparticles, polymers, and proteins. The difference between SLS and DLS is that the dynamic method accounts for the Brownian motion of dispersed particles to calculate their hydrodynamic radii, which provides insight into conformation and mass. Hence the unique capabilities for DLS to carry out particle analysis down to the low nanometer range.
DLS is a non-invasive technique which can be carried out at a fixed angle or at multiple angles. The latter is preferrable for particle analysis in the nanometer size range, but either technique is suitable for assessing samples in their native state, providing truly representative data.
Particle Analysis with RQM+ Lab Services
Interested in carrying out particle analysis via DLS? The RQM+ Lab Services team is well-versed in assessing emulsions and dispersions using both static- and multi-angle DLS. Contact us today to learn more.