Multiple detection angles in dynamic light scattering analysis

Dynamic light scattering (DLS) is a commonly used technology for analyzing nanoparticle size and distribution. Different DLS-based instruments detect light scattered by nanoparticles at different angles, each of which is suitable for analyzing specific types of samples.

One aspect of DLS-based instrumentation is the angle of analysis at which the instrument measures. Most commercially available instruments only detect light scattered at 90° or 175°.

To understand why, one has to take a step back first: Dynamic light scattering (DLS) is a technique that uses a laser to shine light on particles in suspension and then measures changes in the scattered light coming back out of the sample.

The measurement principle enables particle size measurements because the scattered light creates a ‘speckle pattern’ on the detector that changes over time. The rate at which it changes indicates the speed of the particles in suspension which is then related to their size. Now, the light scattering back out of the sample is scattered in all directions, yet often only a few specific angles are measured.

There are a number of benefits to detecting light at specific angles. The term “back angle” or “back scatter” is used to describe light that is scattered back toward the incident laser beam, often measured at 175°. “Side scatter” is the term used for light scattered 90° perpendicular to the beam and “forward scatter” is used for light scattered at 15°, essentially in the same direction as the beam.

Now, why the differences? One of the assumptions with the DLS technology is that a photon of light is scattered once before it exits the sample and is detected. If a photon is scattered by multiple particles (multiple scattering events) the detector will not be able to accurately correlate the rate of the pattern change with the particle size. So, as discussed previously, for samples at relatively high concentration, a “back angle” or “back scattering” mode is useful because the photon has less sample volume to travel through and is thus less likely to encounter multiple particles and undergo multiple scattering events, when measuring close to the wall of the sample cuvette. A “side scattering” mode is often used for weakly scattering samples that contain smaller particles. Such samples can potentially be difficult to analyze using the back angle because the flare caused when the laser hits the cuvette wall overwhelms the scattering signal from the sample. Side angle measurements successfully avoid such issues and lead to significantly better signal-to-noise ratios.

Forward scattering is useful for samples with a mixture of many small particles and a few larger particles or aggregates. This angle enables the optimal measurement to be able to still detect the smaller, less scattering particles at the same time as the fewer, highly scattering larger particles.

More advanced DLS systems can automatically analyze the sample transmittance, or light traveling through the sample, and initiate a measurement using the optimal angle for that specific sample. Other instruments enable measurements at many angles and thus allow full user control over whatever angle might be of interest.

Finally, some instruments use a lens to actually move the focus position of the laser within the sample, to optimize the measurement. For highly scattering, high concentration samples, back scattering is often used, along with moving the focus position of the laser towards the inner wall of the cuvette closest to the laser, lowering the chance of multiple scattering.

The main question that arises when considering a DLS system is which detection angles the instrument can measure and which ones are most suitable for the sample types in question. Also, it is important to know if the focus position of the laser can be adjusted automatically to optimize the measurement, depending on the sample.

The Litesizer™ 500 from Anton Paar enables particle size analysis at three angles (15°, 90° and 175°) and thus facilitates analyzing the broadest range of particle sample types, sizes and concentrations.