Any particle struck by light will deviate its energy in all directions with an intensity depending on the direction. This phenomenon is referred to as scattering. The intensity of light passing through a sample containing particles (e.g. aggregates) will decrease as the light encounters each particle and gets scattered. The Prometheus Panta system measures the scattered light using the DLS optics and the turbidity using the backreflection optics. Both methods are helpful to understand a sample's aggregation behavior.

Light scattering techniques enable fast and non-invasive measurements of important molecular properties in solution, such as the translational diffusion coefficient via dynamic light scattering or the molar mass and the radius of gyration via static light scattering.

There are different types of scattering. The main source of scattering for molecules with a diameter less than 1/10 the wavelength of the incident light is Rayleigh scattering (<40 nm for l=405 nm in Prometheus Panta). Rayleigh scattering is isotropic, which means that particles scatter light equally in all directions in the plane perpendicular to the polarization of the incident light. Furthermore, the amount of scattering is proportional to , which means that light with shorter wavelength is scattered more strongly. This is why the sky appears blue during daylight. Particles between 1/10 the wavelength of light and approximately equal to the wavelength of light induce Mie scattering, which occurs more strongly in the forward direction and is largely independent of the wavelength. This is why clouds, which are composed of water droplets around the size of the wavelength of visible light, appear white or grey.

Figure: Rayleigh scattering is isotropic, while Mie scattering shows angular dependence.

The simplified measurement setup of a light scattering experiment is shown in the image below. Due to the difference between Rayleigh scattering and Mie scattering, detectors at 90° or less are more sensitive to minute amounts of large aggregates or dust particles, while detectors in backscattering orientations with large angles are more sensitive to small particles in the size range of single proteins.

Figure: Schematic overview of a light scattering measurement setup. The incident laser beam with wavelength l is focused into a small area, the scattering volume. Particles in the sample cell struck by light will scatter light in all directions, but only a portion of the scattered light (black arrow with straight line) is collected by a detector positioned at angle q relative to the incident light beam.