Dynamic light scattering detector
When the particles are comparable in size or bigger than the laser light wavelength (like some viruses) we enter the realm of Mie scattering, named after Gustav Mie, the pioneer of study of this kind of scattering in the early years of the 20th century. When the particles are very small compared to the laser light's micron-sized wavelength (like small proteins), we are using Rayleigh scattering-well known in optics, in which the light is scattered in all directions in the horizontal plane to an equal amount, such as you can see in Figure 1 and in the 0.1 µm particle diameter line in Figure 2.įigure 1: Isotropic scattering for particle diameters much smaller than the laser wavelength (from PSI courseware).įigure 2: Scattering intensity as a function of scattering angle for different particle diameters (from PSI courseware). In the DynaPro, polarized laser light, vibrating up and down, shines onto the little particles. It is these particles that the DynaPro can measure, giving their size and molecular weight, for example.
In Dynamic Light Scattering, the little particles are usually any size from around 1 nanometer to around 3 micrometers in size, covering many biological materials such as proteins and viruses. The DynaPro analyzes these signal frequencies with the utmost sensitivity, measuring the changes in signal frequency using just single photons of scattered light from the particles about which we wish to know. This is a dynamic process-so we are talking about Dynamic Light Scattering. The faster the particles move around (for example, smaller particles dance faster than larger particles) the faster the signals changes, so we have a frequency-dependent measure of the particles' size. As the particles are dancing, so the signal strength dances around, changing continuously in strength. At some distance from the particles, all of these little light amplitudes and phases overlap and add up, wherever we choose to detect the light, and this gives rise to the signal 'voltage' that we get from our detector.īecause each little particle is dancing around in the liquid, being pushed by random collisions with the liquid's atoms in a process called Brownian motion, the amplitudes and phases from each scattering particle keep adding up to a different sum at the detector, depending on the relative phases (spatial positions) of the dancing particles. Some of the light bounces off these particles, each particle sending off a small amount of scattered light that has an amplitude and phase. In Dynamic Light Scattering, visible light, which has a wavelength of around 0.5 microns in size, impacts the little particles we want to know about while they are swimming around in large numbers suspended in a liquid.
Brown, Technical Advisor to Protein Solutions Inc. (Charlottesville, VA), explains the basics of light scattering.īy Robert G.W. Brown, inventor of the DynaPro, a molecular sizing instrument offered by Protein Solutions Inc. DLS also allows study of aggregation phenomena and conformational changes as a function of temperature and physiological and non-physiological solutions. It requires very little sample-as low as 12 microlitres-and with it, one can study size and conformational changes of proteins, polysaccharides, and other supermolecular assemblies. While molecular biologists have various tools at hand to look at protein size, shape, and the behavior alone or in combination with other proteins, these tools can be cumbersome and difficult to use.ĭynamic Light Scattering (DLS) is a non-invasive, non-perturbing technique for measuring very small changes in size and conformation of macromolecules. As we move from the genomics era to the proteomics era, large numbers of proteins will be crying out for characterization.