Dynamic Light Scattering The Beckman Coulter Delsa Nano C is a combined particle size and zeta potential analyser with the facility to titrate pH and record zeta potentials of both suspensions and solid phase samples. Principle of Operation Size Measurement Particles suspended in liquids are in Brownian motion due to random collisions with solvent molecules. This motion causes the particles to diffuse through the medium. The diffusion coefficient, D, is inversely proportional to the particle size according to the Stokes-Einstein equation: d T k B 0 3 πη where D is the diffusion coefficient, k B is Boltzmann’s constant, T is absolute temperature, η 0 is viscosity and d is the hydrodynamic diameter When laser light is directed onto the particles, light is scattered in all directions. The scattered light that is observed comes from a collection of scattering elements within a scattering volume that is defined by the scattering angle and detection apertures. The observed intensity of the scattered light at any instant will be a result of the interference of light scattered by each element; and thus, will depend on the relative positions of the elements. If the particles are in motion, the relative positions of particles will change in time; and thus, fluctuations in time of the scattered light intensity will be observed. Because particles in Brownian motion move about randomly, the scattered intensity fluctuations are random. The fluctuations will occur rapidly for smaller, faster moving particles and more slowly for larger, slower moving particles. The fluctuations of the scattered light are recorded and then analyzed using the autocorrelation function to give the size (or distribution of size) of the particles in the sample. Zeta Measurement When an electric field is applied to charged particles in the suspension, particles move toward an electrode opposite to its surface charge. Because the velocity is proportional to the amount of charge of the particles, zeta potential can be estimated by measuring the velocity of the particles. Electrophoretic light scattering is the method most generally used to determine the velocity of the particles. To determine the speed of the particles movement, the particles are irradiated with a laser light, and the scattered light emitted from the particles is detected. Because the frequency of the scattered light is shifted from the incident light in proportion to the speed of the particles movement, the electrophoretic mobility of the particles can be measured.