The Stokes-Einstein relation (1) allows us to calculate the hydrodynamic radius Rh (lower limit ~ 0.5 nm and upper limit ~ 500 nm) from the translational diffusion constant Dt. The solvent dynamic viscosity and temperature must be known. Rh depends only on the physical size and size-related behavior (diffusion, viscosity) of the molecule. Information about concentration or refractive index increment is not necessary for a DLS experiment. DLS is highly sensitive to a small amount of large aggregates (including contamination due to dust etc.).įor a mono-disperse particle size distribution, the normalized intensity autocorrelation function is described by simple exponential function with a decay. For a poly-disperse particle size distribution, the normalized intensity autocorrelation function is described by a sum of the autocorrelation functions of all particle species, weighted by their normalized intensities.ġ. ![]() Fitting the data to a simple exponential function, as is done in Dynamic Astra, does not yield as average of Rh, and in fact will always yield a resulted weighted toward low Rh.Ģ. The method of the cumulants analysis assumes a distribution of diffusion rates, and determinate a mean and width of the diffusion rates. Those values are then inverted to report the corresponding Rh values. The values reported do not correspond to, and are quite different from, the mean and Gaussian width of Rh.ģ. In regularization, it is assumed that the underlying distribution of Rh is smooth. The software determines a number of Rh distributions which all fit the data equally well, and chooses between them based upon the smoothness of the distribution, favoring smooth distributions over spiked distributions. DYNAMIC LIGHT SCATTERING SIZE DISTRIBUTION BY NUMBER SOFTWARE ![]() This is often represented as a histogram. Regularization fits a set of exponentials to obtain the real distribution of Rh. Regularization works well to determine the width of very broad distribution, and can show the presence of a few discrete species. For regularization to see the species as distinct, however, the species must have Rh which different from each other by more than a factor of 5. Any noise in the autocorrelation function will result in an artificial broadening of the peaks, and so even mono- disperse species will appear broadened, as in the data above. ![]() DYNAMIC LIGHT SCATTERING SIZE DISTRIBUTION BY NUMBER SOFTWAREĭiameter of a sphere with the same translational diffusion coefficient D as the particle in the same fluid under the same conditions, defined via the Stokes-Einstein equation.The final particle size distribution is obtained from the sum of diffraction patterns produced by the particles randomly oriented along the direction of the laser beam.angle (diffraction pattern) is a function of the particle size The diffracted/scattered intensity vs.The movement of dispersed particles causes intensity fluctuations of scattered light, which are measured and converted to particle size and particle size distribution (Stokes-Einstein equation).The velocity of particle movement is a function of the particle size.Regularization is incapable of resolving dimers or trimmers from monomers.
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