![]() It is defined as "the size of a hypothetical hard sphere that diffuses in the same fashion as that of the particle being measured". The hydrodynamic size is determined by Dynamic Light Scattering (DLS). This translational diffusion coefficient will depend on the size of the particle "core" and on any surface structure, as well as the type and concentration of ions in the medium. Therefore, the diffusion coefficient defines this Brownian motion of the particle or analyte in that particular solvent environment. Illuminating the molecules or particles with a laser will cause the scattered light intensity to fluctuate at a rate based upon the size of the particles, because smaller particles are "kicked" further by the solvent molecules and they move more quickly.Īnalysis of these intensity fluctuation produces the velocity of the Brownian motion and so the particle size using the Stokes-Einstein equation. ![]() Molecules and particles in suspension/solution undergo Brownian motion, which is produced by the bombardment by solvent molecules that move due to their thermal energy. With regard to a protein analysis, a % polydispersity below 20% points out that the sample is monodisperse. During the Cumulants analysis, a single particle size method is assumed and a single exponential fit is used to autocorrelation function, and the width of the assumed Gaussian distribution is described by the polydispersity. The term polydispersity and % polydispersity in light scattering are derived from the Polydispersity Index, a parameter calculated from a Cumulants analysis of the DLS-measured intensity autocorrelation function. The calculations for these types of parameters are defined in ISO 22412:2008 and the ISO standard document 13321:1996 E. The different size distribution algorithms function with data that falls between these two limits. However, values more than 0.7 show that the sample has an extremely broad size distribution and is perhaps not appropriate for the dynamic light scattering (DLS) technique. This index is dimensionless and scaled in such a way that values smaller than 0.05 are hardly seen other than with monodisperse standards. The Polydispersity Index is a number analyzed from a simple 2 parameter fit to the correlation data (the cumulants analysis). The calculation is defined in the ISO standards therefore, all systems using this calculation as suggested, should provide similar results if the same scattering angle is used. Considered as an Intensity-based calculated value, the Z-Average should not be confused with or compared directly to a Mass or Number mean value generated by other techniques. ![]() Since it is a moments expansion, it can generate a number of values however, in practice, only the first two terms are used - a width parameter called the Polydispersity Index (PdI) and a mean value for the size (Z-Average). The calculation is defined in ISO 22412 and ISO 13321. This is a simple technique for analyzing the autocorrelation function produced by a DLS test. It must to be noted that the Z-average mean is a hydrodynamic parameter and hence relevant to molecules in a solution and particles in a dispersion. only one peak), and the sample is prepared in an appropriate dispersant, because the Z-Average mean size can be susceptible to even slight changes in the sample, for example the presence of a small amount of aggregates. very narrow width of distribution), near-spherical or spherical in shape, monomodal (i.e. The Z-average size can be compared with the size measured by other methods only if the sample is monodisperse (i.e. ![]() When used in a quality control setting, the Z-Average mean is the best value to report as it is described in ISO 13321 and more recently ISO 22412 which defines the Z-Average mean as the 'harmonic intensity averaged particle diameter'. It is the most stable and primary parameter produced by the method. The Z-Average mean or Z-Average size is a parameter used in dynamic light scattering and is also known as the cumulants mean. This article provides a descriptive definition with notes on their particular use in the context of Dynamic Light Scattering. ![]() However, these will not generally provide help in understanding their application in the practical use of the method. There are various sources of information that provide a mathematical explanation of the terms used in light scattering. Sponsored by Malvern Panalytical Feb 6 2018 ![]()
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