how repeatable is the result derived from sequential measurements of the same sample aliquot. When establishing the level of confidence in the size distribution results derived from a dynamic light scattering measurement, DLS users should always first consider the repeatability of the measurement, i.e. Correlation is a statistical method for measuring the degree of non-randomness in an apparently random data set. For particles moving under the influence of Brownian motion, the measured scattering intensity will fluctuate with time. For a collection of solution particles illuminated by a monochromatic light source, the scattering intensity measured by a detector will be dependent upon the relative positions of the particles within the scattering volume. For accurate results, a reusable glass or quartz cuvette must be used and purchased by the user.Dynamic light scattering (DLS), also known as photon correlation spectroscopy (PCS) and quasi-elastic light scattering (QELS), is a technique used to measure the Brownian motion (diffusion) and subsequent size distribution of an ensemble collection of particles in solution. The y-intercept of the plot is equal to the inverse molecular weight of the sample. Different sample concentrations are sequentially measured and a Debye plot is created from the results according to the Rayleigh scattering theory. This technique is used specifically to measure the molecular weight of polymers/molecules in solution. The Zetasizer accessory for surface zeta potential is available for use but must be requested by users as specific training is required. Thus by measuring this effect, the zeta potential of the film can be measured. This is known as the electroosmotic effect and is related to the charge on the film. However, when a charged surface is introduced (film, membrane, etc.), the particles close to that surface will have a different velocity than those far away and a velocity gradient is formed. Similar to standard zeta potential, particles in a suspension are subjected to an alternating electrical field and their velocity is measured. It is a convenient solution for isoelectric point measurements (pH at which zeta potential = 0).
This accessory is available for the Zetasizer and automates the measurement of both particle size and zeta potential as a function of pH, dilution or additive concentration. A specialized capillary cell is necessary for this measurement in the Zetasizer and can be purchased in the MRL storeroom. Analysis of the superposition of incident and scattered beams, called Laser Doppler Velocimetry, enables the calculation of the zeta potential. As a result, the light scattered by the particles exits at a different frequency to the incident beam according to their velocity. The experiment is performed by passing an alternating electric field through the suspension and measuring the change in velocity of the particles, called electrophoresis. It is also related to the stability of the particles in the suspension. Zeta potential is a property of particles in a suspension that relates to the charge on their surface. The Zetasizer can measure particle diameters in the range of 0.6 nm and 6 µm. A forward scattering angle of 13° is also available for particle size measurement. To accomplish this, the Zetasizer measures the scattering of the particles as they cross the incident beam of light at a backscattered angle of 175 °. From this the size (diameter or radius) of the particles can be determined. DLS measures the diffusion coefficient of particles undergoing Brownian motion in a fluid.