Zeta potential quantifies the repulsion between particles in a suspension aiding stability studies.Īnother essential requirement for precise DLS measurements is good temperature control. Detectors can be placed at either 90 o or at a wider angle as exemplified by the NIBS detector shown at here at 173 o.įigure 2.
MALVERN DYNAMIC LIGHT SCATTERING PC
The key elements of a DLS system are the laser (1), measurement cell (2), detector (3), attenuator (4), correlator (5) and data handling PC (6).
The detector(s)- Detectors are one of two types: cheaper, less sensitive photomultipliers or more expensive, higher performance avalanche photodiode detectors (APD).įigure 1. For experiments that require higher sensitivity levels or for more concentrated samples, measuring at wider angles is preferable. The optical arrangement – Measuring only at 90 o enables the production of a simple, cost-effective system that provides an appropriate level of sensitivity for many applications as shown in Figure 1. The laser - A stable laser with low noise characteristics, as exemplified by certain Helium-Neon gas lasers, is the most suitable. Furthermore, light absorption by the dispersant can interfere with detection Dispersant choice – Even though most dispersants are suitable, those with a viscosity greater than 100 mPa.s inhibit reliable measurements. Multiple light scattering - Multiple scattering is when the scattered light from one particle is scattered by another before reaching the detector, and it compromises the accurate calculation of particle size in more concentrated samples. Low resolution - When size populations are closely spaced, less than a factor of three difference in size, DLS will not precisely characterize a polydisperse sample, making it advisable to apply separation prior to measurement. Increasing the density of the dispersant by, for example, introducing sucrose is a helpful strategy but only for density matching up to around 1.05 g/ml. Sedimentation - This is particularly likely with more dense particles. Large particles if present in even small quantities may be accounted during data analysis. This size and concentration range, together with the high reproducibility of the technique makes DLS suitable for a wide range of applications. It is possible to measure both dilute and turbid systems with the concentration range for analysis reaching down as low as 0.1ppm and up to 40% w/v. Its ability to measure small particles is especially valuable, with most systems giving accurate, reproducible data for particles from 2nm and above. DLS is essentially very good at measuring particle size across the range ~ 0.1nm to ~ 10µm. Sample volumes are small, down to just a few microliters, making this an appealing technique for early stage research where valuable materials are involved. Operators can attain usable, detailed data without needing to have significant expertise. This relationship shows how size can be determined from diffusion speed provided that the temperature and continuous phase viscosity of the sample are known. Where D = Diffusion speed, k = Boltzmann’s constant, T = absolute temperature, η = viscosity, and D H = hydrodynamic radius. The relationship between the speed of Brownian motion of a particle and that particle’s size is defined by the Stokes-Einstein equation: The speed of Brownian motion can be directly measured from the scattered light pattern produced by the moving particles, a technique known as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS) but presently referred to as DLS. DLS is driven by collisions with the solvent molecules present, which are in constant movement due to their thermal energy. Small particles in a dispersion or solution are subject to Brownian motion. This article examples the benefits and limitations of DLS, focuses on those aspects of system design that are crucial in defining performance. The observation of scattered light helps determine defining characteristics of a particle dispersion or molecular solution such as particle size, molecular weight and zeta potential. Sponsored by Malvern Panalytical Oct 18 2013ĭynamic light scattering (DLS) is a valued sizing technique for proteins, colloids and dispersions, which comfortably extends to the sub 1 nm region.
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