Dynamic light scattering (DLS) is a noninvasive technique used for characterizing macromolecules in solution and particles in suspension. The technique measures the timedependent fluctuations in the intensity of scattered light that occur because the particles are undergoing Brownian motion. The velocity of this Brownian motion is measured and is called the translational diffusion coefficient D. This diffusion coefficient can be converted into a hydrodynamic diameter (DH) using the StokesEinstein equation.
DLS is inherently a lowresolution technique with limited capability of resolving different sized particles in a distribution. However, distribution algorithms, such as NonNegative Least Squares (NNLS) and CONTIN, do have the ability to distinguish between different size populations in a sample. Whether this is possible in a particular sample is determined by a number of factors including:
As a test of the resolving ability of an algorithm, measurements can be done using mixtures of monosized latex standards prepared at different relative concentrations. This application note details measurements performed on a series of samples where the relative concentrations of different size latex standards were varied.
Measurements were made on mixtures of latex standards at neat concentration on a Zetasizer Nano S (173°).
All latex standards used for the measurements discussed in this application note were obtained from Duke Scientific, Palo Alto, California, now Thermo scientific, Fremont CA, and were traceable to NIST, Gaithersburg, Maryland. Nanosphere™ size standards from the 3000 series were used. Specifically, these were the 60nm (catalogue number 3060A), the 220nm (3220A) and 900nm (3900A) standards respectively. All of these standards are supplied at 1% w/v concentration.
The analysis model used in the Zetasizer Nano software was Multiple Narrow Modes.
The Zetasizer Nano S incorporates noninvasive backscatter optics (NIBS) that allows for the measurement of concentrated samples. Therefore, measurements of mixtures of 60 and 220nm latex standards at different ratios were made at neat concentrations of 1% w/v. Table 1 summarizes the results obtained for these measurements and contains the ratios of the 60 and 220nm latex mixtures, the zaverage diameters, the polydispersity index values, the peak analysis of the intensity and volume size distributions obtained (peak modes and percentages). Conversions of the measured intensity size distributions into volume were done using Mie theory with particle refractive index values of 1.59 and particle absorption values of 0.01. The corresponding intensity and volume size distributions obtained for the various mixtures are shown in figures 1 and 2 respectively.







The zaverage diameters obtained from the latex mixtures decreases as the concentration of 60nm latex in each mixture is increased. Conversely, the polydispersity index values increase as the percentage of 60nm latex contained in each sample increases.
The measured volume ratios are in good agreement with the actual volume ratios of the latex mixtures.
Even though dynamic light scattering is a lowresolution technique, the results presented in this application note have shown that not only can different size particles be resolved, but changes in the relative concentrations of each size population can be monitored.
The noninvasive back scatter (NIBS) optics incorporated in the Zetasizer Nano S allows for measurements to be made on concentrated, turbid samples.
[1] Dynamic Light Scattering: An Introduction in 30 Minutes, Technical Note
[2] International Organisation for Standardisation, International Standard ISO22412:2008, Particle size analysis, Dynamic Light Scattering (DLS)
[3] Dahneke, B.E. (ed) Measurement of Suspended Particles by Quasielastic Light Scattering, Wiley, 1983.
[4] Pecora, R. Dynamic Light Scattering: Applications of Photon Correlation Spectroscopy, Plenum Press, 1985.
[5] M. Kaszuba, M.T. Connah, F. McNeilWatson and U. Nobbmann (2007) Particle and Particle Systems Characterization 24, 159162.