Dynamic light scattering (DLS) is used in many industries for several applications, including for the analysis of organic macromolecules, colloids, polymers, glues, metals, cosmetics, foodstuff and beverages, pharmaceuticals, ceramics, microemulsions, inks, and pigments. Although the technique can deliver faster results, it requires ultra-low concentrations of particles, which is a major limitation. This article discusses (DLS) further and the use of this method for particle analysis.
Image Credit: Login/Shutterstock.com
What is DLS?
DLS is a non-invasive method used to measure the average size and size distribution broadness of submicron particles in a suspension/dispersed in a liquid. DLS is suitable for particle size analysis of dispersed systems, such as liposome formulations and suspensions, as these systems consist of randomly moving particles smaller than one µm.
The method involves the determination of the Brownian motion/diffusion coefficient of particles in the liquid and a hydrodynamic particle diameter through the Stokes-Einstein equation. The viscosity and temperature of the test sample must also be obtained for the evaluation.
The DLS measurement range ranges from 0.3 nm to 10 µm, which overlaps with the laser diffraction measuring range from 10 nm up to the millimeter range. However, the DLS method becomes more effective compared to laser diffraction with the decreasing size of particles.
Particle Analysis Using DLS
During particle analysis using the DLS method, a laser beam is used to illuminate the sample, and the scattered light is recorded at one detection angle, mostly in a backscatter direction, over 30-120 s. The diffusion coefficient and, subsequently, the particle size are determined from the scattered light intensity fluctuations caused by the particle movements.
Submicron particles free from sedimentation are dispersed in a liquid and subject to a perpetual random movement/Brownian motion. The fluctuations in the scattered light intensity depend on the diffusion coefficients of the particles irradiated using the laser beam.
The suspended particles with a specific set of positions in the liquid scatter the radiation to the detector. However, the relative phases of scattered wavelets differ owing to the differing incident phases experienced by the particles at the positions and different particle-detector distances.
The electromagnetic wave can receive or impart momentum and energy from the internal and external motions of the scatterer during the scattering process. Although the energy difference between the scattered and incident photons is neglected due to extremely small frequency shifts caused by Brownian motion, the change in momentum of the photon during the scattering process is a crucial parameter in DLS.
Light scattered at a low angle contains gross information on the structural and dynamic properties of the scatters, while light scattered at a high angle contains similar information at a finer scale.
DLS possesses a greater range compared to other scattering techniques it depends on the distance that a particle diffuses within a time interval. However, obtaining quantitative information from a fluctuating signal is a major problem in data recovery in DLS.
Rapidly diffusing small particles lead to fast fluctuations in the scattered light intensity, while slow-moving aggregates and larger particles yield slow fluctuations. The fluctuation rate can be determined by obtaining the intensity autocorrelation function using the autocorrelation analysis technique.
In DLS measurements, the particle diameter and diffusion coefficient are related by the Stokes-Einstein equation. The intensity of light scattered by the diffusing particles demonstrates a time dependency, which can be described as a spectral frequency shift or as a time-dependent phase shift.
Frequency analysis or photon correlation spectroscopy (PCS) is used based on these concepts to process the time-dependent scattered light intensity. In PCS, the time-dependent scattered light intensity is correlated with a time-delayed copy of itself/autocorrelation function or with the signal from a second detector/cross-correlation function.
Both the cross- and auto-correlation functions of a monodisperse system decay exponentially with correlation time. The decay rate depends on the scattered light fluctuation as a function of particle size. Thus, the decay rate is faster for small particles and slower for large particles. The frequency-based scattered light power spectrum is analyzed in frequency analysis. The power spectrum is a Lorentzian-type function for a monodisperse system.
Both frequency analysis and PCS are mathematically equivalent, as the Fourier transform of the frequency-based power spectrum in frequency analysis is equal to the time-based autocorrelation function in PCS. Thus, the average particle diameter and the polydispersity index that indicates the particle size distribution broadness can be evaluated using both methods.
Several mathematical approaches are employed for data evaluation, including the cumulants method to evaluate the time-based autocorrelation function and a Laplace inversion for particle size distribution.
Heterodyne detection and homodyne detection are the two optical detection types used with DLS instruments. In homodyne detection, the scattered light is measured, while in heterodyne detection, a portion of the incident light and the scattered light are combined for interference.
DLS is effective for analyzing solutions under conditions where particles are spherical and small. The concentration of the suspensions must be sufficiently diluted to ensure that only a single scattering takes place while using this technique. Most of the samples used in DLS are diluted to an extent where they become optically clear.
The use of standard DLS methods is not feasible on more concentrated suspensions as multiple phase shifts occur to the scattered light due to multiple scattering, leading to inaccurate measurements of the particle size.
More from AZoM: Particle Analysis of Different Types of Automotive Fuels
References and Further Reading
Ross Hallett, F. (1994). Particle size analysis by dynamic light scattering. Food Research International, 27(2), 195-198. https://doi.org/10.1016/0963-9969(94)90162-7
Particle size analysis by dynamic light scattering. [Online] Available at https://www.pmda.go.jp/files/000235106.pdf (Accessed on 12 June 2023)
Wang, M., Shen, J., Thomas, J. C., Mu, T., Liu, W., Wang, Y., Pan, J., Wang, Q., Liu, K. (2021). Particle Size Measurement Using Dynamic Light Scattering at Ultra-Low Concentration Accounting for Particle Number Fluctuations. Materials, 14(19). https://doi.org/10.3390/ma1419568
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.