Light scattered from this sample is detected at a defined scattering angle, and the intensity fluctuations are analyzed. It is important to note that the laser and the photon detector sit on the same horizontal plane. 1b).Ī typical Dynamic Light Scattering setup incorporates a vertically polarized laser beam illuminating the sample. Shortening the path length between the scattering volume center and the cuvette surface 9 (Fig. While there are advanced experimental cross-correlation solutions to overcome the influence of multiple scattering 7, 8, the most commonly applied experimental steps to limit or suppress the influence of multiple scattering include. Further observable effects of multiple scattering from higher particle concentrations include a decrease in the Y-intercept of the correlogram, a decrease in apparent average size, and an increase in polydispersity. When multiply scattered light is interpreted as single scattering, this leads to more rapid decay in the correlation function, and thus a smaller apparent size than they really are. Multiple scattering reflects many particles in a typical light path with increasing sensitivity to each other and causing faster fluctuations. Single scattering probes the movement of individual particles. This paper extends the study to more scattering events but cannot directly compare to Sorensen’s work due to differences in the experimental setup.ĭLS analyzes the intensity fluctuations in the scattered light from objects moving in the light path. studied a distinctly lower number of scattering events. 1978, the depolarization ratio of depolarized to polarized intensity is related to the average number of scattering events. Our experimental scattering angle of 173° in this study is very close to that 180°, and thus the depolarized double-scattering should lead to a very similar z-average. They show that the correlation from these double scattering events is independent of scattering angle and approximates the single-scattering correlation function for a scattering angle of 180°. (1976) demonstrate that the lowest order contribution to depolarized scattering in non-interacting samples comes from double scattering events. Initial theoretical and experimental studies of this situation looked at the onset and influence of double and multiple scattering contributions 5, 6. In other words, the path of a typical photon may involve multiple scattering centers, thus termed multiple scattering. As the concentration of scattering centers in a sample gets higher, we then start to encounter a phenomenon where photons face an increased likelihood of scattering off neighboring particles prior to reaching the detector. The concept of single scattering is at the core of practically all commercial particle sizing instruments that are based on dynamic light scattering. Furthermore, the z-average is an intensity weighted mean of the particle distribution and the PDI exhibits the variance in size, indicating the normalized width of a Gaussian distribution. This approximation is particularly valid at short delay times τ and for similar particles. Furthermore, the abundance of single scattering events at measurement positions close to the cell wall results in an apparent increase in uniformity yielding a lower polydispersity index which is more representative of the physical system. The intercept of the correlation function may serve as an indicator of relative strength of single to multiple scattering. We find that the contribution of multiple scattering events effectively reduces both the measured scattering intensity and the apparent size from the autocorrelation function. In this paper, we show how comparison of different polarization components can help ascertain the presence of unwanted multiple scattering, which can lead to false conclusions about a sample’s mean size and polydispersity. With the ease of use of today’s commercial DLS instrumentation comes an inherent danger of misinterpretation or misapplication at the borderlines of suitability. While there are limitations in size range, resolution, and concentration, the technique has found ubiquitous applications from molecules to particles. Dynamic light scattering (DLS) is well established for rapid size, polydispersity, and size distribution determination of colloidal samples.
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