Can I Measure Turbid Samples with DLS?

Many particle-based suspensions are cloudy because they contain a high concentration of particles or strongly scattering particles. There is an ongoing debate about the effectiveness of DLS for analyzing turbid particle suspensions.

A common misconception

The ability to accurately measure the particle size within turbid samples is important in a wide range of industrial applications, including analyzing chemical slurries, developing new coating materials, and assessing drug formulation and stability. However, there is a common misconception that turbid or cloudy samples can’t be analyzed with Dynamic Light Scattering (DLS). This belief stems from the fact that DLS theory assumes that light enters the sample, is scattered once, leaves the sample, and is then detected. We call this a ‘single scattering’ event. In turbid samples, however, the particle concentration can be so high, or the photon’s path length through the sample can be so long, that some photons are scattered more than once. This is termed a ‘multiple scattering’ event and leads to measurement inconsistencies or errors.

Avoiding multiple scattering events

Okay, so is there a way to avoid multiple scattering events and enable DLS analysis of turbid samples? One approach is to use back-scattering analysis, in which the scattered light is detected at a scattering angle of 175°.
In back scattering detection it is possible to adjust the laser focus position close to the inner cuvette wall. This helps because it reduces the photons’ path length within the sample, which in turn reduces the number of particles the photon might interact with, and thus makes it much less likely that the photon is scattered more than once.
Adjusting the laser focus position within the sample may improve the analysis of turbid samples (as seen in Figure 1 below). Both the laser beam and the detector beam are focused by the same lens. The scattering volume, which contains the particles the photon may interact with, is defined as the intersection volume of the laser and detector beam. By moving the lens along the laser direction, the location of the scattering volume can be shifted within the sample. Turbid samples scatter more light, so measuring closer to the cuvette wall reduces the chances of multiple scattering events by minimizing the light’s path length through the sample.

Turbide Sample

Figure 1: Schematic diagram showing the laser focus position for A) small, weakly scattering particles: the laser is focused in the middle of the cuvette, and for B) concentrated, turbid samples: the lens focuses the laser closer to the cuvette wall to shorten the path length of the photons that are detected and to reduce the chance of multiple scattering events.

Transmittance analysis to assess turbid samples

In more advanced DLS instruments, the optimal scattering angle and laser focus position can automatically be determined by assessing the light transmittance through the sample, the intercept of the correlation function, and the intensity of the detected light. These parameters are quickly measured prior to the actual experiment and then guide the instrument to automatically select the best angle for the sample concentration and the best focus position to avoid multiple scattering.

To summarize, there is a common misconception that DLS can’t be used to analyze turbid samples. However, with the optimal angle of analysis and laser focus position, good quality and reliable Dynamic Light Scattering data can be generated from turbid samples. The Litesizer™ 500 from Anton Paar is designed to facilitate the analysis of turbid samples by enabling transmittance analysis, adjustable focus points, and multiple angle analysis.

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