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How do DLS size results differ with angle?

12 January 2016 No Comment

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90-versus-173-thought-bubble

You may have noticed that the Zetasizer instrument range is available in two different optical configurations: the ‘traditional’ 90 degree setup and the ‘NIBS’ 173 degree setup.

An obvious question for a dynamic light scattering (DLS) user is:

“Will I get the same results from both systems?”

This can be a tricky concept to understand, so we’ll do our best to explain here.  First though, we should note that, aside from its larger scattering angle, the NIBS (Non-Invasive Back Scatter) system also has a much larger concentration range than the 90 degree system, meaning the NIBS system can be used to measure more concentrated samples and still generate results which are in agreement with the 90 degree system result.

Perhaps a better way to phrase the initial question is: “Provided there is enough scattering signal for good data acquisition, will the same sufficiently dilute sample give the same size when measured by the 90 and the 173 degree configurations?”

Well, the answer depends on the sample being tested and the distribution observed. In principle, the intensity distribution could be different, but the volume or mass distribution should be the same (although there may be specific conditions where this will not happen due to Mie scattering). “FAQ: Will backscatter and 90 degree results be consistent?” provides more detail on this topic, and a thorough discussion of backscattering is provided in the technical note “Explaining the difference between Non-Invasive BackScatter (NIBS) and Backscatter – they are NOT the same.”

Based on published results, the application note “Effect of Angle on Resolving Particle Size Mixtures Using Dynamic Light Scattering” further elaborates on the issue by analyzing a mixture of two particle species and assessing their resolvability at various scattering angles. The key summary points of this discussion are:

  • DLS is generally considered a fast, low-resolution technique
  • Changes in relative concentrations can be observed, provided contributions are above the noise threshold
  • Conversion from intensity to volume distributions should be independent of angle for select relative concentration range

Light scattering versus angle behavior is handled by Mie theory, which forms an integral part of the intensity-to-volume conversion contained in Malvern light scattering software packages, both for dynamic light scattering and laser diffraction solutions.

Footnotes: 1) When the sample exhibits multiple scattering then the results for different angles will be different, and the above discussion is only valid for single scattering samples, i.e. those samples that are optically clear enough so that light scattered by a particle is not scattered by another particle before reaching the detector.  2) For systems with variable measurement positions, where the effect of multiple scattering can be minimized by measuring closer to the cuvette wall, the measurement should be preformed at the same measurement position as the one for the 90 degree optics and that is the center of the cell. 3) When concentration dependent interactions, structure factor S(q), form factor F(q) or rotational diffusion come into play then investigation versus angle can provide value, and the results will not be the same.

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If you have any questions, please email me at ulf.nobbmann@malvern.com. Thanks! While opinions expressed are generally those of the author, some parts may have been modified by our editorial team.