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How much is very little in a dynamic light scattering experiment?

14 June 2018 No Comment

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Zetasizer-Ultra-Pro_300x270Have you ever tried to explain to someone that the large peak they can see in their dynamic light scattering data, that seems to dominate the size distribution by intensity – is actually only a very tiny part of the sample…and hardly ‘there’ at all?

This has happened to me on numerous occasions, and I always try to show the volume and number size distributions, illustrating how that peak disappears. However, there is often a lingering question hanging in the air: “But if we see it, then it’s there, right?”

In the new Zetasizer® Ultra, there is a feature which really helps me to explain this conundrum. It is called MADLS®, which is short for Multi-Angle Dynamic Light Scattering.  This clever solution automatically combines the size measured at multiple angles, providing a more reliable and complete size distribution of your sample, without the potential artefacts of a single detection angle, which may emphasize one part of the distribution more clearly than the others. This is illustratively explained in the following video.

Beyond this, MADLS can also measure the particle concentration for each resolved population in the sample, which helps with understanding how much very little actually is!

Concentration is given in terms of particles per mL, which allows me to compare numbers. If my small nanoparticle or virus particle population peak is 1013 particles/mL, and the aggregate or dust population peak is 104 particles/mL, it is clear that there is one dust or aggregate particle per billion nanoparticles…so in reality, very few in comparison to my main particle of interest.

It won’t tell us if these aggregates are problematic in the process or in the end product – only comparison with other techniques and data can do that. What it does provide is a more understandable number – which can indicate whether you are likely to see these contaminant particles with other detection techniques.

One in every billion: imagine how many TEM images I would have to analyze to find that one aggregate in my main nanoparticle population!

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