I am evaluating the particle size distribution of a plant protein-polyphenol complex as a function of ionic strength (50-500 mM) and the results are a little bit odd. In general, the particle size distribution gets narrower with the addition of salt but the PDI gets higher. Can I trust in these results? I am using a Malvern Zetasizer.
Hello Cristhian Rafael Lopes Francisco
can you share some example data for this? If the polydispersity index PDI is very large then this might not be a good sign. The data file has the file extension ".dts" and you can create a selection of records as described in https://www.materials-talks.com/blog/2014/08/19/faq-how-can-i-submit-a-data-file-to-the-help-desk/
You may also find the different scenarios of this blog of use https://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/)
Cristhian Rafael Lopes Francisco
This suggests some larger material at the top end that is incorporated in the z-average but not seen in the main peak. As Ulf Nobbmann explains then your .dts files will provide conclusive proof. My expectation is that the far field in the correlogram will rise again and not decay exponentially.
- Are you using the disposable cuvettes?
- Are you filtering your continuous phases to at least as good as you wish to measure? (The size of the plant protein-polyphenol complex)
- Is the addition of higher ionic strengths causing 'salting out'?
All of the above, and many more things beside, can be seen with access to the .dts file...
Hello Ulf Nobbmann and Alan F Rawle
I'll give more details about my samples. I'm facing the same problem described before with all my samples but I'll give you the raw data of my control sample, which is the formulation without polyphenols, containing only the lupin protein. This protein was thermally treated (85ºC and 15 minutes) so I believe they are some kind of protein precipitates (named as C4 in the raw data file). Answering Alan F Rawle's questions,
- I'm using quartz cuvettes (DTS0012).
- I'm not filtering these samples, but I know they contain some populations in the microscale range. I'm diluting the samples in water (1000 fold) before measuring the particle size distribution in the ZetaSizer.
- I'm not sure whether the "salting out" is happening, but I can't see any precipitation.
In the .dts file, you'll find the results for ionic strengths ranging from 50 to 500 mM. Please, let me you if you need more details.
Thank you very much for helping me.
Hello Cristhian Rafael Lopes Francisco ,
thanks for adding the data. The z-average is in a way the overall average, whereas the distribution function may not show very large peaks. The average size and PDI increase with salt, indicating there is a higher tendency to agglomerate at higher salt. The size of the scattering particles is significantly larger than a mono- or oligomer of the protein. It appears that you created some higher order complex through the thermal treatment process, and this produced the "protein nanoparticles" of a few hundred nm.
Your scattering intensity signal is low, probably due to the low concentration and this would make it very difficult to observe any precipitation by eye.
Considering you did not filter, the results are surprisingly nice. A short spin in a table-top centrifuge might improve the results, since there is a baseline present in many correlation functions.
The DTS0012 cuvettes are made of polystyrene. Quartz or glass cuvettes would also be suitable for these measurements at room temperature. The DTS0012 cuvettes were not tested above 70C. They may get deformed when sitting at 85C for a while. [You may have just selected the DTS0012 in the software and still used a quartz cuvette].
Cristhian Rafael Lopes Francisco you can also try transforming your DLS data to volume %. Sometimes that gives a different perspective on the size fractions present.
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