I have a DLS result (done with repetition) of water dispersion of PEG coated nanoparticles using Micromeritics Nanoplus particle analyser where cumulant diameter was shown as 257.5 nm with PDI 0.545. However, if I check intensity and number distribution separately, I see that their Gaussian fitting gives peak at different positions. This is understandable as they are obtained based on different physical principles and detection methods. I also assume that my sample has formed agglomerates in water with different sizes. Now, while presenting in paper, which peak should I consider as the average hydrodynamic size? According to some instrument manuals, intensity peak should be considered especially when the sizes are not too small. Secondly, which PDI should I mention-the one obtained from the cumulant result (mentioned above) or the one obtained from Gaussian fitting of intensity distribution plot using the following relation?
PDI = ((Standard deviation)/(Average diameter))^2
I can get the standard deviation from the σ value of Gaussian fitting. This method gives a PDI value of 0.08.
Can I present average and PDI value from both the distributions using the above equation?
Good morning
I just wont to mention, that is another important parameter of measuring colloidal system by DLS - monodispersity. If the colloidal system is not monodisperse depending on the size and concentration of fraction you may not even see peaks from smaller nanoparticles. We discuss this problem in our paper:
Article Detection Limits of DLS and UV-Vis Spectroscopy in Character...
I hope that this information will be helpful for you.
All the best from Poland
Grzegorz Celichowski
DLS results are highly variable and depend on numerous factors. Therefore, it is good idea to report average values.
The instrument measures using intensity data. The number distribution is calculated from the intensity data.
You PDI is large and so your manual's statement about using the intensity peak isn't valid especially since you are using cumulants which only yields a single peak.
For your presentation, I would report the intensity-weighted average, the number-weighted average and the PDI.
Good morning
I just wont to mention, that is another important parameter of measuring colloidal system by DLS - monodispersity. If the colloidal system is not monodisperse depending on the size and concentration of fraction you may not even see peaks from smaller nanoparticles. We discuss this problem in our paper:
Article Detection Limits of DLS and UV-Vis Spectroscopy in Character...
I hope that this information will be helpful for you.
All the best from Poland
Grzegorz Celichowski
Kashmiri Deka
Although I do not have much experience with using those commercially available benchtop DLS Instruments, I would be rather surprised if they mention in the Manual that you should calculate the average size from the intensity distribution data; I would Always use the number distribution data to calculate the average size if I have to do it manually.
About the peak position difference, I think, John Francis Miller already given the Explanation. But just to elaborate a little more, the scattered intensity is proportional to 6th power of the scatterer size. So, enen if you assume a perfect Gaussian distribution of the scatterers , you will not end up with a Gaussian distribution in terms of scattered intensity ( would be possible if scattered intensity was proportional to the scatterer size).
As I said, I have used multiangle DLS Instruments and not those benchtop 1 angle ones. And if you assume possible agglomeration, I would rather suggest Multi angle DLS for a better average size determination.
The "safest" answer is always the Cumulant size and the overall PDI. The Cumulant size is the intensity weighted average size.
If distribution analysis peaks are repeatable, then the intensity is one most closely related to the actual signal you collect. Number distributions from DLS can be troublesome, especially with a sample of such high overall polydispersity.
https://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/
https://www.materials-talks.com/blog/2017/01/23/intensity-volume-number-which-size-is-correct/
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