Hi Diana,

This is a common question which a quick search on ResearchGate will report a number of answers.

The Z Average result from Cumulants is the recommended result from a DLS measurement according to international standards due to it's lack of reliance on fitting models etc.

It is however limited in use are a measurement of particle size if the sample is multimodal.

The intensity weighted size distribution is therefore the next most reliable particle size measurement if Cumulants is not appropriate. Both of these intensity weighted results do not need the optical properties of the material and are therefore more robust than any subsequent transformation.

Any intensity weighted result may be skewed by low amounts of large material, so here the conversion to number of volume weighting can help, but again there is another level of transformation of the data which may lead to errors.

The fact that you have multiple peaks suggests that you are better off using peak values if you want to report size, however if you are just trying to track qualitative changes in samples, then the Z Average may still be used.

The following links discuss this further.

https://www.materials-talks.com/blog/2017/01/23/intensity-volume-number-which-size-is-correct/

https://www.materials-talks.com/blog/2014/07/10/faq-peak-size-or-z-average-size-which-one-to-pick-in-dls/

Diana Said In addition to Alex's excellent answer and the additional information on RG and on MaterialsTalks, it is important to note that the international standards (ISO22412:2017 and ASTM E2490-09(15)) deprecate the conversion of intensity to number distributions. This is because of the ill-conditioned nature of the deconvolution with small changes in data giving, potentially, large changes in result.

Your results suggest that you have contamination in your sample. Filter to the level you wish to measure. I suspect for exosomes you should be filtering to 0.02 microns or better. Alternatively centrifugation may be an option.

My last comment refers to the number of decimal places you are using, so I ask you one question. What is the diameter of a hydrogen atom?

You should use the ones that are most useful for whatever process you are trying to control/understand.

John Francis Miller Agreed. But one also has to understand the terms themselves and their origin/derivation before trying to apply them to a process. The z-average and PDI given by the poser of the question highly suggest a 'contaminated' sample given that many exosomes lie in the 30 - 150 nm range.