An Nguyễn Hoàng What system are you studying? And its approximate size?
The international standards (ASTM E2490-09(2021) & ISO22412) recommend a concentration ladder to study the effects of concentration on the answer. It's a question of signal-to-noise. We have to balance:
Size - as the size decreases < 100 nm then the scattering decreases as a function of r6 (or volume2)
Double the volume concentration and it will double the scattering generally but we need to avoid multiple scattering
The relative refractive index - metal colloids will always scatter better than proteins, for example
At least one of the above 3 has to be going for you in order to get repeatable answers. Increase the concentration to increase the signal and you may wander into multiple scattering. Reduce the concentration and you may have too little sample for adequate scattering and signal. At worst, in order to get adequate signal for poorly scattering (small, RRI approaching unity) systems then you may have to tightly control the concentration for robustness in the results.
Dear An Nguyễn Hoàng . See the following useful link: https://www.research.colostate.edu/wp-content/uploads/2018/11/Guide_for_DLS_sample_preparation.pdf
Kindly see also the following useful link: https://lsinstruments.ch/en/theory/dynamic-light-scattering-dls/experimental-guidelines-dls-sample-preparation
Aref Wazwaz Fawaz Raad Jarullah I'm not sure just providing a single url multiple times helps the poster of the question. I have tried to fully answer the question at the start and now that answer is lost from repeated urls quoted that in some case need to be requested or hide behind a paywall... Repeat/identical answers can be deleted so that the thread is not full of repetitions...
Thank you so much for your extra suggestions and good references. I am sorry for any inconvenience , I will try to check the updated notifications frequently
An Nguyễn Hoàng RG is a scientific forum and, as such, answers and questions (like yours) should be posted on this forum and not hidden by separate emails to you. In that way, all benefit from the questions and answers.