Molecular scattering of light in aqueous solution occurs as a result of the presence of inhomogeneities in it, for example, local fluctuations of the dielectric constant (fluctuations of density, orientation) and fluctuations of concentration, nanoparticles.Depending on the nature the fluctuations appear and disappear due to the thermal motion of molecules constantly. Their relaxation provides the broadening of the spectral line of light scattering up to 10 MHz (fluctuation of entropy), up to 30 kHz (fluctuations of the solute concentration). Brownian motion of nanoparticles provides an even smaller broadening - from 3 kHz up to several tenths of Hz. In other words, in the case of scattering in solutions the splitting of the incident monochromatic wave into two waves with changed frequencies ω+∆ω and ω-∆ω occurs. Each splitting corresponds to its fluctuation. The decay rate of the fluctuations of the concentration depends on the diffusion coefficient and the falling rate of concentration gradient when distance changes. The attenuation coefficient or the broadening of the scattering line depends on the diffusion coefficient, concentration wave and the direction of propagation of the scattered wave. On the basis of light scattering on the fluctuations R. Pecora proposed the theory of DLS in terms of the autocorrelation function.The device determines the rate of diffusion of nanoparticles, and then by the Einstein-Smoluchowski equation to recalculate the average hydrodynamic radius of the particles.
The best results you will get, if only the gold nanoparticles are in the water. If the nanoparticles are cylindrical, the device will show that they are spherical with a mean hydrodynamic radius. It will add a "coat" around metal particles. Coat may consist of ions, surfactants, etc. If you change the viscosity of the medium, you must specify a different viscosity instrument. In the device a lot of disadvantages. I tried to ask questions to developers, but has not received a clear answer to my question, and I understand that it is not due to the limitations of the method.
https://www.researchgate.net/post/Highly_reliably_data_DLS
Best regards.
When you referred to the quality report, what did it say? What is your system? Are all solutions clean and free from dust? Have you got sedimentation or a broad distribution? Have you got fluorescence in the system? The quality report helps with all of this.
@Yuri With respect, how is your 'answer' helping the poster answer their question? It's long and rambling introduction to a thesis but does not appear to relate to the actual question at all.
As Alan has said, the quality report within the Malvern Zetasizer software aims to help understand your results by interpreting the features of the recorded correlation function and other parameters. There are a range of sample and measurement dependent phenomena that can effect the quality of DLS results, and depending what the problem is, the solution may include changing the measurement settings, some changes to the sample preparation or possibly another characterisation technique.
Every tool has its advantages and disadvantages, depending upon how it is used. Yuri states that there are a lot of disadvantages to DLS, but if you want to perform a quick size/stability measurement of an appropriately sized material in dispersion, then there is nothing wrong with DLS. If you want to measure larger samples which contain a high degree of polydispersity, or very turbid samples, etc, then there are going to be some challenges using DLS. That said, DLS might still be able to tell you something useful, and that is where the quality report page can help.
You can use a hammer to crack a nut, but that wont necessarily give you the outcome you desire. Similarly using a nut cracker to drive a nail is also not going to go very well.
In response to Yuri, looking at your other question that you link to, I note that there are a number of detailed answers, including one by myself. Is there anything else that you would like clarity on with regards to the Malvern Zetasizer?
Dear Alan F. Rawle, Ph.D. Application Manager Malvern Instrument:
Dear Alexander Malm, Mphysics, Doctor of Enterprise –Physics, Development Scientist Malvern Instrument
My answer is not offensive and does not have a commercial nature. I do not get paid for the answers. I help students and researchers. My answer relates to a method DLS and not the Malvern Zetasizer. Any method of research has shortcomings and limitations. To do this, Mr. Afza Shahid need to know the basics of the method. The discussion ends.
Best regards.
The tone of this discussion has taken an interesting turn, and perhaps highlights the hazards associated with written communication in such as forum as tone can not always easily be gauged.
It is sometimes difficult to assess what level of detail needs to be given within an answer, and of course, background detail would always be useful but may cloud the intended question and answer.
In this instance the question posed asked why good quality data was not being reported by an instrument. The reference to a quality report indicates to me that the instrument used is a Malvern Zetasizer as to my knowledge this feature does not appear with other commercial DLS instruments.
The list of phenomena that could cause poor measurement in DLS are numerous, and to list all and their possible remedies I feel would not help Afza, as this could easily lead to a wide study to investigate effects that were never apparent.
Instead more relevant advice could be given if what errors are reported were shared, or alternatively the measurement record or a copy of the autocorrelation function results were shared so that other DLS users could share their insight.
Of course if the posed question was asking how DLS works, or what its limitations are, the structure of the answer would be completely different.
@Yuri, Apologies if any offence was taken but this was certainly not the intent. In the question you linked to earlier you specifically mention a Malvern Zetasizer feature (NIBS) and it appears you still have some questions on this. I'd be happy to discuss these either within that thread or via message if you would like.
Interestingly, we've not heard back from the poster with exact details of the result quality report or even what is being measured.
- Thank you so much everyone for your valueable view point. And yes AlexanderMalm! im using Malvern Zetasizer. And currently im facing some problems in analysis of different samples and therefore asked this question. And i'll definitely share the results as well as the expert opinion given by the software after analysis.
- Alan F Rawle Im working on particle size measurent and will share the results as well as quality report and all other relevant data very soon.
Afza Shahid Glad to hear that you are using DLS. Yes, the Malvern 'quality report' can sometimes err on the side of caution. In other words, your data may not be as bad as you suspect... check the "Size Quality" Report(M) tab in the software [size workspace] and see what recommendations are given. The technical note explaining those test that the quality report does can be found via https://www.google.com/search?q=malvern+size+quality+report
For some tips & tricks on DLS you may also find this of interest
https://www.materials-talks.com/blog/2014/11/17/tips-tricks-for-nanoparticle-characterization/
- Badges
- Science topic