Introduction to Dynamic Light Scattering Analysis 

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@@jeetenderkakkar7570 Don't be rude. Thanks Malvern.

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Try it with the sound off, and turn on closed captions CC.

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Thanks for the question - there is no short answer to that question and it really depends on what the application the particles are being used for and the purpose of measuring them. In some cases, such as for biologic applications, you may already know what particle size to expect and the purpose of measurement is to see if results match your expectations - if they don’t then maybe the sample has agglomerated or changed in some other way. At other times you may want to design a system to have an explicit particle size as theory says it will perform at an optimum for that application - such as reactivity or release profile or availability. At other times you may want your particles to interact, agglomerate or assemble - so presence of monomeric state particles could be bad for the application. Sorry, it’s not an explicit answer - but you need to consider the use of the particles to determine which is “better”.

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Thanks for your comments and we are pleased you liked the video. The diffusion coefficient is obtained from the relation Γ=D t q 2 where q is the scattering vector, given by q=(4πn/λ0)sin(θ/2). A fuller explanation can be found here - www.malvernpanalytical.com/en/learn/knowledge-center/technical-notes/tn101104dynamiclightscatteringintroduction

Thanks for your question Malak. Firstly, I assume you mean the core diameter or hard sphere diameter by real? Both hydrodynamic and core diameters are “real” and which you use depends of course on how you measure them and how you want to use them. What is the important physical parameter in your application? The hydrodynamic diameter accounts for any electric double layer that moves with your particles, if anything is absorbed or bonded to the surface of the particles or if your particles are “soft”. Measuring your samples at high electrolyte concentrations, where the electric double layer is most compressed, will get you closer to the core diameter. For instance we will typically measure standard polystyrene spheres in 10mM NaCl solution to reduce the double-layer. However, this might not always be possible as it could disrupt the chemistry of your sample. If you know the electrolyte composition of your continuous phase then it may be possible to estimate the double-layer thickness using Gouy-Chapman-Stern model for instance. The other, and perhaps easier way, is to measure the particle size by a technique such as SEM or TEM which will typically measure the core size. Hope that helps but difficult to give more specific advice without knowing your application. If you need more help then please contact your MP representative

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Hi Nida, The refractive index of the material is only important in DLS measurements if you want to convert the intensity size distributions to volume or number distributions. Hope this helps.

@@MalvernPanalytical thanks yes it is helpful.

Hi Tapas Pal, size is obtained from the correlation function by using various algorithms. There are two approaches that can be taken (1) cumulants determines the mean size (z-average diameter) and an estimate of the width of the distribution (polydispersity index) and is defined in ISO22412 (2018)) (2) a multiple exponential fit of the correlation function to obtain the distribution of particle sizes. There are various distribution algorithms available such as non-negative least squares (General Purpose or Multiple Narrow Modes or L-Curve). www.malvernpanalytical.com/en/learn/knowledge-center/faqs/FAQ160705CumulantsNNLS Hope this helps.

Thanks for the compliment! So far, this is the first light scattering video in this channel, no ELS or SLS. However, I will share your request with the team.

@@ulfnobbmann6396 thanks a lot!

Thanks for good question Jesky. We use (as all DLS systems do) Stokes-Einstein (S-E) equation to derive particle size from the diffusion coefficient. In S-E the factors affecting the diffusion coefficient are temperature, kinematic viscosity (also temperature related) and particle diameter. Particle density would mainly affect the inertia of a particle and this is a negligible quantity in the sample states typically studied by DLS. Particle density will come into play in terms of defining the upper size limit of DLS. Typically for DLS measurements particles should be freely diffusing. When particles have sufficient mass they will exhibit a gravitational component to their movement and can no longer be thought of as freely diffusing. There will still be a translational diffusion component but with increasing mass (density or size increase) this will become a smaller fraction compared to its gravitational component. So, the upper size limit by DLS for polystyrene beads will be greater than that of a gold sphere for instance. Hope that helps.

@@MalvernPanalytical I understand. Thank you for both answers. Great video by the way! :)

Happy we could help, thank you for the compliment.

@gudipati srinivasarao Makes sense! Thank you!

Thanks for the feedback, it’s a 3 as the formulae is expressed as a diameter and not in radius terms, that you maybe more familiar with.

Thanks for the question. DLS in its simplest form uses Cumulants analysis of the auto-correlation function and that analysis will only provide a mean particle size (the Z-Average size) and a measure of the width of the size distribution (the polydispersity). To extract particle size distribution data a secondary analysis is required and several models for this are available and often proprietary to a manufacturer - these include Non-Negative Least Squares or CONTIN analysis for instance.

Thank you Lina, we will take that into account.

Hi Iluan, thanks for the question. I am not 100% clear what you mean - we rely on the dispersion being free to move/diffuse so that the scattering intensity varies over time. DLS would not work if the particles were static (frozen). But perhaps you meant something else, if so perhaps you can clarify your question?

@@MalvernPanalytical Hello. What I mean is for a sample where the media itself is moving. For instance, a solution that is flowing through a pipe or that is being stirred inside a reactor. I worry that the macro-level movement of the liquid would interfere with the analysis of the movement of the particles.

Hi Iluan, thank you for elaborating. DLS can work in a flowing system, but the flow generally needs to be low (

Thanks for the question. I am not 100% clear I fully understand the terminology you have used. But to try and clarify - we measure the autocorrelation function of the intensity. This is always compared to t0 as the initial point of reference. Subsequent measurements of the intensity are taken at short time intervals from t0 and correlated with the intensity at t0. Obviously we can’t generate the intensity vs time at just one instance. We think the animation clearly shows the autocorrelation function being derived sequentially over several time intervals from t0. Hope this clarifies what is happening in this regard.

Hi, do you mean Mastersizer? Mastersizer uses Laser Diffraction and the video you posted on is around Zetasizer, this uses Dynamic Light Scattering. Both techniques maybe useful in measuring CNT’s - dependent on their size range and agglomeration state. If you can clarify what system you have we can point you in the right direction.

@@MalvernPanalytical Hi, we have Master sizer with EV and MV unit. CNT size is 9.5nm. I have a CNT water dispersion 200ml. Please advise what shall me sample volume and which technique I must use. Many thanks

@@user-vf3mp5nn8l Your expected size of 9.5 nm is below the 10 nm lower limit of the Mastersizer (laser diffraction), but this is well within the capabilities of the Zetasizer (dynamic light scattering). However, aggregation of the CNTs will likely mean you measure sizes larger than 9.5 nm and so the Mastersizer should still be relevant. We cannot advise on volume of sample to be tested as we do not know the concentration etc. Best practice for Mastersizer dictates that you should aim for an obscuration of no more than 5%. As these are small particles, there is a high likelihood of multiple scattering at higher obscuration levels. Try measuring at different obscuration ranges and determine where you see stable sizes versus obscuration - it’s within this range that you should be testing.

@@MalvernPanalytical Many thanks for the reply. The concentration is 1%. please advise the volume of sample.

Hi @user-vf3mp5nn8l I would suggest to contact your local support for more advice.

Hi Erick, the difference will be if you're calculating the hydrodynamic radius or diameter. Hope this clarifies it, but feel free to contact Malvern helpdesk if you have further questions.

@@deowere Thank yout Diogo

Thanks for the enquiry but there is no PowerPoint for the video, welcome to use as long as Malvern Panalytical is credited appropriately.

@@MalvernPanalytical thanks for ur reply it was a very gud presentation

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You can mute the sound on the video and turn on closed captions CC. The autogenerated captions are pretty close.

it means a photograph of the screen or the result in this case, just like the snapshots taken in mobile

Thanks, but why did it give different result when we make the auto correlation graph?

By snapshot we mean a measure of the data at any point in time, typically for correlation this is time zero compared to some other positive increment of time from time zero. It allows us to statistically compare the temporal relationship of the data. I hope that makes sense?

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@@MalvernPanalytical It was a great informative video. The main issue was the music being a bit loud, so focussing on the voice explaining was a little difficult for me.

Thank you for your feedback! We appreciate your honesty and understand that the background music may not be to everyone's taste. We'll take your comment into consideration for future videos and do our best to strike a better balance between the music and the narration so that it enhances rather than detracts from the viewing experience.

Sorry you didn’t like our choice of background music

You can mute the sound and turn CC = closed captions on. Then the distracting music is gone, as well as the voice. The autogenerated captions are pretty close.