Microtrac explain on their website how their instrument can measure concentratied samples using backscattering:

https://www.microtrac.com/dynamic-light-scattering-systems-for-nanoparticle-measurements/

I don't know if you can access the detector signal. Some instruments used to do this but the feature has gone.

It doesn't use a correlator (as the above link explains). It determines a power spectrum. This is how the original instruments operated in the 1960s. Correlation was a way to achieve the same effect but much more cheaply. Modern electronics and computing power means the power spectrum method is trivial. However, the analysis of correlation data has evolved much more than for sizing using the power spectrum.

The instrument also uses "heterodyning" (2 laser beams) which is an odd choice for sizing. The usual way ("homodyning") uses one beam. The latter measures *relative* particle motion. The former measures *absolute" motion. For random diffusion, relative motion is prefered since it is insensitive to other motion such as sedimentation, convection etc. Using heterodyning for sizing means that these kinds of motions will be present in the power spectrum. Most commercial instruments use the homodyning for sizing and heterodyning for measuring electrophoretic mobility. Note, I don't say zeta potential. None of these instruments measure zeta potential. They calculate it using a (wrong) model from the measured mobility.

I believe the instrument may be a good option for particle sizing but it has a significant shortcoming as far as mobility/zeta potential is concerned. It doesn't has phase analysis light scattering (PALS). The major manufactures all offer it. It is essential if you want to look at the electrophoretic mobility of particles in high salt concentration aqueous media or low polarity liquids.

On paper, it can be hard to see differences between instruments. Most use the same basic concepts but have their own ways of implementing them.

Whatever instrument you choose, you have to understand that the results won't necessarily agree with other instruments. One reason is the choice of mathematical models used to take the complicated light scattering signal and distill down to a single number. This is true for zeta potential.

I invented PALS and worked with one company to implement it in a commercial instrument. I'm very familiar with the differences between instruments and know when the manufacturers make unrealistic claims. e.g., Microtrac say "High precision – Achieved by inclusion of Brownian motion frequency spectrum" - this is BS.

Don't take their words for it. Get them to measure *your* samples, not just a nice simple system they like to demonstrate their instruments with. And, please, get a good book on colloid science. Manufacturers like to claim that a single measurement with their instruments will solve all your colloid stability issues. There's so much more to it than that. DLS is just one tool that can help paint a good picture of your product's behavior.

Of course, these commercial push-a-button instruments work well in quality testing environments when you have demonstrated that the technique is discriminating for your purposes.

A nice balanced answer from John Francis Miller

Looking at specifications can be like splitting hairs and it is important to look at how each offering is suited to your application.

I have to say I disagree slightly with John with regards to his comment of " Manufacturers like to claim that a single measurement with their instruments will solve all your colloid stability issues. " This is certainly not always the case.

Myself and my colleagues acknowledge that light scattering is one of many techniques that are necessary to paint the complete picture, and this is a theme of several of my colleague Alan F Rawle 's webinars. Characterisation isn't just about obtaining a number and should be about asking a question. Particularly with the Zetasizer, we promote a workflow where several measurements are taken, and the quality of the data assessed, sample presentation/preparation improved and then remeasuring rather than just dropping in a sample and reading off a number. Light scattering can be that simple, but usually not, and again represents just a jigsaw piece.

Many thanks to all for the answers!

Is it possible to put the correlator on such an optical path as Microtrac uses and measure the correlation function of fluctuations of the intensity of scattered light and determine the hydrodynamic radius of particles from the analysis of such a correlation function?

What do you think will be the advantages or disadvantages of such an optical scheme for measuring correlation functions in comparison with measuring the correlation function of fluctuations in the intensity of scattered light when measuring scattering at 90 degrees?

Vladimir Kuryakov Please refer to my earlier answer regarding homodyne vs. heterodyne detection. The Microtrac uses the latter and has disadvantages over homodyne for sizing.

It principle you could use a correlator but it isn't clear from Microtrac's description exactly how they detect the light. It could be a continuous analog signal or a discrete photon counting approach. Using a correlator would require you developing the necessary software to control it and process the data.

The power frequency spectrum (as generated by the Microtrac) and the autocorrelation function are mathematically related. One is just the fourier transform of the other. I mentioned why correlators tend to be used vs. the power spectrum in my previous answer.

The instruments I have designed and built have all used analog detector signal + fourier transform or phase analysis.

Personally, I consider the autocorrelation method to be like carburetors - the technology had evolved to give an amazing piece of engineering for adding very controlled amounts of fuel to the cylinder. The more obvious - but expensive - way would have been to inject it directly somehow. Well, the technology came along that was affordable enough and carburetors have all but disappeared. Correlators persist because the algorithms have matured to process the data and instrument makers prefer to make incremental changes to their products rather than start over. Microtrac isn't tied to the past that way. Given the mathematical equivalence of correlation and power spectrum analysis, and modern data processing capabilities, there's no reason to stick with correlation. But that is just my biased view!

Vladimir Kuryakov , if you are interested in particle size in concentrated systems, might I recommend Acoustic Spectroscopy. I work for a company called Dispersion Technology (www.dispersion.com) which specializes in Acoustic and ElectroAcoustic Spectroscopy for Particle Size & Zeta Potential in concentrated systems. I would be happy to tell you more about it, should you find it relevant to your application. Regards, Sean.