Edit Nasirov

I know that dimethylpolysiloxane is a food additive (E900) - and also as a condom lubricant and in treatment of head lice on the scalp, but unsure why you'd want to use a relatively expensive (in comparison to water, say) material (with disposal issues) as the continuous phase in a laser diffraction experiment. It's pretty viscous and indeed viscoelastic. What are you measuring that would require this material? It's used as a de-foaming agent so you must be trying to stir the material at a too high rate. You can stop/start the pump stirrer several times to allow bubbles to dissipate but again the viscosity of your medium may make this difficult. It's also a poor thermal conductor so that heat transfer is poor and many minutes may be needed for thermal equilibration and after sonication many minutes may be needed for re-stabilization.

In theory you could kill the first 2 data channels (preferably after measurement) but I always resist killing, or justifying the killing of, information. Best is to get these inner rings to a satisfactory, but more important, stable, level.

My most important question to you is why can't you use another material as the continuous phase or, indeed, measure dry?

In terms of general method development for laser diffraction see:

PST & BDAS - An acronym approach to laser diffraction method development

https://www.malvernpanalytical.com/en/learn/events-and-training/webinars/W190815PST

Vadim S. Gorshkov

Edit Nasirov

You also appended your question to an existing RG question: https://www.researchgate.net/post/How_do_you_minimize_prevent_thermal_artefact_in_particle_size_determination_by_Malvern_Mastersizer_2000_and_3000_using_organic_dispersants

You may wish to go and delete this (to avoid confusion) and I will also delete my posts.

Dimethyl polysiloxane is also used on the Malvern 2000. We want to transfer the Malvern 2000 methods to the Malvern 3000. The use of Dimethyl polysiloxane on the Malvern 2000 is not a problem. We also tried other media such as Isopar G there was also a problem with the background. Sunflower oil has not been tried yet. Do you think it's worth a try? For several substances, Dimethyl polysiloxane is used as for example ... Aciclovir, Potassium chloride ...

Viscous liquids are difficult to use on both MS2000 and MS3000, as entrapped air/bubbles are difficult to remove, and thermal equilibrium may be more difficult to achieve. First, to avoid bubbles, we add the liquid when the stirrer/pump is stopped, then slowly increase the speed. Prior degassing of the liquid, by stirring under vacuum while slightly heating, as USP dissolution media, may also help.

Next, there is a higher power demand and thermal dissipation at the level of dispersion units is more important when using viscous liquids, which has an impact on the thermal equilibrium of the system. For this reason, at lower stir and pump speeds, they may work better from point of view of Background stability and consequently less prone to thermal artefacts.

If possible, avoid using viscous liquids as dispersant.

Isopar G is a less viscous liquid, and thermal artefacts can be avoided if thermal equilibrium is achieved. Best results are obtained at stir and pump speeds over 3000rpm.

Edit Nasirov

There are several factors that may be at play here notwithstanding the fact that the MS3000 is a considerably more sensitive instrument than the MS2000.

• Winter issues - we see more of these problems in winter as solvents such as these are often stored outside for safety reasons. Allow bottles of fluid to reside at least 24 hours at laboratory temperature before use
• The use of a cell/accessory for both aqueous and non-aqueous work meaning much cleaning with an intermediate liquid such as IPA. A little water in the system makes wetting with a liquid such as Isopar or mineral spirit fairly difficult. Some customers reserve an accessory for aqueous work and another for non-aqueous as the cost and usage of IPA can be high. Occasionally I've had to warm the windows in the organic liquid to encourage hydrophobicity - if there's any water trapped in the system it will generally prefer to migrate to the windows rather than stay in suspension increasing the background
• Thermal equilibration. This can be longer in the MS3000 than in the M2000. General rules are to add the liquid slowly (warmer liquid may help too) and increase the pump speed gradually up to close to maximum (I use 3250 or 3300 rpm). Stop and start the pump frequently to allow bubbles to rise out of the system. You may have to wait 15 - 20 minutes (= 1 ICB; ISO Coffee Break) until the background is sufficiently stable for measurement. Be aware that stability of background is far more important than its actual magnitude

In many scenarios, dry measurement circumvents these and other potential issues (e.g. optical properties).

Malvern 3000 has options to cool the cell window, but we haven’t tried it yet. Thermal equilibration might be helped. What do you think?

I found that a simple warm only thermostat set at a temperature slightly above the cell temperature can get the background low and stable and prevent artefacts when using MS3000, even at lower than 2500-3000rpm, which is not normally possible when using organic liquids. It still needs a thermal equilibration time for dispersant and cell, but it is quite shorter in this way, especially at higher rpm.

I did not try the cooling device (sold separately), but I assume that we still need to understand that a thermal equilibrium between dispersant and cell components should be reached and this depends on cell temperature, dispersion unit type, flow regime (rpm), dispersant thermal properties and ambient temperature. Therefore, I don't expect it solves all the problems. Ideally, we would need a dispersion unit which keeps the temperature of the dispersant constant, at the same level as the measurement cell, to avoid temperature gradients. For now, it may seem an unnecessary sophistication.