There is an important difference between the temperatures of the measuring cell which is located inside the optical system and the dispersion unit which is at room temperature, especially for MS3000 for which the cell is at above 30°C. Additionally, the dispersion units (e.g., Hydro MV) generate heat by stirring (depending on speed) or ultrasonication (depending on intensity and time). Temperature gradients created inside the cell simulate the behavior of large particles, increasing the signal on the first detectors. The MS3000 cell provides for optionally connecting a thermostat, recommended when using internal ultrasound treatment. To me it looks like this is not optional when using organic dispersant. In the case of MS2000, the cell is closer to room temperature, but avoiding temperature differences and subsequent artefacts with organic dispersants can be tricky, depending on room temperature, stirrer speed, sonication and even MS2000 series.
Thank you for the question. My response is lengthy and thus put in this attachment form.
You need to connect the Hydro to an external refrigerated circulation bath using the two ports on the front.
Thank you @John Francis Miller! I am experimenting with a thermostat coupled to the cell with Hydro MV /MS3000. This, in fact, needs to increase the temperature when there is no sonication, but MS2000 does not provide for thermostating facility.
Thank you for the question. My response is lengthy and thus put in this attachment form.
By the way, I forgot to mention that this phenomenon was first observed in the late 1970's in 'dry' measurements of aerosol sprays and fuel injectors (evaporating solvent), and MDI's (volatile suspension medium such as CFC's). It can be seen in dry powder measurements (long measurement times or evaporating residual solvent).
Thank you so much for taking the time to provide a detailed answer. It is certainly helpful to have your insight. There is little information available on this topic so far, but I assume the interest for it is going to increase gradually as the older instruments are being replaced by Mastersizer 3000. I started having feedbacks from colleagues from other organizations who are confronted with similar problems. Therefore, I think that we, end users, would appreciate having information regarding these issues and solutions to be easier accessible.
Hi Mircea I'm not sure what you mean by 'easier accessible'.
The right 'solution' depends on the material that is being measured (size distribution), the utilized dispersant liquid, the size range of the instrument especially the top end, the optical concentration/obscuration, the specified parameter(s) on the material itself and so on. The result can always be 'cropped' by using an instrument with a less wide specification but then genuine large material may not be seen.
This subject has been discussed and raised at the ISO TC24/SC4 (WG6) meetings and thus is common to a number of manufacturers and instruments when volatile solvents and thermally poorly conductive dispersant liquids are employed. I would agree that ISO meetings are not open to all but the outcome in terms of the standards is. High sensitivity specifications from end users means that instruments may need much more care and attention in setting up methods - I can (with care) overcome these issues but I would be considered an experienced user and many users want the systems to be 'dumbed down' for inexperienced users. This I can understand too - there's just too much pressure in laboratories with the limited number of personnel. Older instruments not providing this (validated) sensitivity will be more robust to artifacts like this, but one could still generate these with non-understanding practice. The more sensitive systems will require more accurate optical properties (due to higher angular capability) and will be less robust to changes in these parameters. In particular the imaginary part of the RI will need to be determined for example by application of the Beer-Lambert (Bouguer) relationship. See (this older presentation can ONLY be opened in Internet Explorer. It is not accessible to mobile devices):
Determination of the optical properties of CeO2 and CaCO3 by the volume concentration experiment - the 3 S's, Beer-Lambert and Mie
https://www.brainshark.com/malvern/OpticalPropertiesCeO2
Hi Alan!
I am following for long time the publications by Malvern with great interest and found the webinars extremely interesting and useful. Especially the ones presented by you are loaded with valuable information. Also, I am so grateful for the time you spend answering my questions. It is fascinating having first hand information, learning directly from the source!
Regarding the term "easier accessible" I used above, I was thinking that it would be good and time saving to have a mention about the potential issues related to the use of organic solvents and the possible approaches to solve them right in the user manual. Also, I think that sharing this information by other media like webinars published by the manufacturer would be welcome.
I am aware of the new features of the MS3000 software which allow to circumwent thermal artefacts, by killing the response of detectors, forcing a single mode result, etc. However, it appears to me that, in the case of MS3000 wet procedures, using a thermostat is promissing as a solution to have a low and stable background even at low stirer speeds and to avoid thermal artefacts. I was curious to confirm my findings that temperature differences are the sources of the problems and learn from other's experience. If it is the case, the solution appears to be relatively simple. I am going to further explore this way.
Hi Mircea Thank you for your thoughtful comments. The actual 'reason' for beam steering is the change in refractive index caused by the thermal fluctuations in the volatile solvents (or similar considerations in a gas flow). It's a property of the liquid (or gas) rather than the technique. This happens in a beaker of the material as well as in an accessory. A similar effect can be mimicked by pouring water into whisky and observing the mixing fluctuations due to RI changes.
Standard stirrer speed is 90 - 95% of maximum to keep material in suspension. The only exception would be emulsions where high shear is to be avoided (and correct dilution in the mother liquor employed).
It's differences in temperature that give rise to the issue rather than the value of the temperature itself. Getting this thermal stability is key and there's no short cut here obviously. Obviously looking for a solvent of lower volatility would be a good move.
Thank you, Alan!
It is the explanation which I supposed myself regarding the effect of temperature gradients, imagined like a shimmering landscape abobe a hot road.
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