Back to Basics of Laser Diffraction – Q&A from Masterclass Part 1

Thank you to everyone who attended our recent masterclass webinar series on laser diffraction using the Mastersizer instrument range. Your participation and engagement were truly inspiring, and we are grateful for the excellent questions you posed throughout the sessions. In this series of blogs, we will address some of the key topics raised during the back-to-basics webinar, providing further insights and clarifications to enhance your understanding. Let’s dive into the first installment of the Q&A blog series!

Shedding light on optical models and properties

In this section, we delve into the fascinating world of optical models and properties. Your questions highlighted the complexity and importance of understanding how light interacts with particles in laser diffraction. Let’s explore these concepts further.

Q – Are there any other optical models like Mie or Fraunhofer?
Q – Where in the software can we choose the optical model?

A – Within the Mastersizer Xplorer software you have the option to choose between two implementations of Mie theory (spherical or non-spherical) or the Fraunhofer approximation. You select the optical model via the ‘Particle Type’ section of the measurement and SOP settings as shown below. 

Q – Do you have any solutions to determine the refractive and absorption properties for particles with not known chemical structures?

A – When analysing a novel material, we recommend starting with an approximation of the required optical properties, the refractive index (RI) and absorption index (AI). This first approximation can be informed by broad categories of material types as shown in the tables below:  

Once you have taken some measurements using these approximate optical properties you can assess the suitability of your choice of parameters using the fit report, the residuals and a visual assessment of the particle size distribution. The Optical Property Optimiser tool allows users to quickly test different combinations of RI and AI to assess the sensitivity of the result to the optical properties. For more guidance on choosing the most appropriate optical properties, please see this recorded masterclass.

Q – What values of residual would you consider appropriate to accept a result as correct? Usually, we have differences between residual and weighted residual.

A – Generally, a residual and weighted residual below 1% is recommended and both residuals (weighted and unweighted) should generally be within 0.4% of each other. This reflects good agreement between the measured data and data fitted using the optical model. However, there are circumstances where it will be challenging to achieve values close to or lower than 1%. This tends to be the case when we have noise on inner detectors, which can result from having very narrow size distributions (spanning much less than a decade in size), and/or sub-micron distributions. Obtaining residual values less than 1% can also be challenging when particles are strongly coloured, owed to their complex optical properties.

Q – You showed circular particles in this presentation, what about irregular shapes?

A – The mathematical models used in the Mastersizer 3000(+) to produce a PSD assume that particles are either disc-shaped (Fraunhofer) or spherical (Mie). While perfectly spherical particles produce symmetrical scattering patterns, non-spherical particles can cause asymmetrical scattering due to their orientation and surface roughness. To address this, the software provides an option to select non-spherical particles in the ‘Particle Type’ settings which is an implementation of Mie theory that considers the depolarization of the scattered light caused by non-spherical particles. The sensitivity to the choice of particle type is particularly pronounced for sub-micron PSDs.

Q – The measurement of monomodal particles can be described by Mie theory well, but in case that particle size distribution is broad (let say 30-300 nm with even bigger aggregates) the measurement is just approximation, and the exact particle size is difficult to resolve. Am I correct?

A – Mie theory can be implemented for an ensemble or group of particles as well as being able to resolve single particle scattering, therefore, it can provide a full solution for broad PSDs (Wriedt 2012, Mackowski 2012). The size range you quote, however, 30-300 nm is towards the lower end of the dynamic range of the Mastersizer 3000(+), and sub-micron PSDs are particularly sensitive to the optical model and optical properties chosen which is where uncertainties may be introduced when resolving the PSD using Mie theory.

Q – What are the scattering considerations when analysing particle adsorbed dispersions such as Pickering emulsions?

A – Scattering from Pickering emulsions depends on the interaction of light with both the particles at the surface and the core droplet. It may not be as straightforward as using the optical properties of one component alone. Generally, start with the optical properties of the core and assess the data fit across all detectors. Then, compare results using the refractive index (RI) and absorption index (AI) of the adsorbed particles. If differences are negligible, the optical model is robust. If differences are significant, compare data fits to determine the most suitable set of optical properties to use. There is no perfect solution, but by experimenting with different combinations and utilizing the fit report, you can approximate the optical properties effectively.

Further resources and webinar recordings

Thank you again for your engagement throughout the Mastersizer Masterclass. If you would like to revisit the webinars please find the recordings on our website. For more information on laser diffraction applications and method development, please visit our knowledge center where you can find a library of application notes, technical notes and blog posts on a range of topics. And if you would like to ask more specific questions, please do get in touch via our customer support portal where your enquiry will be passed on to the relevant technical expert. Stay tuned for the next blog in the series where we will investigate how to master dispersion methods and materials.

Further reading

• Back to Basics of Laser Diffraction – Q&A from Masterclass Part 3
• Back to Basics of Laser Diffraction – Q&A from Masterclass Part 2
• Back to Basics of Laser Diffraction – Q&A from Masterclass Part 1
• Precision on the Loch: delivering Mastersizer 3000+ training on the Isle of Skye and exploring its scientific wonders