Reports of how laser diffraction instruments are used to measure particle size often lack detail. This can make reproducing results from literature challenging and can also lead to frustrations when trying to measure similar samples. Key information, such as dispersant used (and whether any additives or surfactants have been used), choice of optical model, optical properties, dispersion conditions, and optical concentration are often missing. A typical example from the literature may be:
Although a good starting point, this statement lacks vital information, making it difficult for readers to reproduce the results accurately. We will now look at what else needs to be considered and why:
Log in or sign up for free to continue reading.
Reports of how laser diffraction instruments are used to measure particle size often lack detail. This can make reproducing results from literature challenging and can also lead to frustrations when trying to measure similar samples. Key information, such as dispersant used (and whether any additives or surfactants have been used), choice of optical model, optical properties, dispersion conditions, and optical concentration are often missing. A typical example from the literature may be:
Although a good starting point, this statement lacks vital information, making it difficult for readers to reproduce the results accurately. We will now look at what else needs to be considered and why:
Whilst all laser diffraction instruments for particle size analysis work using the same principles, differences in hardware and software mean that results may sometimes vary between manufacturers. It is therefore imperative to report the name of the instrument used to perform the measurement. Laser diffraction systems will also use a dispersion device to disperse (and circulate in the case of wet dispersions) the particles. These will contain different volumes of dispersant, with larger volume units more typically used for polydisperse materials and samples containing large particles.
The dispersant is the dispersing medium in which our particles are suspended during the analysis. This typically might be water, alcohol or air, but laser diffraction instruments can support a wide range of liquids. The interaction between the particles and dispersant is vital. The smaller our particles are, the more important this interaction becomes when trying to achieve a stable and reproducible dispersion of our particles. The use of stabilising reagents and surfactants are often commonplace when performing laser diffraction measurements as they can promote wettability and dispersion stability.
A laser diffraction system measures the scattering pattern produced by an ensemble of particles as they pass through a laser beam. This is, however, only half the story. To best match the particle size distribution to the scattering pattern, an appropriate optical model should be applied. There are two options that can be considered – the Fraunhofer approximation and Mie theory. The former, is a basic model, valid for large particles (defined here as particles > 50 µm, as per ISO 13320), whilst the latter is valid for particles of all sizes. If Mie theory is to be applied, then additional information, such as the particle’s refractive and absorption indices, must be input. Due to the Fraunhofer approximation’s more basic approach, no information about our particles is required. The use of different optical models for the same samples may produce different particle size distributions. This is particularly true for particles <50 µm, where it becomes important to model the secondary scattering effects that are considered by Mie theory.
If using Mie theory, then three optical properties are required:
The choice of these can have a dramatic influence on the particle size distribution, with this being especially true for small and transparent particles, therefore the chosen values should be reported accordingly.
The feed rate is typically first optimised to ensure that a good signal to noise ratio is achieved without causing choking of the material (when too much material is passing through the venturi at once it cannot be adequately dispersed). The particle size distribution achieved for a dry dispersion measurement can be highly dependent on the dispersion pressure. As the pressure is increased, more energy is used to disperse the particles. This can lead to more dispersion of agglomerates but may also mill primary particles. This dependence on pressure highlights the need to report the value used.
Stirring is required to circulate the sample and keep it suspended within the measurement volume. A suitable stirrer speed should be selected such that all particles are suspended whilst maintaining the integrity of the sample e.g., doesn’t destroy emulsions or promote further agglomeration. If a sample is agglomerated the use of ultrasound is often investigated. If ultrasound is to be used, then the user should state the method of sonication (e.g., bath, external sonicating probe or dispersion accessory sonication), power and the duration.
Laser diffraction results should be independent of the optical concentration of particles present. Too low a concentration and users may struggle to get reproducible results, owed to poor sampling of the material, or artifact peaks arising from a low signal to noise ratio. On the other hand, at too high a concentration we may begin to introduce multiple scattering to the measurement. Ideally, the incoming photon hits a single particle before going on to the detector. If the concentration becomes sufficiently high, this photon will interact with multiple particles before reaching the detector. This in turn leads to an increase in the scattering angle and an overestimation of the percentage of fines present. Optical concentration is typically reported using obscuration or transmission values.
We can now use this guidance to generate a more appropriate report of how a measurement was performed. This information can be incorporated into the main text or provided in an appendix or supplementary methods material to ensure the method can be reproduced.
The particle size distribution of the CaCO3 particles was measured using a Mastersizer 3000+ Ultra (Malvern Panalytical). Particles were dispersed in a Hydro MV dispersion unit (120 mL) containing water (refractive index = 1.33). Five drops of a 5 wt.% solution of Igepal CA-630 were added to the dispersion unit to promote the wettability of the sample. The size distribution was derived using Mie theory. For the particles, a refractive index of 1.54 was used alongside an absorption index of 0.01. Samples were prepared by forming a slurry with the dispersant. An aliquot of the slurry was then added to the dispersion unit until the desired obscuration (5-10%) was achieved. Thirty seconds of ultrasound (100% power) using the Hydro MV’s in-built sonication capabilities was carried out prior to the measurement.
The particle size distribution of coffee granules was measured using a Mastersizer 3000+ Ultra (Malvern Panalytical). Particles were dispersed using an Aero S dispersion unit, which uses air to disperse the particles. The size distribution was derived using the Fraunhofer approximation. Measurements were performed using a dispersion pressure of 2 bar. The feed rate was optimised in order to achieve a steady flow of material within a suitable obscuration range (0.5-8%).
Whilst not completely exhaustive, this technical note serves to highlight the key parameters to report. Further information on other factors to consider can be found in ISO 13320: Particle size analysis – Laser diffraction methods.
