(467b) Offline Size and Shape Characterization of Crystalline Powder through a Combined Imaging and Chromatic Confocal Microscopy Technique

The particle size and shape distribution (PSSD) has a significant impact on the processing, handling, and quality of end products of powders in various sectors. Traditionally, a single characteristic length of the particles is measured, assuming a spherical shape for any arbitrarily shaped particle.¹ However, this approach is inadequate in accurately characterizing irregular morphologies such as plate-like particles that are often encountered in fine chemicals. To accurately characterize such particles using a generic particle model, three characteristic lengths (length, width, and thickness) are required. Techniques such as X-ray Computed Tomography,² offline stereoscopic imaging,³ digital holography,⁴ and fluorescence confocal microscopy combined with image processing⁵ can provide accurate 3D reconstruction of a single or several particles, but they have associated disadvantages in terms of minimum resolution or the time taken to characterize an ensemble of particles.

In an online fashion, this challenge has been addressed once by a dual-projection dynamic ex situ measurement technique.⁶ The integration of a second camera solves problems arising from particle orientation inside the solution. The approach can accurately extract the mean of the characteristic lengths of plate-like particle populations. However, it finds it challenging to accurately measure the width of the distribution which is important if one wants to predict the powder behavior through the downstream processes.^7,8 In an offline fashion, laser scanning confocal microscopy (LSCM) can extract all three characteristic lengths from dry powder samples. Due to its specialisation on surface profiling, LSCM can measure thickness with nanometer range but compared to the aforementioned dynamic technique can be slower by orders of magnitude. While the dynamic technique can obtain measurements in a matter of minutes, LSCM might require several hours to measure an ensemble of particles.

To address the shortcomings of existing techniques, a novel offline characterization device combining two independent techniques, single-projection imaging, and chromatic confocal microscopy, has been conceptualized and built. The former is used to estimate the length and width of crystals dispersed on a substrate, while the latter estimates the thickness of the crystals. The proposed method complements existing techniques used both in an academic and industrial setting, providing a more accurate measurement than dynamic techniques and potentially faster characterization than commercial techniques for ensembles of plate-like particles. The proposed technique has been evaluated and validated using LSCM as a reference, by characterizing crystalline compounds that exhibit differences in size and shape characteristics and optical properties. The proposed technique can accurately obtain three characteristic lengths for thousands of particles, making it a valuable addition to existing process analytical technology toolbox.

References

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(7) Wilson, D.; Bunker, M.; Milne, D.; Jawor-Baczynska, A.; Powell, A.; Blyth, J.; Streather, D. Particle engineering of needle shaped crystals by wet milling and temperature cycling: Optimisation for roller compaction. Powder Technol. 2018, 339, 641–650.

(8) Perini, G.; Avendano, C.; Hicks, W.; Parsons, A. R.; Vetter, T. Predicting filtration of needle-like crystals: A Monte Carlo simulation study of polydisperse packings of spherocylinders. Chem. Eng. Sci. 2021, 230, 116151.