Interlaboratory Comparison of Particle Size Analysis Methods for Dry and Wet Nicotine Pouch Fillers: A Proficiency Testing Study

Particle size distribution (PSD) is a key parameter in understanding the physical characteristics of nicotine pouch products. This study presents the results of a proficiency testing (PT) study conducted across eight laboratories (Labs 1–8) to assess the consistency and accuracy of particle size analysis methods for four commercially available nicotine pouch filler samples, two dry (CP1 and CP3) and two wet (CP2 and CP4). Three analytical techniques were employed: sieving, dynamic image analysis (DIA), and laser diffraction. The study aimed to evaluate inter-laboratory variability and method comparability, providing insights into the most suitable techniques for different nicotine pouch filler types. Four laboratories used sieving, two laboratories used dynamic image analysis, and two laboratories used laser diffraction, providing particle size distributions and distribution intercepts at 10%, 50%, and 90% (Dx[10], Dx[50], and Dx[90]). Sieving showed consistent Dx[50] results across laboratories but was inefficient for high sample volumes and unsuitable for wet samples. DIA, while offering high sensitivity and detailed shape characterization, exhibited significant inter-laboratory variability due to differences in shape parameters and sample introduction methods. Laser diffraction provided rapid measurements with a broad dynamic range, suitable for both wet and dry samples, though it requires accurate refractive index and non-fibrous samples. Statistical analysis revealed significant differences within and between methods, particularly for Dx[10] and Dx[50] values. For dry products, sieving yielded similar Dx[50] values (315.3 μm to 325.4 μm for CP1), while DIA showed larger differences (327.66 μm to 381.14 μm for CP1). Laser diffraction showed good consistency, especially for wet samples, with low percent differences (1.9% for CP2 Dx[90]). The study underscores the importance of method selection based on sample nature, desired sensitivity, and application requirements, with sieving showing consistency for dry products, laser diffraction being robust for wet samples, and DIA valuable for shape characterization. The results support the development of standardized protocols for PSD analysis in nicotine pouch formulations, fostering better understanding in the development of these products.