The application of E171 (titanium dioxide) as a food additive has been an issue of debate in the European Union. A detailed physicochemical characterization of the E171 particles can objectify the discussions and is essential in the context of risk analysis. This work focuses on the physicochemical characterization of 15 pristine E171 materials by transmission electron microscopy (TEM) and single particle inductively coupled plasma mass spectroscopy (sp-ICP-MS) following CEN/TC 352 guidelines. The E171 samples were purchased on the Belgian market or were obtained from European producers. To measure the minimal external dimension of the constituent particles of E171, sample preparation protocols influencing particle dispersion (pH, probe sonication and centrifugation) were tested and optimized. In optimized conditions, representative TEM micrographs could be recorded, and it was demonstrated that all examined E171 food additives contained a significant amount of nanoparticles. The large majority of constituent particles, confirmed to be TiO2 by energy dispersive X-ray spectroscopy (EDX), were reliably detected and measured using the ParticleSizer software. In the most dispersed state, the particle size measurements by TEM and sp-ICP-MS agreed well. The measurement uncertainty budgets of particle sizing by TEM and sp-ICP-MS are in the order of 10% and 16 % (Ucx, k=2), respectively, based on validation studies of a series of representative test materials. Electron diffraction demonstrated that both anatase and rutile TiO2 particles were found in pristine E171. Eleven E171 materials consisted of anatase. Three materials consisted of smaller rutile TiO2 particles (20-40 nm) coated on mica. One material contained a mixture of anatase and rutile particles. In future research, the methodology will be implemented in a systematic and larger scale study of E171 food additives and food items containing E171, available on the market.
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