Validation of methodology and uncertainty determination of Co and Ni ferrites by WD-XRF

Ferrite materials, particularly CoFe₂O₄ and NiFe₂O₄, are of significant interest as redox materials in thermochemical water-splitting cycles for hydrogen production. This study presents the development and validation of a cost-effective, faster than acid digestion and reliable analytical methodology for the determination of iron (Fe), nickel (Ni), and cobalt (Co) in spinel ferrites using wavelength-dispersive X-ray fluorescence (WD-XRF). The validation, of the proposed method, referred as FENICO, was performed by evaluating the main analytical performance parameters, namely linearity, limits of detection and quantification, specificity, precision (repeatability and reproducibility), and accuracy. To minimize matrix effects and improve analytical accuracy, fused bead preparation was employed as the sample preparation approach. Excellent linearity was obtained for all analyzed elements, with correlation coefficients higher than 0.998. The method demonstrated good repeatability and reproducibility, confirmed through statistical tests such as Cochran's and Student's t -test. Accuracy was assessed by comparison with both ICP-AES and the semi-quantitative OMNIAN XRF method, showing no statistically significant differences between results. In addition, a comprehensive uncertainty assessment was performed, considering both Type A (sample preparation and calibration fit) and Type B (weighing) contributions. The results indicate low expanded uncertainties (below 11%), with identified calibration fit as the main source of uncertainty. Overall, the validated WD-XRF methodology provides a robust and efficient alternative for the quantitative characterization of ferrite materials, particularly in applications related to solar thermochemical hydrogen production.