Quantitative measurement of oxygen by Electron Probe X-ray Microanalysis for estimation of iron oxidation state

• EPMA methodology, when improved, enables direct, accurate oxygen quantification. • It allows accurate determination of transition metal oxidation states. • Methodology is validated on Fe-O samples, demonstrating necessary improvements. • Benefits of EPMA over “wet chemical methods“ and XPS for multiphase materials are explained. Electron Probe X-ray Microanalysis (EPMA) is a key method for studying phase equilibria in oxide materials, offering an effective balance of precision, accuracy, cost, speed and spatial resolution. It provides valuable compositional data for single phase and multiphase materials, particularly those with complex microstructures. Historically, high absorption of characteristic X-rays has limited the accuracy of oxygen quantification, leading to reliance on metal cation measurements and assumed oxidation states, often resulting in inaccuracies. This study highlights recent advancements in EPMA sample preparation and measurement techniques that significantly enhance oxygen quantification. To validate the method, Fe-O system samples were synthesized at 1300 °C (1573 K) under controlled oxygen partial pressures. A wide range of P (O 2 ) was employed to cover the Fe 3+ /(Fe 2+ + Fe 3+ ) ratios from 0.1 to 1.0 in wüstite, spinel, and hematite phases. The results were compared with existing thermodynamic models and experimental data, demonstrating excellent agreement. The enhanced EPMA methodology offers potential for direct quantitative measurement of oxygen, which in turn, opens an opportunity to estimate Fe 3+ /(Fe 2+ + Fe 3+ ) ratios in complex chemical systems, including quenched slags. The approach is applicable to other metals with variable oxidation states, provided a single element primarily drives oxidation state changes.