Homogeneity Guarantee of Nickel Reference Material in Soybean Matrix: Influence Mechanism of Particle Size Distribution

In response to the demand for reference material under the EU Maximum Levels for Nickel (Ni) limit in soybeans (15 mg/kg) in 2024, this study explored the technical difficulty of ensuring the homogeneity of Ni reference material in the soybean matrix. Multi-scale characterization (LA-ICP-MS, ICP-MS, FT-IR, etc.) verified that Ni was specifically enriched in embryo and the finer powder (mainly embryo). Based on this finding, we innovatively proposed the span [(D90 − D10)/D50] as a rapid predictor to evaluate homogeneity, offering a potential screening tool to optimize grinding conditions and reduce reliance on time-consuming traditional homogeneity assessments (Ni-RSD by ICP-MS). A positive correlation between span and homogeneity was observed, which was attributed to the inhomogeneous distribution of low-Ni tissue (seed coat). By optimizing the crushing process (hammer cyclone milling, room temperature: 20 °C, 15,000 r/min, ≤ 0.45 mm sieve), a homogeneity uncertainty of 1.00% was obtained. This finding helps in ensuring the homogeneity of reference materials from other high-fat oilseed matrixes.