2019 年 68 巻 2 号 p. 133-137
The homogeneity and stability of reference materials for the determination of Cd in white and brown rice grains were evaluated using atomic absorption spectrometry (AAS) after HNO3-H2SO4 digestion. Two kinds of rice grain reference materials were prepared by adding 100 g each of white and brown rice grains (both from Niigata prefecture, Japan) to 750 mL of methanol containing 40 μL of 1000 mg L−1 Cd standard solution (40 μg as Cd), and then heating them at 300°C for 1 h on a hot plate to evaporate the solvent. Each rice grain reference material (100 g) was packed in 10 glass bottles (10 g each). The homogeneity of Cd in the rice grain reference materials was estimated by an analysis of the variance after the Cochran test. A total of 9 samples (3 samples each from 3 bottles) were subjected to the homogeneity test. There was no significant difference between the within-bottle and between-bottle variances, indicating that both rice grain reference materials were sufficiently homogeneous. The stability of Cd in the rice grain reference materials was estimated by regression analysis. For a stability study, 6 bottles were stored in the dark at room temperature for a period of 6 months. The rice grain reference materials were stable for 5 months for white rice grain and for 6 months for brown rice grain. Elemental mapping images of the Cd of a longitudinal section of a white rice grain and a brown rice grain, using a micro-X-ray fluorescence instrument, suggested that the added Cd is present in the outer layers of the rice grain reference materials. The rice grain reference materials with a Cd concentration of approximately 0.31 mg kg−1, prepared in this study, can be used for validating the AAS determination of Cd in white and brown rice grains.