天然镉是与铜矿共生的高挥发性元素,含有8个稳定同位素.测量其同位素丰度的不确定度只能达到0.9%～4.8%.因此,必须按国际理论和应用化联合会原子量和同位素丰度分会(IUPAC CAWIA)的要求来测定天然丰度镉的新镉原子量.IUPAC CAWIA要求的条件和步骤是采用高真空蒸馏法纯化浓缩镉和天然镉样品,测量和计算这些样品中杂质元素的含量,用计量学称重法制备同位素混合样品,用多接收电感耦合等离子体质谱(MC-ICP-MS)和热电离质谱(TIMS)测定镉的同位素比值,计算镉原子量并评估相关的不确定度.一些国家实验室用MC-ICP-MS获得了稻米中镉的含量,可用作对照的关键参考值是14.48±0.087 nmol·g-1.%Natural cadmium is a chalcophile and highly volatile element and consists of eight stable isotopes. The values of the uncertainties for the isotope abundances have been assessed by the IUPAC and amount to 0.9%～4.8%. Therefore, the determination of a new and improved atomic weight value for natural abundance cadmium has to be in accordance with the IUPAC CAWIA conditions and steps. These conditions and steps are purification of isotopically enriched and natural abundance cadmium by high vacuum metal distillation, determination and calculation of the amount in the metallic and no-metallic impurity mass fractions, gravimetric preparation of isotopic mixtures by metrological weighing, determination of isotope ratios of cadmium by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) and thermal ionization mass spectrometry (TIMS), calculation of the atomic weight including evaluation of its combined uncertainty.Some national reference laboratories have now obtained the values for the cadmium amount content in rice. A key comparison reference value is 14.48±0.087 nmol*g-1.
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