A fit-for-purpose nongaseous impurity assay procedure for potential inorganic primary reference materials by inductively coupled plasma mass spectrometry

Purity-evaluated inorganic materials, which are potential primary reference materials (PRMs) in chemistry, are crucial to the production of inorganic calibration solutions with metrological traceability to the International System of Units (SI). Purity evaluation is typically performed by individually assaying all possible impurities (gaseous and nongaseous) in a material, but technical challenges and the lack of resources are creating bottlenecks in developing effective purity assay procedures. This study describes a nongaseous impurity assay procedure developed and used at Korea Research Institute of Standards and Science (KRISS) to evaluate the purities of inorganic PRMs. Inductively coupled plasma mass spectrometry (ICP-MS) is used to assay more than 60 impurity elements in a single day, and external glow discharge mass spectrometry (GDMS) data are used to complement ICP-MS data. All aspects of the impurity assay procedure, including sample preparation, instrument operation, and data treatment, are described in detail. It is shown that the procedure is applicable to high-purity Al, As, Cu, NaCl, Se, Si, and Zn, and that a target relative uncertainty of 0.005 % for purity values can be satisfied even if individual impurity data are quite uncertain. Particular emphasis is on clearly specifying the measurand and target uncertainty for developing a fit-for-purpose assay procedure, as they determine the acceptable level of assay quality and help preventing misinterpretation of assay results.