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The preparation and characterisation of certified reference materials of four selenium-rich rocks (GBW07397 to GBW07400) are described in this paper. The raw materials were derived from selenium-rich rocks in two famous seleniferous regions, Enshi Prefecture, Hubei Province and Ziyang County, Shaanxi Province, China. Sample homogeneity and stability were tested by inductively coupled plasma-mass spectrometry and atomic fluorescence spectrometry. The determined element mass fractions included selenium, arsenic, copper, zinc, molybdenum, cadmium, lead, vanadium and silver. Except for silver, the results of analysis of variance (ANOVA) and the relative standard deviations of the element mass fractions showed that the four materials exhibited good homogeneity and stability. Ten laboratories were involved in an interlaboratory comparison scheme for certification. Eight element mass fractions in the selenium-rich rocks were assigned as certified values, while only indicative values were obtained for Ag mass fractions. The certified values and expanded uncertainties for the selenium mass fractions in GBW07397-GBW07400 are 0.96 ± 0.05, 1.03 ± 0.05, 49 ± 4 and 38.5 ± 1.9 lg g -1 , respectively.

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