Determination of trace ruthenium in geochemical sample by antimony- copper fire assay graphite furnace atomic absorption spectrometry
FAN Lei1,2,3, WANG Tiantian1,2,3, YAO Mingxing*1,2,3, SUN Qiliang1,2,3, GUO Xiaorui1,2,3
1. Zhengzhou Institute of Multipurpose Utilization of Mineral Resources, CAGS, Zhengzhou 450006, China; 2. Key Laboratory for Polymetallic Ores' Evaluation and Utilization, MNR, Zhengzhou 450006, China; 3. Key Laboratory of Comprehensive Utilization of Gold Resource in Henan Province, Zhengzhou 450006, China
Abstract：Ruthenium (Ru) is an effective precious metal catalyst in the fields of industry and scientific research. The determination of Ru content in geochemical samples is essential for the exploration of precious metal minerals. Antimony oxide and copper nitrate were used together as collectors. Meanwhile, the mixed reagent with same composition as the flux was added as the covering agent for melting. The obtained antimony-copper particle was turned over in a magnesia cupel for cupellation. In this process, antimony was oxidized into antimony oxide and volatilized, realizing the removal of much antimony. But copper served as a cupellation protective agent to avoid the volatilization loss of ruthenium, thereby enriching ruthenium in milligram level in antimony-copper particles. The antimony-copper particles were dissolved with 20% (V/V) aqua regia by microwave digestion and then determined by graphite furnace atomic absorption spectrometry. Therefore, the determination method of trace ruthenium in geochemical samples by antimony-copper fire assay graphite furnace atomic absorption spectrometry(GFAAS) was established. The certified reference materials of olivinite and pyroxene peridotite were selected as experiment object. The comparison tests with and without covering agent were designed. The results showed that the measured results of Ru after adding covering agent were in good agreement with the certified values, while the measured results without covering agent were lower compared to the certified values. It indicated that the addition of covering agent significantly improved the collection efficiency of Ru. The blank values of Ru in the collectors of three fire assay methods, i.e., antimony-copper fire assay, lead assay, and nickel sulfide fire assay, were compared. The results showed that the blank values of antimony-copper fire assay were at least one order of magnitude lower than the other two methods. It was found in experiments that the chromite samples, which were hardly decomposed, should be pretreated with sodium peroxide and calcium oxide by sintering. After breaking the agglomerate, the Ru in sample was separated and enriched according to the antimony-copper fire assay method. Using this method, the determination results were relatively accurate. Under the optimal conditions, the mass concentration of Ru in range of 0.01-50 ng/mL and the corresponding absorbance were fitted with least square method of quadratic equation to obtain the calibration curve. The linearity of the calibration curve was good, and the determination coefficient was 0.999 6. The characteristic concentration was 2.34 ng/mL. The proposed method was applied for the determination of trace Ru in certified reference materials of several rock samples (olivinite, chromite), stream sediment and soil. The found results were consistent with the certified values. Three geochemical samples were selected and determined according to the experimental method. Meanwhile, the recovery tests of standard addition were conducted. The relative standard deviation (RSD, n=5) of the results were between 4.6% and 6.8%, and the spiked recoveries were between 95% and 105%.
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