锍镍试金-微波消解-高分辨率连续光源石墨炉原子吸收光谱法测定岩石矿物中超痕量铂钯钌铑铱

Pt、Pd、Ru、Rh和Ir属于铂族元素,它们物理化学性质接近,是一组重要的地球化学和环境化学元素。然而,铂族元素在自然界矿石中丰度很低且分布不均匀,具有块金效应,准确测定其含量一直是分析化学中的难题。实验采用锍镍试金法对样品中Pt、Pd、Ru、Rh和Ir进行分离和富集,用微波消解法处理Pt、Pd、Ru、Rh和Ir这些金属的硫化物沉淀以及滤纸,用高分辨率连续光源石墨炉原子吸收光谱法(HRCS-GFAAS)对样品溶液中这些元素进行测定,建立了岩石矿物中超痕量Pt、Pd、Ru、Rh和Ir的测定方法。实验表明:运用传统的锍镍试金富集岩石矿物中Pt、Pd、Ru、Rh和Ir时,用实验制备提纯的NiCO₃代替商品试剂NiO作为捕集剂,可大幅度降低锍镍试金流程中待测元素Pt、Pd、Ru、Rh和Ir的空白值,且空白值非常稳定;采用微波消解仪在优化的加热消解程序下以10mL王水(1+1)溶解Pt、Pd、Ru、Rh和Ir这些金属的硫化物沉淀以及滤纸,Pt、Pd、Ru、Rh和Ir可被完全溶解在溶液中,Ru不会形成RuO₄挥发;采用连续光源原子吸收光谱仪的高分辨率分光系统,各待测元素的连续光源原子吸收光谱分辨率均不大于0.00223nm/pixel,可将Pt、Pd、Ru、Rh和Ir的原子吸收谱线与干扰谱线完全分开,故样品中共存元素对Pt、Pd、Ru、Rh和Ir所测谱线的干扰可忽略。在选定的实验条件下,Pt、Pd、Ru、Rh和Ir的吸光度与其对应的质量浓度运用二次方程最小二乘法拟合校准曲线,各元素校准曲线的相关系数分别为0.9999、0.9998、0.9998、0.9999、0.9997;特征浓度分别为2.98、0.47、0.98、0.48、2.76ng/mL。将实验方法应用于铬铁矿原矿中Pt、Pd、Ru、Rh和Ir的测定,测定结果的相对标准偏差(n=5)介于4.2%~13.4%之间。按实验方法测定土壤和岩石(橄榄石、辉石)等标准物质中的Pt、Pd、Ru、Rh和Ir,测定值与标准值吻合良好。

Determination of ultra-trace platinum,palladium, ruthenium,rhodium and iridium in rocks and minerals by high resolution continuum source graphite furnace atomic absorption spectrometry with nickel sulfide fire assay enrichment and microwave digestion

Pt, Pd, Ru, Rh and Ir belonged to platinum group elements with similar physical and chemical properties. They were a group of important elements in geochemistry and environmental chemistry. However, the abundance of platinum group elements in rocks in nature was very low and the distribution was non-uniform. Due to the nugget effect, the accurate determination of their contents was always a problem in analytical chemistry. After the elements including Pt, Pd, Ru, Rh and Ir in the samples were separated and enriched by nickel sulfide fire assay method, their sulfide precipitates and the filter paper were treated by microwave digestion. Then the contents of these elements in sample solution were determined by high resolution continuum source graphite furnace atomic absorption spectrometry (HRCS-GFAAS). Thus, a determination method of ultra-trace platinum, palladium, ruthenium, rhodium and iridium in rocks and minerals was established. The experiments showed that when Pt, Pd, Ru, Rh and Ir in rocks and minerals were enriched by traditional nickel sulfide fire assay method, the blank values of Pt, Pd, Ru, Rh and Ir could be greatly reduced in the process of nickel sulfide fire assay with commercial reagent NiO replaced by the purified NiCO₃ as the trapping agent. Moreover, the blank values were very stable. The sulfide precipitates of Pt, Pd, Ru, Rh and Ir as well as filter paper were dissolved with 10mL of aqua regia (1+1) in microwave digestion system with optimized heating procedures. The results showed that Pt, Pd, Ru, Rh and Ir could be fully dissolved in solution, and Ru would not form volatile RuO₄. The high resolution beam splitting system of continuum source graphite furnace atomic absorption spectrometer was adopted. The resolutions of continuum source graphite furnace atomic absorption spectra for test elements were all not more than 0.00223nm/pixel. The atomic absorption lines of Pt, Pd, Ru, Rh and Ir could be fully separated from interference lines. Therefore, the interference of coexisting elements in sample with the spectral lines of Pt, Pd, Ru, Rh and Ir could be ignored. Under the selected experimental conditions, the relationship between absorbance and mass concentration of Pt, Pd, Ru, Rh and Ir was fitted with least square method for quadratic equations to prepare the calibration curves. The correlation coefficients of calibration curves were 0.9999, 0.9998, 0.9998, 0.9999 and 0.9997, respectively. The characteristic concentration for Pt, Pd, Ru, Rh and Ir were 2.98, 0.47, 0.98, 0.48 and 2.76ng/mL, respectively. The proposed method was applied for the determination of Pt, Pd, Ru, Rh and Ir in chromite raw ore. The relative standard deviations (RSD, n=5) of determination results were between 4.2% and 13.4%. The contents of Pt, Pd, Ru, Rh and Ir in certified reference materials (CRMs) such as soil and rock (olivine and pyroxene) were determined according to the experimental method. The results were well consistent with the certified values.