碱熔-电感耦合等离子体质谱法测定钨矿石和钼矿石中稀土元素

钨矿石和钼矿石具有难熔的特征,常需要用碱熔法才能将其溶解完全,但若用碱熔法对样品进行前处理,样品溶液会因加入大量熔剂引入钠离子从而导致盐度过高进而造成基体干扰及锥孔堵塞,不利于电感耦合等离子体质谱法(ICP-MS)对其中痕量稀土元素进行测定。针对这一问题,实验提出了用NaOH和Na₂O₂高温熔融样品、热水提取后过滤的方法将稀土元素富集在稀土氢氧化物沉淀中,从而与大量的Na及K、W、Mo等金属离子分离,加入酒石酸-HCl体系溶解沉淀后稀释,可实现ICP-MS对钨矿石和钼矿石样品中稀土元素的测定。实验表明:称取0.500 0 g样品,加入3.0 g NaOH和1.5 g Na₂O₂,在马弗炉中于700 ℃熔融20 min可将样品分解完全;通过选择合适的同位素及数学校正方法可消除质谱干扰,以10 μg/L¹⁰³Rh为内标可有效地监测和校正分析信号的漂移和基体效应。在优化的实验条件下,各元素校准曲线的相关系数均大于0.999 5,方法检出限为0.004～0.08 μg/g。采用实验方法对钼矿石和钨矿石成分分析标准物质中的稀土元素进行测定,结果与标准物质认定值对数误差(ΔlgC)的绝对值均小于0.1(地质矿产行业标准DZ/T 0130-2006的要求),相对标准偏差(RSD,n=6)小于5%。

Determination of rare earth elements in tungsten ore and molybdenum ore by inductively coupled plasma mass spectrometry with alkali fusion

The tungsten ore and molybdenum ore are refractory and usually dissolved by alkali fusion method. However, a lot of sodium ions in the flux would be introduced into sample solution and lead to high salinity with the pretreatment by alkali fusion. The high salinity could cause matrix interference and cone-hole blocking, which is disadvantageous to determination of trace rare earth elements by inductively coupled plasma mass spectrometry (ICP-MS).In order to solve this problem, the sample was fused with NaOH and Na₂O₂ at high temperature followed by leaching and filtration with hot water. Then the rare earth elements were enriched into rare earth hydroxide precipitates, realizing the separation from much sodium and other metallic ions such as potassium, tungsten and molybdenum. The precipitate was dissolved with tartaric acid-HCl system. After dilution, the content of rare earth elements in tungsten ore and molybdenum ore was determined by ICP-MS. The results showed that 0.500 0 g of sample could be fully decomposed with 3.0 g of NaOH and 1.5 g of Na₂O₂ in muffle furnace at 700 ℃ for 20 min. The mass spectrometry interference could be eliminated by selecting proper isotopes and mathematic correction methods. The analytical signal drift and matrix effect could be effectively monitored and corrected with 10 μg/L ¹⁰³Rh as internal standard. Under the optimal conditions, the correlation coefficients of calibration curves of all elements were all higher than 0.999 5. The detection limit of method was 0.004-0.08 μg/g. The experimental method was applied to the determination of rare earth elements in component analysis certified reference materials of molybdenum ore and tungsten ore. The absolute values of logarithm error (ΔlgC) between the results and the certified values were all less than 0.1 (the requirements according to industrial standard of DZ/T 0130—2006 for geology and mineral resources). The relative standard deviations (RSD, n=6) were all less than 5%.