电感耦合等离子体串联质谱法测定煤灰中痕量稀土元素

Determination of Trace Rare Earth Elements in Coal Ash by Inductively Coupled Plasma Tandem Mass Spectrometry

煤灰是稀土等战略性关键金属资源的重要资源来源之一,目前国内外尚未建立煤中稀土元素的标准分析方法。相比其他类型的传统稀土矿产,煤灰基体组成更为复杂。针对现有酸湿法消解-电感耦合等离子体质谱(ICP-MS)测试煤灰中稀土元素存在的前处理效率低、质谱干扰及其消除方法繁琐复杂、测试结果不够准确等问题,提出一种基于石墨消解-(动态反应池)电感耦合等离子串联质谱(ICP-MS/MS)的煤灰中微量稀土元素快速、准确测定含量的方法。通过比较不同消解体系(硝酸、硝酸-盐酸、硝酸-氢氟酸、硝酸-盐酸-氢氟酸)与消解温度(100、140、180℃)对测试结果的影响,发现以硝酸-盐酸-氢氟酸为消解介质、140℃温度条件下煤灰消解效果最好,氢氟酸的加入可以显著提升稀土提取效率,二次加水消解赶去HF避免了难溶氟化物的生成及HF对质谱测试的不利影响。ICP-MS/MS测定过程中分别将Rh、Re元素作为内标在线加入补偿基体效应,通过对比无气体质量原位模式(M-SQ-N/A)、氦气碰撞和动能歧视模式(M-SQ-KED)、氧气质量转移模式(M-TQ-O_(2))、氨气质量转移模式(M-TQ-NH_(3))四种测定模式,发现M-SQ-KED模式下测试值更接近真实值,质谱干扰最小,各元素测试回收率为92.49%~112.88%,检出限低于0.0052μg·g^(-1),相对标准偏差低于1.71%。以煤灰测定过程中常见的Ba对Eu干扰为例,模拟高Ba本底溶液中Eu测试,揭示出M-SQ-KED模式通过反应气与待测元素和干扰离子碰撞概率不同可极大程度减弱多原子离子干扰。将以上方法应用到我国不同电厂燃煤飞灰样品分析,测定得到的稀土标准化曲线分布均一平滑,表明建立的方法稳定、可靠,相比于微波消解前处理,具有成本低、操作简便、效率高可大批量测定等优点;而动态反应池技术可在线消除质谱干扰,极大提高了工作效率。

Coal ash is one of the most important alternative rare earth elements(REEs)sources to conventional REEs ores.Standard methods for determining REEs in coal ash have still not been established.Compared with other traditional ores,the chemistry and mineralogy of coal ash are more complicated.The reported analytical methods have severe problems in the pretreatment process(low efficiency)and are difficult to reduce the mass interferences when using inductively coupled plasma mass spectrometry(ICP-MS)as an analytical tool to detect REEs in the liquid digested sample.This study proposed a new efficient and accurate REEs determination method based on graphite digestion-(dynamic reaction cell)inductively coupled plasma tandem mass spectrometry(ICP-MS/MS).By comparing the effects of different digestion reagents(nitric acid,nitric acid-hydrochloric acid,nitric acid-hydrofluoric acid,nitric acid-hydrochloric acid-hydrofluoric acid)and different digestion temperature(100,140,and 180℃)on the test results,it was found that the HNO_(3)-HCl-HF digestion at 140℃was the best pretreatment method.The addition of HFcould significantly improve the extraction efficiency of REEs,and the secondary addition of water to eliminate HF avoided the formation of insoluble fluoride and hydrofluoride,which decreased adverse effects of F^(-1) on mass spectrometry.ICP-MS/MS measurement process,Rh and Re elements were added online as internal standards to compensate for the matrix effect,respectively.By comparing the gas-free mass in situ mode(M-SQ-N/A),helium collision and kinetic energy discrimination mode(M-SQ-KED),oxygen mass transfer mode(M-TQ-O_(2)),ammonia mass transfer mode(M-TQ-NH_(3)),it was found that the detected value in M-SQ-KED mode is closer to the standard value with minimum mass spectrum interference.The recovery rate of REEs(compared to values reported in standard coal ash)is 92.49%~112.88%,the detection limit is lower than 0.0052μg·g^(-1),and the relative standard deviation is less than 1.71%.Tracing Eu in a high-Ba solution revealed that the M-SQ-KED mode can greatly reduce the ion interference because the He was likely to collide with interfering polyatomic ions with a larger collision probability than that of the target analytes.Finally,the established method was applied analyse fly ash samples from different coal-fired power plants.The results showed that the distribution of the normalized REE curves(relative to UCC)was uniform and smooth,indicating that the results were stable and reliable.Compared with microwave digestion,the proposed method has the advantages of low cost,simple operation,and high efficiency and can be used for vast amounts of ash sample determination.The dynamic reaction cell technology can eliminate the interference of mass spectrometry online,which greatly improves the determination efficiency.