Determination of 42 Trace Elements in Silicate Glass Reference Materials by High Spatial Resolution LA-ICP-MS
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摘要:
近年来，随着人们对关键金属（稀有金属、稀土金属、稀散金属和稀贵金属）的成矿机制、分布规律和绿色利用等研究日益加深，建立原位测定地质样品中关键金属元素（如REEs、Cr、Co、Ga、Ag、Cd、In、W、Tl等）分析方法对于研究关键金属元素的地球化学行为、分布规律和成矿机制具有重要意义。由于关键金属在地壳中丰度极低（一般为ng/g～μg/g级别），赋存矿物非常细小（粒径μm级别），因此需要建立高空间分辨率微区原位分析技术实现低含量（ng/g～μg/g）微量元素的定量。本文提出了高频剥蚀模式与Ar-N2等离子体技术相结合提升LA-ICP-MS对微量元素的检出能力，使之能够满足地质样品中关键金属元素的检测需求。结果表明：在Ar-N2等离子体条件下，采用高频（20Hz）剥蚀模式，LA-ICP-MS分析中大部分元素灵敏度提升了1.5～9倍。在使用高灵敏度X型截取锥时，高频剥蚀模式与氮气增敏技术相结合可以显著减小氧化物产率和降低U-Th分馏，获得更宽的载气流速区间（0.9～1.08L/min）以满足测试的仪器分析条件（ThO+/Th+＜0.5%和U/Th=1）。本研究开发的高空间分辨率LA-ICP-MS关键金属分析方法具有较低的检出限（在剥蚀束斑24μm条件下，30种元素的检出限＜0.02μg/g），在高空间分辨率（10～24μm）条件下，通过对8种国际硅酸盐玻璃标准物质中42种微量元素进行定量分析，34种微量元素的测试结果的准确度优于10%，精密度优于15%，实现了在高空间分辨率条件下对微量元素的准确定量分析。
Abstract:BACKGROUND Critical metal elements are a group of metal elements including rare metal elements (e.g., Li, Be, Rb, Cs, Nb, Ta, Zr, Hf, W), rare earth elements (REEs), rare disperse elements (e.g., Ga, Ge, Se, Cd, In, Te, Re, Tl) and rare precious elements (e.g., PGE, Cr and Co), which are important for the development of emerging industries. In recent years, the critical metal elements have shown great economic characteristics in emerging industries such as advanced materials, new energy resources and national defense and military industry uses, which is important strategic significance for the development of the national economy and technology. Therefore, it is necessary to investigate the geochemical properties and metallogenic mechanism of critical metal elements. How to accurately determine trace elements in geological materials is a prerequisite for these investigations. Critical metal elements in geological materials can be determined by conventional chemical wet digestion methods. However, chemical wet digestion methods can only obtain an average chemical composition without spatial distribution information of critical metal elements. Compared to digestion methods, in situ microanalysis technology can obtain micrometer scale elemental distribution in silicate minerals, omit tedious chemical processing processes and avoid the use of a large amount of chemical reagents. However, the abundance of critical metal elements in the crust is low (μg/g level) and the carrier minerals containing critical metal elements are at the micrometer scale. Therefore, it is necessary to establish a high spatial resolution in situ analysis technique to determine trace elements in geological materials.
OBJECTIVES To improve sensitivity of LA-ICP-MS for the determination of critical metal elements (ng/g-μg/g level) in silicate minerals by high-frequency ablation mode combined with Ar-N2 mixed plasma technique.
METHODS Experiments were carried out using a single collector ICP-MS (Element XR Thermo Fisher Scientific, Bremen, Germany) in combination with a 193nm excimer laser ablation system (GeoLas 2005, Lambda Physik, Gttingen, Germany) at the Ministry of Natural Resources Key Laboratory of Gold Mineralization Processes and Resources Utilization. The X skimmer cone was used to improve sensitivity of ICP-MS. To obtain high sensitivity and reduce oxide interference, a small amount (0-10mL/min) of nitrogen was added into the carried gas, downstream from the ablation cell by a T junction. The ablation frequency was 5Hz or 20Hz. The ablation spot size was 10-24μm. Each measurement consisted of 18s of acquisition of the background signal, followed by 10s ablation signal acquisition. The washing time was 20s between each measurement. The standard reference materials NIST 610, NIST612 and NIST614 were used as calibration standards. The comparison of signal intensity, sensitivity, oxide yield and U/Th ratio in LA-ICP-MS were investigated at low and high frequency ablation modes in Ar plasma or Ar-N2 mixed plasma. Before testing, the signal of 232Th and 238U were higher than 1×106cps when ablating NIST 612 at 24μm. Moreover, U/Th was close to 1 and ThO+/Th+ was lower than 0.5%. At optimum condition, an in situ elemental quantitative method with high spatial resolution (10-24μm) was established to determine 42 trace elements in MPI-DING and USGS silicate glass reference materials.
RESULTS Sensitivity in LA-ICP-MS is the primary factor for the elemental quantitative analysis with high spatial resolution. Compared to Ar plasma, sensitivities of most elements were improved by a factor of 1.5-9 when using Ar-N2 mixed plasma at high-frequency (20Hz) ablation mode. In LA-ICP-MS analysis, analytical results can be influenced by oxide yield and elemental fractionation. When using X skimmer cone in SF-ICP-MS, the oxide yield and elemental fractionation was significantly reduced in Ar-N2 mixed plasma at high-frequency (20Hz) ablation mode. There was a wide range of carrier flow rate (0.9-1.075L/min) for obtaining good analysis conditions (ThO+/Th+<0.5% and U/Th=1). The limits of detection for 30 trace elements were lower than 0.02μg/g when ablation spot and ablation frequency were at 24μm and 20Hz, respectively. At optimum conditions (ablation spot 10-24μm and ablation frequency 20Hz), 42 trace elements in MPI-DING and USGS silicate glass reference materials were analyzed by LA-ICP-MS in Ar-N2 mixed plasma. The accuracy of analytical results for 34 trace elements was better than 10% and the precision was better than 15%, which suggested high-frequency ablation mode combined with Ar-N2 mixed plasma technique can be used to achieve the determination of critical metal elements (ng/g-μg/g level) in silicate minerals with high spatial resolution.
CONCLUSIONS Compared to low-frequency ablation mode, high-frequency ablation mode combined with Ar-N2 mixed plasma technique can improve sensitivity and reduce oxide yield and elemental fractionation. Moreover, the analysis time of high-frequency ablation mode is very short, which can improve the analysis efficiency of LA-ICP-MS. Due to the high sensitivity and high spatial resolution of LA-ICP-MS at high-frequency ablation mode, LA-ICP-MS can be applied to quantify trace elements and complex internal chemical compositions in micrometer level minerals, such as distribution of trace elements in mineral growth zones. Furthermore, this high-frequency ablation mode may also be applied to the development of in situ accessory mineral dating and isotope ratio analysis.
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图 1 不同剥蚀频率（5Hz和20Hz）和等离子体条件（N2=0或 10mL/min）单点剥蚀NIST612产生的元素139La信号随氩气流速（a）和剥蚀时间（b）的变化情况
Figure 1.
图 3 不同剥蚀频率（5Hz和20Hz）和等离子体条件（N2=0或 10mL/min）下Th信号强度、氧化物产率（ThO+/Th+）和分馏效应（U/Th）随着载气气流的变化
Figure 3.
图 5 在剥蚀束斑为10μm （a）、16μm （b）和24μm （c）条件下采用高频剥蚀（20Hz）模式结合Ar-N2等离子体分析获得硅酸盐玻璃标准物质MPI-DING和USGS中42种微量元素分析结果的相对偏差（n=15）
Figure 5.
表 1 LA-ICP-MS仪器参数
Table 1. Instrumental operating conditions
高分辨电感耦合等离子体质谱
（Thermo Scientific Element XR）激光剥蚀系统
（Geolas 2005准分子激光器）参数 工作条件 参数 工作条件 RF 功率 1200W 波长 193nm 冷却气（Ar）流速 16.00L/min 脉冲宽度 15ns 辅助气（Ar）流速 0.8L/min 能量密度 14J/cm2 载气（Ar）流速 0.9～1.5L/min 剥蚀束斑（直径） 10μm、16μm、24μm 采样锥和截取锥类型 标准采样锥+X型截取锥 剥蚀频率 5Hz、20Hz 分辨率 M/ΔM=300 载气（He）流速 0.54L/min 检测器模式 Triple 同位素个数 79 每个元素积分时间 0.010s 总积分时间 1.2s 
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