激光剥蚀电感耦合等离子体质谱小激光斑束线扫描定量分析技术

摘要

采用激光剥蚀-扇形磁场电感耦合等离子体质谱(LA-SF-ICP-MS)技术建立了小激光斑束(<15μm)线扫描定量分析方法.对比了硅酸盐矿物LA-ICP-MS分析中不同激光进样模式(点剥蚀和线扫描)对于元素信号强度和分馏效应的影响.小激光斑束点剥蚀分析元素信号强度随时间下降明显,并且剥蚀过程中元素深度分馏效应影响明显.深度分馏效应主要是由于各元素倾向于富集在不同粒径颗粒中,而不同大小颗粒在剥蚀坑附近发生冷凝沉淀的几率差异造成.实验结果表明,相对于内标元素Ca,Na、K、Cr、Co、Cd和U等元素富集在更小颗粒中;Cu、Zn、V、Mn、Fe、Ni、Tl、W、Rb、Cs等元素与Ca富集行为相似;Al、Y、Sc、Zr、Nb、Hf、Ta、Th和REE等元素易进入大颗粒中.线扫描分析具有高且稳定的元素信号强度,分析过程中剥蚀行为一致,不受深度剥蚀效应的影响.采用双剥蚀池结构进样系统研究单脉冲激光剥蚀信号结构,不同元素信号强度降低至50％ 需0.8～1.2 s;降低至20％ 需1.2～1.6 s;降低至背景值需2～3 s.本研究通过优化仪器参数降低信号叠加作用的影响,在均质和非均质样品(榍石)线扫描分析中,获得了准确的元素含量和元素比值.线扫描定量分析技术可有效降低激光斑束(≤15μm),相对于采用线扫描元素强度分布研究,数据更加直观,可表现元素比值的变化特征.通过调整激光斑束大小和扫描速度可在不同分辨率尺度下全面了解矿物中元素的分布特征.%Line scanning quantitative analysis method on silicate with small laser beam ( < 15 μm) was developed using laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS). Differences on signal intensity and elemental fractionation induced by different laser sampling patterns were compared. While spot ablation with small laser beam, the elemental signal intensity decreased with time significantly, and the elemental fractionation was obvious. In contrast, the elemental signal intensity by line scanning was higher and more stable and line scanning was free of elemental fractionation. Therefore, identical ablation pattern and condition should be used for the standard and the unknown sample in LA-ICP-MS quantitative analysis. A single pulse experiment was carried out to investigate the washout time when coupled to two-volume ablation cell. The result indicated that the elemental intensity decayed to the background value needed 2-3 s. The optimal parameters on SF-ICP-MS were set to reduce the effect of signal overlapping. Homogeneous sample KL2-G and titanite grains with composition zoning were analyzed by this method. Accurate element contents and element ratios indicated that fast washout time and optimal instrument parameters made it feasible to perform line scanning quantitative analysis accurately. Comparing to traditional microanalysis, line scanning quantitative analysis could reduce the laser beam size (<15 μm) and improve the spatial resolution efficiently. The potential of the technique to unveil compositional complexities in greater detail would help to improve our understanding of geochemical processes in mineral scale.