地质样品中痕量贵金属的火试金富集与测定技术研究进展

Research Progress in Fire Assay Preconcentration and Determination Techniques for Trace Precious Metals in Geological Samples

  • 摘要: 贵金属(金和铂族元素)在自然界中丰度极低、分布不均,且易产生“粒金效应”,长期以来对其准确测定一直是分析领域的难点。火试金法凭借取样量大、样品代表性好等优势,已成为地质样品中痕量贵金属分析的主要前处理方法之一。近年来,地质分析工作者在降低火试金全流程空白值、多元素同时测定、干扰元素校正及改善检出限等方面开展了大量研究,显著提升了整体分析效率。本文系统评述了铅试金、镍锍试金、铋试金、锑试金、锡试金、铜试金等前处理方法的优势与局限;探讨总结了灰吹保护剂的研究现状及作用机理;归纳比较了火试金与电感耦合等离子体质谱法(ICP-MS)、激光剥蚀电感耦合等离子体质谱法(LA-ICP-MS)、电感耦合等离子体发射光谱法(ICP-OES)及原子吸收光谱法(AAS)等联用测定技术的特点。在火试金中,铅试金、镍锍试金与锑试金法捕集效果好,但毒性大、流程空白较高,且锑扣后续除杂分离困难;铋试金、锡试金法毒性低、空白低,但铋试金成扣效果较差,锡扣处理繁琐;铜试金法成扣效果良好,但扣体坚硬难处理,基体铜干扰严重且校正流程冗长。对于灰吹保护剂,单一贵金属保护剂虽可提升目标元素的富集效果,但难以实现多种贵金属同时测定;不完全灰吹法(Incomplete Cupellation)的过程终点不易控制,灰吹效果高度依赖操作人员经验;而复合保护剂在增强保护效果的同时,会引入额外基体,增加仪器干扰风险。在仪器分析方面,ICP-MS灵敏度高,可实现多元素同时分析,但存在质谱干扰问题,可通过碰撞/反应池等技术有效校正;LA-ICP-MS虽可避免溶液进样干扰,却严重依赖基体匹配标准物质,且仍存在基体效应与元素分馏;ICP-OES线性范围宽、抗干扰能力强,但检出限较高;传统线光源AAS设备相对简单、分析成本低,但分析通量(尤指石墨炉法)和抗干扰能力有限;而新型高分辨率光源AAS则在抗干扰能力和多元素分析效率方面显著提升,但设备复杂度与成本也相应增加。在此基础上,本文对火试金富集与测定方法的未来发展方向进行了展望,指出应重点加强绿色低毒捕集剂研发、固体进样技术应用、全流程自动化及分析方法标准化等方面的研究。

     

    Abstract: Precious metals, including gold (Au) and platinum-group elements (PGEs), are characterized by extremely low abundances, heterogeneous distribution and susceptibility to the “nugget effect” in natural geological matrices, making their accurate quantitative determination a long-standing challenge in the field of analytical geochemistry. Fire assay (FA) has established itself as one of the primary pretreatment methods for trace precious metal analysis in geological samples, offering distinct advantages such as large sample intake and excellent sample representativeness. This review systematically evaluates the merits and limitations of six prominent fire assay approaches: lead, nickel sulfide, bismuth, antimony, tin and copper fire assay. Furthermore, it discusses and summarizes the current research status and mechanisms of cupellation protective agents. A critical comparison is presented of the analytical techniques hyphenated with fire assay, including inductively coupled plasma-mass spectrometry (ICP-MS), laser ablation ICP-MS (LA-ICP-MS), inductively coupled plasma-optical emission spectrometry (ICP-OES) and atomic absorption spectrometry (AAS). Among the FA methods, lead, nickel sulfide and antimony fire assay exhibit superior precious metal collection efficiency but are plagued by high toxicity and elevated procedural blanks; moreover, impurity removal and separation from antimony buttons are particularly difficult. In contrast, bismuth and tin fire assay offer low toxicity and low blanks but are constrained by poor button formation integrity (for bismuth) and cumbersome post-processing procedures (for tin). Copper fire assay produces buttons of satisfactory integrity yet results in intractable buttons, severe copper matrix interference and lengthy signal correction procedures. Regarding cupellation protective agents, single precious metal agents can improve the enrichment efficiency of specific target elements but are insufficient for the simultaneous determination of multiple precious metals. The incomplete cupellation suffers from poor endpoint controllability and is highly dependent on operator experience. However, while composite agents enhance the overall protective effect, they inevitably introduce additional matrix components, increasing the risk of instrumental interferences. In terms of instrumental analysis, ICP-MS provides ultra-high sensitivity and multi-element capability but remains vulnerable to spectral interferences, particularly from polyatomic ions, which can be effectively corrected using collision/reaction cell technologies. LA-ICP-MS eliminates solution-based matrix interferences but is strongly dependent on the availability of matrix-matched reference materials and is still subject to inherent matrix effects and elemental fractionation. ICP-OES features a wide linear dynamic range and strong resistance to spectral interferences but is limited by relatively high detection limits. Conventional line-source AAS offers simple instrumentation and low operational cost, but suffers from limited analytical throughput (especially graphite furnace AAS) and poor anti-interference performance. In contrast, novel high-resolution continuum-source AAS exhibits significantly improved anti-interference ability and multi-element analytical efficiency, with increased instrumental complexity and corresponding cost. On this basis, this review outlines the future development directions of fire assay-based enrichment and determination methods. Emphasis is placed on the development of green and low-toxicity collectors, the application of solid sampling techniques, full-process automation and the advancement of analytical standardization.

     

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