硼同位素分析测试技术研究进展

A Review on the Development of Boron Isotope Analytical Techniques

硼(B)是一个质量较轻的流体活动性元素。它有2个稳定同位素:10B和11B,两者之间相对质量差较大,导致自然界显著的硼同位素分馏。因此,硼同位素作为强有力的非传统稳定同位素示踪工具,在化学、环境、生物、地球及行星科学等研究领域具有广泛的应用。近二十年来,国内外硼同位素分析测试技术不断改进并取得了诸多重要进展。然而,获取高质量硼同位素数据,在样品消解、分离纯化以及质谱测试三个主要环节中仍然存在很多挑战。因为硼具有易挥发性及其在不同pH值环境中因配位不同导致同位素分馏,样品消解和分离纯化对硼同位素准确测量有很大影响。样品消解法主要有高温水解法、酸溶法、碱熔法和灰化法,其中酸溶法与碱熔法是最常用的方法。分离纯化法主要包括离子交换法、硼酸甲酯蒸馏法和微升华法。这些样品前处理方法各有利弊。质谱测试方法主要有两类:一类是溶液法,即热电离质谱法(TIMS)或多接收电感耦合等离子体质谱法(MC-ICP-MS);另一类是微区原位分析法,即二次离子质谱法(SIMS)或激光剥蚀法(LA)-MC-ICP-MS。不同的测试方法对样品前处理要求不同:溶液法要求去除基质;微区原位分析法要求样品与标样的成分匹配。这些测试方法也存在不同技术挑战:TIMS分析过程中容易产生同位素分馏。而SIMS和LA-MC-ICP-MS分析过程中存在缺少标准样品、样品表面污染、低含量样品精度有限及高含量样品重现性差等问题。基于MC-ICP-MS测量低含量样品中硼同位素的独特优势,本文深入探讨了基体效应、记忆效应和质量歧视效应三方面的现存挑战,通过梳理文献和数据对比,在总结现有硼同位素地球化学研究方法的基础上提出一些分析测试技术发展方向的建议。

Boron(B)is a light and fluid-mobile element.It has two stable isotopes:10B and11B.The two isotopes fractionate significantly in nature due their relatively large mass difference.Therefore,B isotopes are one of the non-traditional stable isotope tracers,which have been used in the research areas of chemistry,environmental,bioscience,earth and planetary sciences.In the last twenty years,the analytical methods of B isotopes have been continuously improved and many important advances have been made.However,there are still some challenges to obtaining high-quality B isotope data.The techniques of B isotope analysis are quite different among laboratories,which arise principally from three stages:sample digestion,purification,mass spectrometry.Because B is volatile and isotopic fractionation may be induced by different coordination in different pH environments,sample digestion,and purification have a great impact on the high-precision measurement of B isotopes.Four digestion methods have been applied for extracting B from samples,including pyro-hydrolysis,acid dissolution,alkali fusion,and ashing.Pyro-hydrolysis requiring large volumes of water is time-consuming.Acid dissolution is one of the most popular techniques due to the small volumes of reagents needed and hence lower levels of contamination.Samples are dissolved with different acids such as hydrochloric,nitric,hydrofluoric,and perchloric.Painstaking attention is required with hydrofluoric acid since BF3is highly volatile and easily lost in nature.Suitable amounts of mannitol are added during acid dissolution to form a stable boron-mannitol complex to prevent the loss of B and avoid B isotope fractionation.Alternatively,alkali fusion is a dissolution method for solid rock samples.High purity fluxing agent is needed,such as K2CO3,Na2CO3,NaOH,NaOH,and Na2O2.As all the B would be present as borate in the resulting alkaline solution,alkali fusion eliminates the risk of B isotope fractionation due to evaporation.The advantage of this method is that it is rapid and relatively large numbers of samples can be processed.The ashing is mainly used to digest plant samples.Ashing was chosen for plant sample decomposition because ashing removes the organics and avoids the use of reagents carrying a B blank or generating isobaric interferences.Once a sample is dissolved,it is necessary to purify B before analysis.There are two principal methods currently in use,which are ion exchange and microsublimation.The ion exchange techniques can be divided into those involved in using B-specific resin Amberlite IRA 743 and those using cation(AG50W-X8/AG50W-X12)or anion(Bio-Rad AG MP-1)cation exchange resins.Microsublimation is an effective and simple method to purify B.It is used to purify B from organic-enriched solutions.Microsublimation appears advantageous in terms of matrix removal efficiency and low procedural blank,however the technical challenges involved are also great.There are two main types of B isotope analytical methods:in-situ and solution methods.Solution methods analyse B ratios using thermal ionization mass spectrometry(TIMS)method or multiple collector inductively coupled plasma-mass spectrometry(MC-ICP-MS).The in-situ method,uses secondary ion mass spectrometry(SIMS)method or laser ablation multiple collector inductively coupled plasma-mass spectrometry(LA-MC-ICP-MS)to measure samples with high B concentration.The accurate and precise determination of the B isotope composition is still a difficult task.For solution methods,the difficulty arises principally from the near ubiquitous level of B contamination in most standard clean laboratories,the light mass of the element,the occurrence of only two stable isotopes,and the large mass difference between them.For in-situ approaches,the difficulty arises principally from a lack of reference materials,surface contamination,limited precision in low-concentration samples,and limitations in reproducibility in high-concentration samples.On the whole,MC-ICP-MS is the dominant method for B isotopic analysis,which is still has the challenges of matrix effect,memory effect,and mass bias.The relevant techniques inherent to the three stages of B isotope analysis are summarized and the advantages and disadvantages of the different techniques are discussed.The aim of the work contained in this paper is to further promote the progress and development of domestic and foreign scholars in the research of B isotope geochemistry.