Research Progress on the Geological Application of a Flow-Through Time-Resolved Analysis System
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摘要:
流通式-时间分辨分析系统(简称FT-TRA)是二十一世纪初新发展起来的一种快速反应(溶解)-在线分析系统,由淋洗液混合单元、反应单元与分析单元组成,核心功能是通过特定流动相淋洗样品池中的微量样品,分离或去除样品中的特定组分,并监测样品不同元素和矿物组分的出溶特征,实现高分辨的在线过程分析。本文综述了FT-TRA系统的技术原理与软硬件组成、实验方法与操作要点及地质应用发展过程,重点对该系统在地质应用过程中出现的争议点进行阐释与分析,并基于其发展现状展望其未来发展方向与潜力。FT-TRA系统目前主要的地质应用包括古海洋学与古环境学研究代用指标的验证(如有孔虫、介形虫淋洗)、矿物溶解过程与反应动力学研究、环境样品的元素相态分析等。FT-TRA系统以溶解并提取有孔虫/介形虫壳体的元素组成信号作为还原古海洋指标的重要手段,与传统批处理法相比,该方法被认为具有实时监测清洁程度、降低损失率并实现差异溶解的优势,能够获取更精细的壳体化学组成信息;测试矿物溶解态与溶解参数也是FT-TRA系统的重要功能之一,该系统的实验室模拟能够与模型结合探究不同类型矿物在稳态下的溶解动力学,为研究矿物在自然状态下的溶解过程提供启示;近些年来该系统还逐渐被用于矿物反应性测试,其中将气相CO2作为淋滤液与矿物反应的研究可能在全球变暖及CO2的人工捕捉课题上具有潜在应用价值。FT-TRA系统运行中涉及的不同组分的溶解机制是其应用过程中亟待解决的重要问题,进一步完善其溶解动力学原理必然将为该系统的未来发展提供更多如多类型地质样品溶解、矿物的模拟合成等新思路。
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关键词:
- 流通式-时间分辨分析系统(FT-TRA) /
- 在线地球化学分析 /
- 地质应用 /
- 环境研究
要点(1) FT-TRA是一种基于自动色谱基础,以“流动淋洗(溶解),实时监测”为优势的快速反应-分析系统,适用于多类型流动相与固定相的反应分析。
(2) FT-TRA的主要地质应用包括古海洋学与古环境研究代用指标的验证、矿物溶解过程与反应动力学研究、环境样品的元素相态分析等。
(3)目前FT-TRA发展过程中的争议与难点是对于其内部溶解动力学原理的解释,与其他原位技术手段比较并完善基础理论是拓展其未来发展方向的基础。
HIGHLIGHTS(1) FT-TRA is a fast reaction-analysis system based on automatic chromatography, with the advantage of “flow leaching (dissolution), real-time monitoring”, which is suitable for the reaction analysis of multiple types of mobile phase and fixed phase.
(2) The main geological applications of FT-TRA include the verification of proxy indexes for paleoceanography and paleo-environment research, the study of the mineral dissolution process and reaction kinetics, and the analysis of the elemental phase of environmental samples.
(3) At present, the controversy and difficulty in the development of FT-TRA is the explanation of the principle of its internal dissolution kinetics. The comparison with other in situ technical means and the improvement of the basic theory are the basis for expanding its future development direction.
Abstract:FT-TRA is a rapid reaction (dissolution)-on-line analysis system newly developed in the early 21st century. It consists of an eluent mixing unit, a reaction unit and an analysis unit. Its core function is to wash trace samples in the sample reactor with a specific mobile phase, separate or remove specific components in the sample, and monitor the exsolution characteristics of different elements and mineral components of the sample to achieve high resolution online process analysis. In this article, the technical principle, hardware and software composition, experimental method, operation key points and geological application development process of the FT-TRA system are reviewed. The controversial points in the geological application of the system are explained and analyzed, and its future development direction and potential are forecasted based on its development status. At present, the main geological applications of this system include the verification of proxy indicators for paleoceanography and paleo-environment research (such as foraminifera and ostracod leaching), the study of the mineral dissolution process and reaction kinetics, and the analysis of the elemental phase of environmental samples. The dissolution mechanism of different components involved in the operation of the FT-TRA system is an important problem to be solved in its application process. Further improvement of its dissolution dynamics principle will inevitably provide more new ideas for the future development of the system, such as multi-type geological sample dissolution and mineral simulation synthesis. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202304130048.
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Keywords:
- flow-through time-resolved analysis system (FT-TRA) /
- online geochemical analysis /
- geological applications /
- environmental research
BRIEF REPORTThe flow-through time-resolved analysis system (FT-TRA) system, developed in the early 21st century, is a rapid reaction-analysis tool designed to dissolve, separate, or remove targeted components in natural samples using a specific mobile phase (reaction liquid or eluent) [1]. It was created to support the development and verification of geochemical proxies related to paleo-ocean and paleo-environment studies and to address geochemical dynamic processes. Currently, the FT-TRA system is mainly used for cleaning and dissolving microbiological shells, like those of foraminifera and ostracods, and for monitoring their exsolution components as indicators of paleo-oceanographic and paleoenvironmental conditions. It has also proven useful for measuring dissolution parameters of minerals like forsterite[2-4] and testing mineral reactivity[5]. The system’s key advantages are its “flow response” and “real-time monitoring” capabilities, though understanding the dissolution mechanisms of different components remains a challenge.
Since the FT-TRA system is currently in a self-fabricated stage and lacks commercial availability, it has not yet been widely adopted by the scientific community, especially in China, where foundational research is limited. In recent years, our research team has collaborated with international experts to pioneer the system’s development and conduct exploratory studies. To promote its broader application in China, this article reviews the system’s technical principles, hardware and software components, experimental methods, operational procedures, and application development, while also analyzing the current challenges in its use.
1. Main technical principles and components of FT-TRA
The FT-TRA system is an integrated platform designed around the principles of “flow cleaning (dissolution) and real-time monitoring”. It comprises three primary components: an eluent mixing unit, a reaction unit, and an analysis unit. The eluent mixing unit regulates the composition and concentration of the eluent, which is then continuously introduced into the reaction unit containing the solid sample. The resulting product from this reaction is mixed with an internal standard in a tee and subsequently directed to the analysis unit for measurement.
The core hardware components of the FT-TRA system include the gradient pump, the reaction cell, and the analytical instrument (or fraction collector). In offline test mode, the fraction collector is employed to sample the continuous flow at regular intervals. In online test mode, the eluate after reaction is continuously pumped into the analytical instrument, which measures and provides component information at fixed time intervals. This data acquisition is integrated with the time-resolved analysis (TRA) mode of quadrupole-ICP-MS to facilitate comprehensive analysis.
2. Experimental methods of FT-TRA system for geological sample analysis
The stability and reproducibility of the FT-TRA system are fundamental to ensuring the accuracy and reliability of test data, with appropriate standard samples serving as crucial evaluation tools. For FT-TRA, the selection criteria for standard samples should include: (1) solid standard samples; (2) homogeneity; and (3) alignment with the main mineral composition of the geological samples being studied. Currently, there is no standardized sample universally adopted for this system. For instance, in the study of foraminifera using FT-TRA, standard samples utilized include North American Shale (NASC)[1], NIST 1c Marl[18], PDB[19], and ECRM752-1 Limestone[20-23]. These selected standard samples generally exhibit chemical compositions similar to the study samples and are widely used in elemental analysis. Given that samples with varying physical and chemical properties impact FT-TRA stability differently, it is essential to thoroughly explore and evaluate different types of geological samples by selecting appropriate standard samples before conducting formal testing.
The FT-TRA system is applicable to the dissolution and compositional analysis of various solid samples, primarily including biological shell samples and mineral samples, each requiring distinct selection and preparation procedures. In paleo-oceanographic and paleo-environmental studies, foraminifera, ostracod, and other biological shells are analyzed to serve as proxies. For these applications, it is crucial to ensure that sample size and quantity are controlled to maintain data comparability and to accurately obtain the original shell information while avoiding interference from secondary components. Therefore, before conducting FT-TRA dissolution tests on biological shells, samples should be meticulously selected based on size, quantity, and completeness through microscopic characterization and cleaned according to the specific experimental objectives[1,7,19-20,24-26]. In mineralogy, the FT-TRA system is primarily used to assess mineral dissolution parameters and reactivity. To explore dissolution kinetic parameters, the pre-treatment process typically involves grinding, cleaning, verifying mineral phase purity, determining stoichiometric numbers, and measuring surface area. For studies on mineral reactivity, the pre-treatment procedures should be tailored to align with the specific goals of the experiment.
The analysis mode and adjustment of test parameters for the ICP-MS and FT-TRA systems are critical application aspects, with the sample flow rate being a particularly significant parameter. The sample flow rate is governed by the eluate flow rate within the reaction cell, which in turn influences the dissolution kinetics of the sample[4]. A flow rate that is too high may prevent adequate contact between the sample and the eluate, leading to incomplete reactions, while a flow rate that is too low can cause the dissolution products to accumulate in the reaction cell, thereby not accurately reflecting the real-time dissolution of chemical components. Existing literature lacks detailed recommendations on optimal flow rates. Currently, Haley et al.[1] reported that the eluate flow rate entering the sample pool was set at 4mL/min but did not provide a detailed rationale for this choice. In our laboratory, we conducted exploratory experiments using dilute nitric acid (pH=2) to dissolve foraminifera shells at various flow rates. The results of these experiments are illustrated in Fig.4, although the underlying mechanisms require further investigation.
3. Progress in geological application of FT-TRA
The FT-TRA system was developed from the study of foraminifera leaching and the verification of environmental proxies. Sedimentary foraminifera are central to paleo-oceanographic and paleo-environmental research. An essential requirement for accurately extracting paleo-ocean and paleo-environmental information from foraminifera is to obtain biogenic calcite data formed during the organism’s lifetime, while minimizing interference from post-burial pollution and secondary composition[41]. To address this, Boyle et al.[42] introduced the traditional batch method in the 20th century, which has since undergone continuous refinement and optimization[26,29,42-44]. Although this method can somewhat remove contaminants, it often results in sample loss and poor reproducibility due to extended cleaning times and manual handling. Moreover, it is challenging to verify the complete removal of contaminants and to distinguish between original and secondary calcite. In response to these limitations, Haley et al.[1] from Oregon State University designed the FT-TRA system in the early 21st century. This system enables real-time monitoring of the cleaning process for foraminifera shells, helps prevent the loss of key mineral phases, and mitigates the re-adsorption of rare earth elements during dissolution[6]. Experimental results demonstrate that the FT-TRA system effectively monitors the cleaning process of foraminifera in real time and can adjust for contamination based on concentration fluctuations of elements such as Al and Fe during dissolution (Fig.5). Furthermore, the system is capable of differential dissolution according to the solubility of different mineral phases, thereby facilitating the extraction of the most original biological calcite information.
However, further studies by other teams found that the kinetic behavior of foraminifera shell dissolution was complex, and more rigorous controlled experimental methods were designed to investigate the cleaning and “differential dissolution” effects of the FT-TRA system. Compared with the traditional batch method and in situ methods such as EPMA, SEM and SIMS, there is no selective dissolution of Mg-enriched components of foraminifera shell under natural environmental or laboratory simulation conditions, and the samples follow the dissolution order from outside to inside in most cases. Although these conclusions affirm the reliability and convenience of the FT-TRA system in the rapid analysis of foraminifera composition[53,56-58], they also reveal the complexity of the application of the FT-TRA system to biological shell leaching and component analysis.
Subsequently, de Baere[4]explored the dissolution characteristics of two different minerals, forsterite and calcite, within the FT-TRA system, explaining the dissolution kinetics model from both theoretical and experimental perspectives. de Baere[2]conducted targeted experiments using calcite and aragonite to test the ideas of Haley and Klinkhammer[1], confirming that the FT-TRA system has limitations in achieving differential dissolution of foraminifera and distinguishing between calcite of different origins. However, the study also demonstrated the system’s applicability in measuring mineral dissolution rates, dissolution parameters, and stoichiometry under both steady-state and transient leaching conditions. It proved that the FT-TRA system can be an effective supplementary tool for studying mineral dissolution.
As the dissolution kinetics theory within the FT-TRA system gradually improved, the system began to be used to explore the potential of ostracod shells as paleoenvironmental indicators[22,71-72]. Additionally, Power et al.[5] utilized a NaCl solution equilibrated with CO2 gas as a leachate to test the reactivity of tailings with CO2 in the FT-TRA system, aiming to assess the potential of these deposits to sequester CO2 by directly capturing it from the atmosphere. This research aligns with international academic trends and offers an innovative approach to the application of the FT-TRA system, holding significant practical significance and reference value.
4. Summary and outlook
The FT-TRA system exhibits certain unique features in the extraction of paleo-environmental and paleo-oceanographic signals, as well as in the testing of mineral dissolution parameters. However, its major limitation lies in the lack of a comprehensive theoretical foundation. Currently, the only available theory is de Baere’s[2] proposition of differential dissolution states of phases with varying solubility within the FT-TRA system, but this theory still fails to fully explain all the phenomena observed in experiments. Future research should focus on a deeper exploration of the dissolution mechanisms within the FT-TRA system. Potential development directions include the dissolution of various geological or synthetic samples, the synthetic modeling of minerals, and the integration of FT-TRA with multiple analytical instruments, including isotope analysis tools.
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流通式-时间分辨分析系统(简称FT-TRA)是在二十一世纪初发展起来的一种快速反应(溶解)-分析系统,其原理基于自动色谱技术,利用特定流动相(反应液或淋洗液,如不同浓度的硝酸等)对固定相(自然样品)中的混合物进行淋洗(溶解),从而达到分离和去除固定相中特定组分的效果[1]。FT-TRA系统的构建源于古海洋、古环境等相关地球化学代用指标开发验证的需求以及地球化学动力学过程的迫切需要,目前主要应用于清洗和溶解有孔虫、介形虫等微体生物壳体,并监测其出溶成分作为古海洋、古环境指标,另外也被证明可用于部分矿物(如镁橄榄石)的溶解参数测量[2-4]及矿物反应性测试[5]。“流动反应,实时监测”是该系统的最显著优势,其中“流动”能够使其规避传统原位封闭反应的副作用(如元素再吸附[6]等),而“实时”则能够协助研究者快速精准获取样品反应成分变化的信息,将原本以“点”为单位的信息连点成线,因此被认为在水岩反应、表生过程模拟研究中具有一定的应用潜力。
2002年Haley等[1]首次设计搭建FT-TRA系统,作为一种新的清洗与溶解有孔虫壳体的手段。有孔虫作为古海洋古环境研究的重要载体,传统批处理法是其应用较为广泛的前处理手段。因此在开发前期,FT-TRA系统的研究主要聚焦于验证其在提取有孔虫壳体环境信号过程中相对于批处理法的优势。FT-TRA较批处理法可以带来更精细的信息,原因有两点:第一,在传统批处理法中,无论前处理清洗的程度如何,最终的溶解都只能得到“一个点”,这个“点”的数据代表了所有壳体物相的混合信息;而在FT-TRA方法中溶解得到的数据是样品化学组成信息随时间变化的一条曲线,研究者可以根据出溶的实时信息来判断是否有污染相的混入,增加了单一样品的信息量与可分析性;第二,FT-TRA系统是一个开放的溶解体系,可以用溶解动力学去解释样品在分析过程中的溶解行为,例如根据溶剂不变的条件下不同物相出溶时间的差异来进行特定物相信息的提取,对于有孔虫壳体来说或许能够更精准地提取原生生物方解石信息。所以研究者在早期研究中就提出,FT-TRA系统可以展现有孔虫壳体的不均一性,并根据不同成因方解石溶解度的差异针对性地提取其中生物方解石的信号。例如Benway等[7]运用批处理法与FT-TRA法提取相同点位与层位的有孔虫壳体Mg/Ca值并将其运用不同校正公式[8-9]还原为表层海水温度(SST),结果显示FT-TRA法所得温度结果比批处理法高1.5~2.6℃(6.9%~10.9%),更接近数据集记载中当时表层海水温度的真值。造成这一差异的根本原因在于,批处理法在处理一个样品时只会得到一个Mg/Ca值,这个Mg/Ca数据是所有最后溶解在酸中的物相的混合,而FT-TRA得到的是一条随溶解时间变化的Mg/Ca曲线,研究者选取了该曲线前期表现出的Mg/Ca高值作为原生生物方解石的信号,因其认为生物方解石较次生方解石对酸更敏感(即生物方解石在稀酸中的溶解度更高),故在淋滤前期就会出溶,而这个Mg/Ca高值显然与批处理法得到的壳体的Mg/Ca混合值不同,也就造成了还原出来的表层海水温度的差异。
但由于缺乏系统解释FT-TRA内部溶解动力学的研究,其“差异溶解”(即不同溶质因为在相同溶剂中溶解度的大小不同而发生的选择性溶解)的功能在早十年间持续受到质疑,直至2015年de Baere等[3]运用该系统研究不同类型矿物的溶解相与溶解参数,初步从理论层面阐释了碳酸盐相与硅酸盐相在该系统不同pH介质中的溶解行为差异,基本否定了其对于有孔虫壳体的“差异溶解”,但同时也以镁橄榄石为例,强调了其作为研究硅酸盐溶解参数手段的潜力。近十年来,FT-TRA系统的研究领域从古环境学逐渐扩展至矿物学,理论基础逐渐完善,但差异溶解假设被推翻后仍然缺乏对于碳酸盐生物壳体在连续溶解过程中不均一性的解释,而这可能正是利用该系统针对性提取矿物相或分析元素赋存状态的关键。
虽然过去二十多年,国外研究者一直在探索FT-TRA系统在不同研究方向的应用,但相关理论研究仍有待完善,且相关装置都处于自制阶段,没有商业化的产品,因而尚未被科学界广泛采用,尤其在中国缺少相关分析研究基础。过去几年,本研究团队和国际同行合作,率先构建了FT-TRA系统并开展了探索性实验研究。为了更好地推动该分析技术在中国不同研究领域的应用,本文综述了FT-TRA系统的技术原理与软硬件组成、实验方法与操作要点及应用发展过程,分析目前应用中存在的问题,并展望该系统未来在古海洋古环境学指标建立、矿物学参数测试及研究地球表生过程中元素赋存状态的发展潜力。
1. FT-TRA系统技术主要原理与软硬件组成
1.1 FT-TRA系统技术的主要原理
FT-TRA系统是以“流动清洗(溶解),实时监测”为指导概念所搭建的组合式系统,由淋洗液混合单元、反应单元与分析单元构建而成。系统通过淋洗液混合单元控制淋洗液的成分与浓度,而后将淋洗液连续泵入固体样品所在的反应单元中,两者反应后的产物与内标在三通中混合后直接通入分析单元中进行测量(图1)。FT-TRA最大优势是能够进行高分辨的过程分析,给出样品淋洗过程中元素与矿物变化的精细地球化学信息。
1.2 FT-TRA系统的硬件组成与软件控制
FT-TRA系统三个主要部分(淋洗液混合单元、反应单元与分析单元)相应的核心硬件分别为梯度泵、反应池、分析仪器(或馏分收集器)。
(1)梯度泵(Advanced gradient pump,AGP):常被用于分析过程中对流动相液流的比例进行动态调整的部件[11],在FT-TRA系统中的作用是将不同成分的淋洗液按照实验需求进行混合后连续泵入反应池中。高度的重现性及精准的流量控制是这一组件需要达到的常规要求。
(2)反应池(Flow-through reactor):作为FT-TRA系统反应单元的核心,用于承载与淋洗液反应的固体样品。反应池的样式与规模,可根据不同的实验需求进行设计,如直径为13mm的PTFE注射器过滤器配以鲁尔接头与反应池盖托可作为微量样品的反应池,具备25μL的内体积(适用于粉末样品)和50μL的覆盖体积(适用于块状样品)(图2a),而对于体积或数量较大的样品,一般设计以传统流式反应器的造型为基础,内部空间采用聚四氟乙烯材质,同时外部使用铝支架来提供反应池盖和反应池之间的压力(图2b)[2]。反应池设计过程中需重点关注其对外密闭性与对内连通性。
(3)馏分收集器(Fraction collector):与分析仪器分别应用于FT-TRA系统分析单元的离线模式与在线模式。
在离线模式中,系统借助馏分收集器收集反应溶液与样品在反应池中经过反应产生的淋滤液。馏分收集器具备在多个容器中定期采样连续流出物的能力,并能根据固定时间间隔切换容器,且其包含的分流阀会根据需求自动将废水引导至废液桶[12-13]。之后再使用ICP-MS或ICP-OES等分析仪器来得到溶液的各种元素信息[14]。
在线模式以“连续流动溶解,实时监测”为特点,反应后的淋滤液将以连续流的状态被直接泵入分析仪器中,仪器则以固定的时间间隔测试并获取淋滤液的成分信息。该系统在在线模式下需与四极杆ICP-MS的TRA(Time-resolved analysis)模式联用,达到高分辨率监测淋洗液空白、样品清洗以及样品溶解成分变化的目的。
FT-TRA系统使用色谱仪中常用的梯度泵控制系统控制淋洗液的混合比例。该类软件能够在计算机上实时调整双泵模块连接的多种洗脱液的混合比例,使系统能够满足多样化的实验需求。同时,与FT系统联用时的ICP-MS TRA模式无法像LA状态下一样引入固态标样并在系统内完成自动单位转换[15-16],需应用软件对梯度泵进行预先编程,自动生成与时间相关的不同浓度标液曲线,而后通过Iolite软件提取标线数据并建立标线cps与ppb的转换方程(图3),最后通过转换方程将测试样品的cps数据转换为浓度数据使用。
2. FT-TRA系统分析地质样品的实验方法
本节将简要介绍系统的性能评估方法与标准样品的选取、待测地质样品的选取与制备,以及在测试过程中仪器模式参数的调试。
2.1 FT-TRA系统的重现性与稳定性评估
仪器或系统的稳定性与重现性是确保其测试所得数据科学、有效的基本前提。鉴于ICP-MS 在地球科学等领域的广泛应用,其系统稳定性和重现性都已得到普遍认可[17],本小节将简单说明如何运用标样对FT-TRA与ICP-MS联合使用状态下系统整体的稳定性与重现性进行评估,并给出两者可能的影响因素。
合适的标准样品是评估测试系统重现性与稳定性的重要载体。对于FT-TRA系统,标样的选取应遵循以下要求:①为了评估样品在反应池中的溶解过程,需选用固态标样;②标样需具有均质性(或较弱的非均质性);③不同类型的地质样品需根据其主要矿物组成选择相对应的标样。目前该系统的应用过程中并无统一标样。以FT-TRA研究有孔虫壳体为例,目前已被使用的标样包括北美页岩(NASC)[1]、NIST 1c泥质灰岩[18]、PDB[19](北美南卡罗来纳州白垩系皮狄组内的美洲似箭石)以及ECRM752-1灰岩[20-23](一种来自英国的灰岩标样),基本选择的是与研究样品化学组成相近的,且在元素测试中被广泛使用的标样。
对于具有不同物理化学性质的样品,影响FT-TRA系统稳定性的因素存在一定差异,如样品主要成分为碳酸盐时,酸溶解过程中产生的CO2气泡可能导致仪器的测量波动,在cps随时间变化的曲线上呈现出尖锐而突出的峰,且这一波动出现的概率随酸浓度的升高而显著提升。所以不同类型的地质样品在进行正式测试前都需选用合适的标样做好充分的探索与评估。而标样的均质程度与质量差异都可能在一定程度上影响对系统重现性的评估,需要引起注意。
2.2 样品选取与制备
作为一种流通式反应-分析系统,FT-TRA系统主要适用于不同类型固体样品的淋滤溶解与成分分析。目前应用较为普遍的是对有孔虫、介形虫等生物壳体与较纯矿物相的出溶成分监测。
2.2.1 生物壳体样品的选取与制备
对有孔虫、介形虫等生物壳体的分析,多数来源于古海洋古环境学代用指标的需求,期望通过不同时期赋存在生物壳体中的化学成分差异重建地质历史时期的环境参数。在此类应用中,除了控制不同组样品的大小、数量使其数据具有可比较性之外,精确获取壳体的原生生物信息,避免次生组分的干扰也是实验的重点之一。因此,在使用FT-TRA系统进行生物壳体的溶解测试之前,应先用显微表征手段手动选取大小、数量及完整度相似的样品,而后根据不同的实验目的进行壳体的清洗工作。
多数研究者选择在有孔虫样品进入FT-TRA反应池之前对其进行磨碎与物理清洗,以保证去除壳体内外附着的黏土与黄铁矿等杂质[1,7,19-20],其流程大致为:①将挑选好的有孔虫壳体在63μm的筛子中轻轻压碎,打开房室;②用去离子水与乙醇采用超声振荡[24-26]进行物理清洗;③在体视镜下检查壳体是否洗净,未洗净则重复第二步;④将洗净的壳体放入反应池内准备测试。传统的有孔虫清洗法是在物理清洗之后进行批量化学清洗,主要包括用还原性试剂(盐酸羟胺等)去除表面氧化层,以及用弱酸去除次生方解石相。但在FT-TRA系统的测试中,化学清洗可以通过系统来进行,并实时监测清洗过程中的出溶成分。FT-TRA系统常用的清洗剂组合包括0.5~1.0mol/L盐酸羟胺(清洗氧化层)、去离子水(清洗样品池)与1mol/L硝酸(清洗管路)[1],与批处理法相比减少了用氢氧化钠与双氧水的复配溶液清除附着黏土与有机质的过程,因为这两者在FT-TRA系统测试的数据中可以被察觉并予以校正。而对于封闭性较高的介形虫壳体,在前处理过程中更是省略了打开房室的步骤[22],只进行物理清洗,以达到监测壳体由外至内成分差异的目的。
2.2.2 矿物样品的选取与制备
目前FT-TRA系统在矿物学方面的应用,主要为测试矿物的溶解速率与反应性。
以de Baere等[4]研究镁橄榄石的溶解动力学参数为例,矿物样品在进入反应池前的制备及前期准备如下:①将矿物研磨至大小相近的颗粒(63~150μm);②用丙酮进行超声清洗,得到澄清的上清液后置于60℃烘箱中干燥;③使用扫描电镜(SEM)确保矿物颗粒表面不附着细颗粒;④使用射线衍射(XRD)定量法确定样品纯度;⑤采用消解与元素分析法确定待测矿物的化学计量数;⑥使用多点BET法测试表面积。完成以上流程后可综合矿物化学计量数、矿物颗粒表面积与FT-TRA系统测得的实时溶解元素浓度来确定某一矿物的溶解参数。
当用于矿物反应性测试时,样品的前处理制备则更为灵活。Power等[5]用FT-TRA系统测试尾矿与富CO2流体的反应性,仅控制进入样品池的尾矿的质量为500mg,反而对洗脱液的制备较为严谨,在实验开始前,根据弗式地球化学模型[27]保证洗脱液至少在48h内与一定浓度的CO2气体平衡,以确保pH稳定在预期值(pH=4.41)附近。
由于FT-TRA系统应用于矿物学研究的实例并不多,且实验目的有差异,所以并未产生固定的样品选取与制备流程,使用者可根据自身需求进行调整。
2.3 仪器分析模式与测试参数
如前文1.1节所述,FT-TRA系统的主要优势是能够利用四极杆ICP-MS的TRA(Time-resolved analysis)模式,实现连续实时地在线分析。四极杆ICP-MS的TRA模式广泛应用于激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)分析。该模式由于其快速跳峰的能力,能够完成测定质量范围内的快速扫描[28]。但由于TRA模式不常用于直接的溶液进样测试,因此FT-TRA系统在与ICP-MS进行联用测试时,需对ICP-MS仪器参数重新进行优化调整。
样品流速是FT-TRA应用过程中较为重要的参数。样品流速由反应池中的洗脱液流速决定,而洗脱液流速同时也影响着样品的溶解动力学[4],过快的流速会导致样品无法与洗脱液充分接触反应,而过慢的流速则会使得出溶成分在反应池中堆积,无法真实反映样品化学成分的实时出溶情况。
针对流速的选择,前人并未给出详细的建议,目前只有Haley等[1]在文章中阐述了洗脱液进入样品池的流速为4mL/min,但并未解释具体的原因。本研究团队在实验室用pH=2稀硝酸在不同流速下溶解同一有孔虫壳体进行探索实验,实验结果如图4所示,可以看出随着流速的增加,各元素浓度逐渐增加,当流速达到 0.3mL/min 时43Ca和44Ca达到峰值,而24Mg呈持续上升态势。为了排除切换流速瞬间系统波动造成的影响,本文计算了各元素浓度在不同流速的平均浓度变化,发现在0.4mL/min时43Ca和44Ca浓度均值达到最大,这可能表明此时有孔虫钙质壳体的出溶程度最高。但总体来看该探索实验并没有完全满足控制变量法条件(因为有孔虫壳体本身会在流速增加的过程中被逐渐溶解),且无法从根本原理上解释这一变化,因此对FT-TRA系统使用过程中流速的选择依然有待讨论。
3. FT-TRA系统在地质领域的应用进展
FT-TRA系统在地质样品中的应用目前主要集中在三个领域,分别是古海洋与古环境指标研究、矿物溶解参数测试、矿物反应性测试,其关键应用发展节点如表1总结。其中,古海洋学指标开发过程中清洗、溶解有孔虫壳体并监测其出溶成分是FT-TRA系统应用的起源,也是目前使用较广泛的方向。
表 1 FT-TRA系统应用实例总结Table 1. Summary of FT-TRA system application examplesFT-TRA
应用领域文献来源 研究方法 研究指标 研究亮点 对于FT-TRA系统的意义 以有孔虫壳体成分为指标的古海洋学研究 Haley等
(2002)FT-TRA 浮游有孔虫壳体REEs、Mg、Sr、Cd、Ba、Mn、Mg/Ca、Sr/Ca 提出FT-TRA系统作为一种新的清洗溶解有孔虫壳体的技术手段 FT-TRA系统的设计与最初应用 Benway等
(2003)FT-TRA 浮游有孔虫壳体Mg/Ca 首次评估该系统提取有孔虫壳体Mg/Ca作为还原古海水表层温度指标的可行性 首次提出FT-TRA系统可根据溶解度差异提取不同成因方解石的化学信号 Klinkhammer等 (2005) FT-TRA 浮游有孔虫壳体Mg/Ca 进一步提出采用在FT-TRA系统中溶解最敏感的方解石部分作为反映表层海水温度的准确值 实现了系统自动化运行;肯定了前处理/清洗过程对于FT-TRA系统提取有孔虫Mg/Ca的非必要性 Haley等
(2005)FT-TRA 底栖及浮游有孔虫壳体REEs 运用FT-TRA系统分析底栖及浮游有孔虫壳体中的REEs作为古环境指标的潜力 / Hoogakker等 (2009) FT-TRA、
电子探针、
传统批次
溶解法浮游有孔虫壳体Mg/Ca 对比高盐度与正常盐度环境中的浮游有孔虫壳体Mg/Ca,证明盐度高低不会影响有孔虫壳体Mg/Ca,变化是由次生生长导致 首次质疑FT-TRA系统可根据溶解度差异提取生物方解石的观点,但提出可通过Mg/Ca斜率变化进行判断 Klinkhammer等 (2009) FT-TRA 有孔虫壳体Mn/Ca 测定钻孔中不同种属有孔虫壳体Mn/Ca在过去3万年间的变化趋势,得出陆源输入改变、表层生产力改变等结论 / Sadekov等
(2010)FT-TRA、
LA-ICP-MS有孔虫壳体Mg/Ca 针对部分溶解对有孔虫壳体Mg/Ca的影响作了针对性研究 提出有孔虫壳体在FT-TRA系统内部的溶解仅是由外至内进行,并不能提取特定组分方解石 Torres等
(2010)FT-TRA 有孔虫壳体Mg/Ca、Ba/Ca、
Sr/Ca、Mn/Ca测定受次生矿化影响的有孔虫的El/Ca,并结合C/O同位素识别其是否生长在甲烷泄露区 / Boussetta等
(2011)FT-ICP-AES、SEM、XRD 浮游有孔虫壳体Mg/Ca 对地中海钻孔浮游有孔虫Mg/Ca进行研究,证明地中海有孔虫Mg/Ca高于开阔海洋不是由于盐度效应,而是与成岩作用有关 提出FT-TRA系统并没有分离出次生方解石的效果 Haarmann等 (2011) 离线FT-TRA 清洗有孔虫壳体 使用FT-TRA系统清洗样品以防止壳体损失,用于测试单个有孔虫壳体的Mg/Ca变化探究季节温度变化 / Kraft等
(2013)FT-TRA、
传统批次
溶解法有孔虫壳体Al/Ca、Mn/Ca和Nd 同位素组成 详细比较了手动批次清洗和 FT-TRA 清洗有孔虫壳体的效果,证明两者清洗效率相同且批次清洗法对于Nd的重吸附效应更小 认为FT-TRA系统清洗有孔虫并不具备优势 Mckay等
(2015)SIMS、FT-ICP-OES 底栖有孔虫壳体Mn/Ca 评估底栖有孔虫壳体Mn/Ca作为记录东北大西洋低纬度上升流系统底层水氧化还原条件变化的替代物的潜力 / 矿物溶解动力学研究 de Baere等
(2015)FT-TRA 镁橄榄石溶解
参数以镁橄榄石为例,探索FT-TRA在恒定和瞬态洗脱条件下测量矿物溶解速率、确定溶解参数和溶解化学计量学的适用性 首次将FT-TRA系统应用于矿物溶解动力学领域 de Baere等
(2016)FT-TRA、孔隙尺度建模 镁橄榄石、
方解石溶解态对镁橄榄石和方解石的溶解态进行了经验测定表明FT-TRA系统能够判断矿物溶解态 提出方解石在系统内部溶解是受转运控制,首次从理论上反驳FT-TRA系统进行差异溶解的能力 矿物反应性研究 Power等
(2020)FT-TRA 尾矿与CO2的反应性 使用与CO2气体达到平衡的NaCl溶液作为洗脱液,在FT-TRA系统中测试尾矿与CO2的反应性 FT-TRA系统首次运用于矿物反应性研究;首次将气相作为系统洗脱液组分 以介形虫壳体成分为指标的古海洋学研究 Börner等
(2017)FT-TRA、
LA-ICP-MS介形虫壳体El/Ca 与LA-ICP-MS的测试结果对照,探究FT-TRA提取介形虫壳体环境信号的潜力 首次根据FT-TRA系统由外至内的溶解特性进行介形虫壳体研究 Rodríguez等 (2021) FT-TRA、
电子探针介形虫壳体微量元素 探究微量元素在介形虫壳体内的分布,评价其作为古海洋学指标的潜在价值 / 对比传统的有孔虫批处理方法,FT-TRA系统能够根据溶解敏感性(或者说溶解度)的不同来区分矿物相,具备更精确地提取成分信息的优势。但这一优势并不绝对,其涉及的不同组分溶解机制一直是该系统运用过程中的重要问题。
3.1 应用起点:有孔虫淋洗与环境代用指标验证
3.1.1 理论提出与方法初探
沉积有孔虫是古海洋及古环境研究的主要对象。通过分析不同时期(不同层位)海洋有孔虫壳体中元素比值及C-O同位素组成,可以重建地质历史时期的海水温度(Mg/Ca、18O)[29-33]、盐度(Sr/Ca、Ba/Ca)[30,34-35]、碳酸盐化学(U/Ca、B/Ca)[36-38]、生产力及沉积氧化还原环境(Cd/Ca、Mn/Ca、V/Ca)[25,39]等指标;近些年,海洋有孔虫的Li-Sr等同位素组成也被应用于地质历史时期尤其是新生代表生风化与海水化学演化研究[40]。但有孔虫中的古海洋古环境信息可靠提取的一个重要前提是必须尽量提取其在活体时期形成的生物成因方解石信息,避免埋藏沉积后的污染相(残留有机体、沉积黏土附着)及次生相(壳体氧化形成的富镁铁氧化膜、溶解重结晶形成的次生方解石)对原始壳体成分的干扰[41],为此早在二十世纪Boyle[42]就提出严格的清洗方法并经过不断地修改优化[26,29,42-44]后沿用至今。这种先清洗污染相而后一次性溶解获取壳体信息的方法被称为批处理法,其包括若干连续的步骤以消除不同阶段的混染,主要包括:①去除碎屑黏土;②去除有机物;③去除铁锰氧化物包膜等,最后将清洗“干净”的壳体放入稀硝酸中溶解,并吸取上清液作为该壳体的原生方解石信号。传统批处理法虽然可以在一定程度上清洗污染相,但由于清洗时间长、受人为操作等因素影响,可能会导致样品损失(清洗试剂会导致钙质壳体的部分溶解)、可重复性差等问题,且很难验证污染相是否被有效清除,更难以区分原生生物方解石与次生方解石的信号差异。
基于上述情况,二十一世纪初,美国俄勒冈州立大学Haley等[1]为了在线监测有孔虫壳体的清洗过程,防止关键矿物相的损失,同时规避稀土元素在溶解过程中的再吸附效应[6],设计了FT-TRA系统。该系统的反应模块(Flow through reactor)遵循批反应器(Batch reactor)和混合流式反应器(Mixed flow reactor)的基本原理,并在此基础上进一步缩小了反应池的体积(20~50μL),大幅度减少淋洗液在反应池中的滞留时间,以实现高时间分辨率下分析淋滤液成分。实验结果证明,FT-TRA具备实时监测有孔虫清洗流程的功能,并能够依据有孔虫溶解过程中Al、Fe等元素的浓度波动对污染相进行校正(图5),且使用该新型手段所测得的有孔虫壳体El/Ca与前人通过批处理法所得数据具有可比性[38,45-47]。另外,该团队认为虽然缩小后的流式反应器的溶解反应在绝大多数情况下无法达到平衡,但溶解动力学表明反应模式仍然处于“稳态”,其证据是不同大小、数量的有孔虫样品在相同溶剂作用下都在同一时间窗口内溶解[1,7](图5)。他们进一步提出处于稳态的系统可以根据不同矿物相的溶解敏感性(即溶解度差异)来实现行差异溶解,从而获得最原始的生物方解石信息,即可以记录原始海洋环境信息的组成特征(图6)。
作为一种可监控的清洗手段,FT-TRA系统结合不同溶剂对有孔虫壳体的淋滤实验结果显示,壳体主要含有两种污染相,分别是富Mn、Cd、Mg的氧化包膜和富稀土元素(REEs)和Ba的顽固相(Refractory phase),还揭示了不同清洗溶剂对于壳体REEs产生的不同效果[1,6],继而评估有孔虫稀土元素作为古环境指标的潜力[6];同时作为一种具有连续性的化学信号提取手段,FT-TRA系统也可提取壳体的元素与Ca比值来重建古海洋古环境信息[19,23],如用Mg/Ca值重建古海水的温度(图7),用Mn/Ca值重建海水的氧化还原条件变化等。其中值得注意的是,多次测试结果都指示了壳体具有显著的Mg不均一性,存在Mg/Ca值起始高而后稳定减少并在溶解后期显示出低值的现象[1,7,18](图7);推断是由于FT-TRA系统能根据不同成因方解石的溶解度大小进行差异溶解造成的,并建议使用对溶解最敏感的高Mg部分作为计算壳体钙化温度的参数(图6黄色区域)[7]。
图 7 FT-TRA系统与批处理法提取有孔虫壳体Mg/Ca还原古海洋温度(修改自Benway等[7])(a) FT-TRA方法数据点选择;(b)两种方法测试的有孔虫壳体Mg/Ca数据;(c)使用b图数据计算的对应表层海水温度。Figure 7. FT-TRA system and batch method extracting Mg/Ca of different foraminifera shells to reestablish paleo-ocean temperature. Figure a shows how to use FT-TRA dissolution curve to selectively extract the foraminifera shell biological calcite signal; Figure b shows Mg/Ca of foraminifera shells obtained by the two methods; Figure c shows the corresponding surface seawater temperature calculated using the data in Figure b. Modified from Benway, et al[7].3.1.2 实验对照与对“差异溶解”优势质疑的提出
俄勒冈州立大学团队作为FT-TRA系统的最初设计者,认为该系统具有分离不同矿物相及不同成因方解石的优势,但其他团队的深入研究发现有孔虫壳体溶解动力学行为具有复杂性,并设计了更严谨的对照实验方法来探究FT-TRA系统清洗及溶解效果,从而更可靠地提取有孔虫壳体的环境指标。针对有孔虫壳体的清洗效果,Kraft等[26]详细比较了手动批清洗和FT-TRA清洗对于有孔虫Al/Ca、Mn/Ca和Nd同位素测试结果的影响,认为两种方法的清洗效率是相同的,甚至批清洗法对于Nd同位素的重吸附效应更小[48-51];同时,通过与传统有孔虫溶解测试手段及电子探针(EPMA)、扫描电镜(SEM)、二次离子质谱(SIMS)等原位手段的对比,证明使用FT-TRA系统溶解有孔虫壳体所提取的化学成分信息(元素与Ca比值等)具有良好对照性[20,52-55]。而对于FT-TRA系统的差异溶解优势,Sadekov等[55]使用FT-TRA系统联合LA-ICP-MS比较研究部分溶解作用对于有孔虫壳体Mg/Ca的影响,发现不论是实际海底环境还是实验室模拟条件下都不存在有孔虫壳体富Mg组分的选择性溶解,在多数情况下样品遵循从外至内的溶解顺序,如果外层为低Mg层,则溶解初期会显示低Mg值,反之则会显示高Mg值(图8)。Hoogakker等[52]和Boussetta等[53]在探究高盐度环境对有孔虫壳体Mg/Ca值的影响的实验中也印证了这一结论,但另外提出可以通过溶解过程中Mg/Ca的斜率变化来对不同成因方解石进行区分。这些研究结论虽然肯定了FT-TRA系统在有孔虫组成快速分析方面的可靠性和便捷性[53,56-58],但也揭示出FT-TRA系统应用于生物壳体淋洗和组分分析的复杂性。
图 8 LA-ICP-MS与FT-TRA测试不同种类有孔虫壳体Mg/Ca值变化比较(a) LA-ICP-MS分析三种有孔虫壳体由内至外Mg/Ca值的变化;(b) FT-TRA系统溶解同样三种有孔虫所得Mg/Ca值的变化。修改自Sadekov等[59]。Figure 8. Comparison of LA-ICP-MS and FT-TRA tests on Mg/Ca of foraminifera shells of different species: (a) LA-ICP-MS analysis of Mg/Ca changes from inside to outside of three foraminifera shells; (b) FT-TRA analysis of Mg/Ca changes from inside to outside of three foraminifera shells. Modified from Sadekov, et al[59].3.2 应用进展:FT-TRA内部溶解动力学与矿物溶解参数测试
3.2.1 矿物溶解动力学基本原理
虽然FT-TRA系统最初应用经历了从“稳态” 反应模式提出到实验质疑的发展过程,但一直缺乏理论层面对FT-TRA系统是否能依据矿物溶解度进行差异溶解这一问题的阐释,因此对该系统溶解过程和机制的认知始终较为模糊。近年来,加拿大英属哥伦比亚大学de Baere[2]对原始FT-TRA系统进行了硬件升级与软件自动化,发掘其在探究矿物溶解动力学方面的潜力。他们研究指出,矿物溶解反应由一系列不同的物理化学过程组成[60],包括:①将反应物从水相转移到矿物表面;②将反应物吸附在矿物表面;③将反应物迁移到矿物表面的“活性位点”;④被吸附的反应物与矿物反应(溶解);⑤将产物从反应位置转移走;⑥将反应产物从矿物表面解吸附;⑦将解吸附的反应产物从矿物表面转移至水相中。而矿物溶解动力学的一个重要概念就是,以上步骤中最慢的一步将会成为溶解过程的“速率限制”步骤[61]。其中,步骤①和⑦涉及反应物和产物在矿物表面和水相之间的运输,因此如果这两步的速率被限制,则被称为“转运控制”(Transport-controlled);而步骤②~⑥发生在矿物表面,如果这其中某一步的速率被限制,则被称为“表面控制”(Surface-controlled)。
根据该理论,溶解“稳态”代表反应产物脱离矿物表面的速率等于其通过扩散从矿物表面转运的速率。在测试矿物的溶解速率时,如果溶解模式为转运控制,则矿物的溶解速率随着流速的降低(滞留时间增加)而降低,这时得到的矿物溶解速率是不准确的,因为单位时间内从矿物表面剥离下来的反应产物会受到扩散边界层(DBL)的阻碍而并不能被马上检测到[62];但当溶解模式变为表面控制时,单位时间内检测的出溶的元素浓度会随着滞留时间增加(流速降低)而线性增加,此时矿物溶解速率不随流速/滞留时间的改变而改变,计算得到的矿物溶解速率才是有效的[2,63-65]。
3.2.2 FT-TRA系统内部不同类型矿物溶解行为特征
基于上述溶解动力学理论, FT-TRA系统根据矿物相的溶解度不同来进行差异溶解的前提是,它必须能真实地反映某一矿物相的溶解速率,即该矿物相在FT-TRA系统中的溶解是表面控制的。因此只有像镁橄榄石之类溶解度较小的矿物才能在系统中反映真实溶解参数,而方解石类的碳酸盐矿物在酸的作用下是无法在系统中达到表面控制的[66]。为证实该理论,de Baere[2]以方解石与文石为例对Haley等[1]的想法做了针对性实验,结果显示:不论是方解石颗粒还是文石颗粒,两者在FT-TRA系统中的溶解度都与其实际的溶解度相差一个至几个数量级,根本无法体现出其真实溶解度的差异[63,65,67-68](图9),这可能是由于方解石和文石在系统中的溶解模式以转运控制为主所导致的。而将方解石和文石粉末按一定比例混合后在FT-TRA系统中进行的实验结果显示,无论是以表面控制为前提的模型还是被转运控制主导的实际溶解,都无法从时间上识别两种粉末的端元成分;不同矿物相在系统中的溶解主要还有由溶质与溶剂的接触主导的,即先接触到溶剂的溶质先发生溶解,而探测器得到的不同相的混合信息也没有规律可循。
上述结果从理论和实践层面均证实FT-TRA系统很难实现有孔虫的差异溶解,区分不同成因方解石,也展示了FT-TRA系统在恒定和瞬态淋洗条件下测量矿物溶解速率、溶解参数和溶解化学计量数的适用性,可作为研究矿物溶解的有效补充手段,与孔隙尺度建模相结合,获取扩散边界层厚度、pH、流速和滞留时间对运移的影响等单一方法无法获取的信息[4],赋予FT-TRA系统新的地学应用方向。
3.3 更多应用:介形虫与矿物反应性测试
介形虫是一种常见于大陆、河口和海洋水域中的双壳纲微体生物,其壳体与有孔虫类似,也被作为古环境重建中地球化学分析的重要来源资料[69-70]。在de Baere[2]否定了FT-TRA系统对有孔虫壳体差异溶解的优势后,部分研究者利用该系统由外至内的溶解特性分析封闭性更强的介形虫壳体[22,71-72]。介形虫壳体FT-TRA与LA-ICP-MS对比分析表明,两个方法所得的除了Sr以外介形虫壳体的元素与Ca比值具有很好对照性[22,73],结合EPMA分析也进一步显示浅海介形虫壳体的微量元素具有古海洋学指标的潜在价值[71]。
除了在古海洋指标方面的发展,Power等[5]使用与CO2气体达到平衡的NaCl溶液作为淋洗液,在FT-TRA系统中测试尾矿与CO2的反应性,以确定该矿床通过直接捕捉大气中的CO2形成富含CO2的流体来进行碳酸化过程从而隔离CO2的潜力。虽然目前利用该系统进行矿物反应性测试的只有零星研究,但这一研究聚焦国际学术热点——全球变暖及CO2的人工捕捉,为FT-TRA的应用提供了一种创新的思路,具有很强的实践意义和参考价值。
4. 总结及展望
FT-TRA系统是因为古海洋与古环境研究需要在二十一世纪初才发展的测试技术。截至目前,发展初期的FT-TRA系统重点应用于有孔虫研究,除了基础的清洗、溶解及监测功能外,能提取生物方解石信号的“差异溶解”一直被认为是其不可取代的优势。直到有研究者从理论和实验层面几乎推翻了这一优势后,FT-TRA系统的地学应用也在同时遭遇瓶颈。然而,虽然de Baere[2]否定了FT-TRA系统在有孔虫研究中“差异溶解”的功能,但仍有许多溶解实验表明,有孔虫壳体在FT-TRA系统中的溶解并不是毫无规律,依然存在一定趋势(如几乎所有样品的Mg/Ca在弱酸溶解时都呈现出从一个从峰值缓慢下降的趋势),所以目前并不能完全否定FT-TRA系统在提取有孔虫信息方面的独特性。另外de Baere等[4]运用该系统测试的镁橄榄石的溶解度接近其真实溶解度,说明该系统对于溶解度低的矿物相(如硅酸盐)的差异溶解存在一定的可能性。最新有关矿物反应性方面的应用更是将气相作为淋洗成分之一加入FT-TRA系统中,极大地拓宽了其应用潜力。
FT-TRA系统目前存在最大的问题是缺乏完善的理论基础,目前仅有de Baere[2]根据溶解动力学提出的不同溶解度物相在FT-TRA系统中的溶解态差异,但该理论依旧不能完全解释实验观察到的所有现象。理论不仅是发展应用方向的指导,更是科学解释实验数据不可缺少的一环。今后应深入研究揭示FT-TRA系统内部的溶解机制,其未来发展方向包括多类型地质或人造样品的溶解、矿物的模拟合成以及与多种测试仪器(包括同位素分析仪器)的串联使用等。
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图 7 FT-TRA系统与批处理法提取有孔虫壳体Mg/Ca还原古海洋温度(修改自Benway等[7])
(a) FT-TRA方法数据点选择;(b)两种方法测试的有孔虫壳体Mg/Ca数据;(c)使用b图数据计算的对应表层海水温度。
Figure 7. FT-TRA system and batch method extracting Mg/Ca of different foraminifera shells to reestablish paleo-ocean temperature. Figure a shows how to use FT-TRA dissolution curve to selectively extract the foraminifera shell biological calcite signal; Figure b shows Mg/Ca of foraminifera shells obtained by the two methods; Figure c shows the corresponding surface seawater temperature calculated using the data in Figure b. Modified from Benway, et al[7].
图 8 LA-ICP-MS与FT-TRA测试不同种类有孔虫壳体Mg/Ca值变化比较
(a) LA-ICP-MS分析三种有孔虫壳体由内至外Mg/Ca值的变化;(b) FT-TRA系统溶解同样三种有孔虫所得Mg/Ca值的变化。修改自Sadekov等[59]。
Figure 8. Comparison of LA-ICP-MS and FT-TRA tests on Mg/Ca of foraminifera shells of different species: (a) LA-ICP-MS analysis of Mg/Ca changes from inside to outside of three foraminifera shells; (b) FT-TRA analysis of Mg/Ca changes from inside to outside of three foraminifera shells. Modified from Sadekov, et al[59].
表 1 FT-TRA系统应用实例总结
Table 1 Summary of FT-TRA system application examples
FT-TRA
应用领域文献来源 研究方法 研究指标 研究亮点 对于FT-TRA系统的意义 以有孔虫壳体成分为指标的古海洋学研究 Haley等
(2002)FT-TRA 浮游有孔虫壳体REEs、Mg、Sr、Cd、Ba、Mn、Mg/Ca、Sr/Ca 提出FT-TRA系统作为一种新的清洗溶解有孔虫壳体的技术手段 FT-TRA系统的设计与最初应用 Benway等
(2003)FT-TRA 浮游有孔虫壳体Mg/Ca 首次评估该系统提取有孔虫壳体Mg/Ca作为还原古海水表层温度指标的可行性 首次提出FT-TRA系统可根据溶解度差异提取不同成因方解石的化学信号 Klinkhammer等 (2005) FT-TRA 浮游有孔虫壳体Mg/Ca 进一步提出采用在FT-TRA系统中溶解最敏感的方解石部分作为反映表层海水温度的准确值 实现了系统自动化运行;肯定了前处理/清洗过程对于FT-TRA系统提取有孔虫Mg/Ca的非必要性 Haley等
(2005)FT-TRA 底栖及浮游有孔虫壳体REEs 运用FT-TRA系统分析底栖及浮游有孔虫壳体中的REEs作为古环境指标的潜力 / Hoogakker等 (2009) FT-TRA、
电子探针、
传统批次
溶解法浮游有孔虫壳体Mg/Ca 对比高盐度与正常盐度环境中的浮游有孔虫壳体Mg/Ca,证明盐度高低不会影响有孔虫壳体Mg/Ca,变化是由次生生长导致 首次质疑FT-TRA系统可根据溶解度差异提取生物方解石的观点,但提出可通过Mg/Ca斜率变化进行判断 Klinkhammer等 (2009) FT-TRA 有孔虫壳体Mn/Ca 测定钻孔中不同种属有孔虫壳体Mn/Ca在过去3万年间的变化趋势,得出陆源输入改变、表层生产力改变等结论 / Sadekov等
(2010)FT-TRA、
LA-ICP-MS有孔虫壳体Mg/Ca 针对部分溶解对有孔虫壳体Mg/Ca的影响作了针对性研究 提出有孔虫壳体在FT-TRA系统内部的溶解仅是由外至内进行,并不能提取特定组分方解石 Torres等
(2010)FT-TRA 有孔虫壳体Mg/Ca、Ba/Ca、
Sr/Ca、Mn/Ca测定受次生矿化影响的有孔虫的El/Ca,并结合C/O同位素识别其是否生长在甲烷泄露区 / Boussetta等
(2011)FT-ICP-AES、SEM、XRD 浮游有孔虫壳体Mg/Ca 对地中海钻孔浮游有孔虫Mg/Ca进行研究,证明地中海有孔虫Mg/Ca高于开阔海洋不是由于盐度效应,而是与成岩作用有关 提出FT-TRA系统并没有分离出次生方解石的效果 Haarmann等 (2011) 离线FT-TRA 清洗有孔虫壳体 使用FT-TRA系统清洗样品以防止壳体损失,用于测试单个有孔虫壳体的Mg/Ca变化探究季节温度变化 / Kraft等
(2013)FT-TRA、
传统批次
溶解法有孔虫壳体Al/Ca、Mn/Ca和Nd 同位素组成 详细比较了手动批次清洗和 FT-TRA 清洗有孔虫壳体的效果,证明两者清洗效率相同且批次清洗法对于Nd的重吸附效应更小 认为FT-TRA系统清洗有孔虫并不具备优势 Mckay等
(2015)SIMS、FT-ICP-OES 底栖有孔虫壳体Mn/Ca 评估底栖有孔虫壳体Mn/Ca作为记录东北大西洋低纬度上升流系统底层水氧化还原条件变化的替代物的潜力 / 矿物溶解动力学研究 de Baere等
(2015)FT-TRA 镁橄榄石溶解
参数以镁橄榄石为例,探索FT-TRA在恒定和瞬态洗脱条件下测量矿物溶解速率、确定溶解参数和溶解化学计量学的适用性 首次将FT-TRA系统应用于矿物溶解动力学领域 de Baere等
(2016)FT-TRA、孔隙尺度建模 镁橄榄石、
方解石溶解态对镁橄榄石和方解石的溶解态进行了经验测定表明FT-TRA系统能够判断矿物溶解态 提出方解石在系统内部溶解是受转运控制,首次从理论上反驳FT-TRA系统进行差异溶解的能力 矿物反应性研究 Power等
(2020)FT-TRA 尾矿与CO2的反应性 使用与CO2气体达到平衡的NaCl溶液作为洗脱液,在FT-TRA系统中测试尾矿与CO2的反应性 FT-TRA系统首次运用于矿物反应性研究;首次将气相作为系统洗脱液组分 以介形虫壳体成分为指标的古海洋学研究 Börner等
(2017)FT-TRA、
LA-ICP-MS介形虫壳体El/Ca 与LA-ICP-MS的测试结果对照,探究FT-TRA提取介形虫壳体环境信号的潜力 首次根据FT-TRA系统由外至内的溶解特性进行介形虫壳体研究 Rodríguez等 (2021) FT-TRA、
电子探针介形虫壳体微量元素 探究微量元素在介形虫壳体内的分布,评价其作为古海洋学指标的潜在价值 / -
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