Research Progress on the Geological Application of a Flow-Through Time-Resolved Analysis System

The 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.