Liu Xi, Deng Wenfeng, Wei Gangjian
State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.
College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049, China.
Rapid Commun Mass Spectrom. 2019 Mar 15;33(5):411-418. doi: 10.1002/rcm.8365.
Carbon and oxygen isotopic compositions of individual minerals in calcite-dolomite mixtures could be measured using a selective acid extraction technique based on the different reaction rates of calcite and dolomite with phosphoric acid. However, poor accuracies of calcite are usually obtained when mixtures of low calcite content are analyzed, which may result in incorrect conclusions. To overcome this shortcoming, improvements are needed in accuracy and precision when using the technique with mixtures of low calcite content.
Calcite and dolomite were analyzed under different reaction conditions using a GV Isoprime™ II isotope ratio mass spectrometer equipped with a dual inlet (DI) and an automatic sampler, to study the relationships between isotopic compositions and CO yield, and reaction time and temperature. Artificial mixtures with varied calcite:dolomite ratios were prepared, and their calcite isotopic compositions were determined with the aim of optimizing analytical conditions.
The CO yields and isotopic compositions of calcite and dolomite were obtained on-line under different experimental conditions using DI-isotope ratio mass spectrometry (DI-IRMS). Based on the analysis results of artificial mixtures, the reaction conditions for analyzing isotopic composition of calcite in mixtures were optimized as follows. Samples should have grain sizes of 75-80 μm; and samples with calcite contents of >50%, 30-50% and < 30% should be reacted with acid for 10-15 min at 50°C, 6 min at 50°C and 45 min at 25°C, respectively.
Optimized analytical conditions are suggested for the determination of the isotopic compositions of calcite in mixtures with improved accuracy by an automatic selective acid extraction technique.
基于方解石和白云石与磷酸的反应速率不同，可采用选择性酸萃取技术测定方解石 - 白云石混合物中各矿物的碳和氧同位素组成。然而，分析低方解石含量的混合物时，方解石的测量精度通常较差，这可能导致得出错误结论。为克服这一缺点，在对低方解石含量的混合物使用该技术时，需要提高准确性和精密度。
使用配备双入口（DI）和自动进样器的GV Isoprime™ II同位素比率质谱仪，在不同反应条件下对方解石和白云石进行分析，以研究同位素组成与CO产量、反应时间和温度之间的关系。制备了不同方解石:白云石比例的人工混合物，并测定其方解石同位素组成，旨在优化分析条件。
使用DI - 同位素比率质谱法（DI - IRMS）在不同实验条件下在线获得了方解石和白云石的CO产量及同位素组成。根据人工混合物的分析结果，将混合物中方解石同位素组成分析的反应条件优化如下。样品粒度应为75 - 80μm；方解石含量>50%、30 - 50%和<30%的样品应分别在50°C下与酸反应10 - 15分钟、在50°C下反应6分钟和在25°C下反应45分钟。
建议采用自动选择性酸萃取技术，优化分析条件，以提高混合物中方解石同位素组成测定的准确性。
