Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, India.
Department of Earth Sciences, Indian Institute of Science Education and Research-Kolkata, Nadia, India.
Rapid Commun Mass Spectrom. 2020 Jul 15;34(13):e8790. doi: 10.1002/rcm.8790.
Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO for measurement in a gas-source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of C- O bonding preference in the mineral lattice, which is captured in the product CO , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO to mass 47 in CO ) during the acid digestion.
We studied the reaction mechanism for the phosphoric acid digestion of calcite using first-principles density functional theory. We identified the transition state structures for each reaction involving different isotopologues and used the corresponding vibrational frequencies in reduced partition function theory to estimate the Δ acid fractionation. Experimental Δ data were acquired by processing the sample CO gas through the dual-inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer.
We showed that the acid digestion reaction, which results in the formation of CO enriched with C- O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ acid fractionation and reaction temperature as Δ = -0.30175 + 0.57700 × (10 /T ) - 0.10791 × (10 /T ) , with T varying between 298.15 and 383.15 K.
We propose a reaction mechanism that shows a higher slope (Δ acid fractionation vs. 1/T curve) for the phosphoric acid digestion of calcite than in previous studies. The theoretical estimates from the present and earlier studies encapsulate experimental observations from both "sealed vessel" and "common acid bath" acid digestion methods.
碳酸盐团簇同位素分析涉及碳酸盐矿物与磷酸反应，以释放 CO 进行气相源同位素比质谱仪测量。虽然团簇同位素示踪剂基于矿物晶格中 C-O 键合偏好随温度的变化，但在酸消化过程中，磷酸反应机制和团簇同位素分馏（CO 中的质量 63 到 CO 中的质量 47）的幅度信息有限。
我们使用第一性原理密度泛函理论研究了方解石磷酸消化反应的反应机制。我们确定了涉及不同同位素的每个反应的过渡态结构，并使用相应的简正振动频率在约化配分函数理论中估计 Δ 酸分馏。通过将样品 CO 气体通过 ThermoFinnigan MAT253 同位素质谱仪的双入口外围处理，获得实验 Δ 数据。
我们表明，在水的存在下，碳酸钙的质子化导致了富含 C-O 键的 CO 的形成，从而开始了酸消化反应。我们的模拟得出了 Δ 酸分馏与反应温度之间的关系，Δ=-0.30175+0.57700×(10/T)-0.10791×(10/T)，其中 T 在 298.15 到 383.15 K 之间变化。
我们提出了一种反应机制，表明方解石的磷酸消化比以前的研究具有更高的斜率（磷酸消化的 Δ 酸分馏与 1/T 曲线）。本研究和早期研究的理论估计包含了来自“密封容器”和“普通酸浴”酸消化方法的实验观察结果。
