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Isotopic fractionation during acid digestion of calcite: A combined ab initio quantum chemical simulation and experimental study
Author(s) -
Pramanik Chirantan,
Chatterjee Swastika,
Fosu Benjamin R.,
Ghosh Prosenjit
Publication year - 2020
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.8790
Subject(s) - chemistry , isotope fractionation , phosphoric acid , calcite , carbonate , fractionation , mass spectrometry , equilibrium fractionation , reaction mechanism , inorganic chemistry , analytical chemistry (journal) , mineralogy , chromatography , organic chemistry , catalysis
Rationale Carbonate clumped isotope analysis involves the reaction of carbonate minerals with phosphoric acid to release CO 2 for measurement in a gas‐source isotope ratio mass spectrometer. Although the clumped isotope proxy is based on the temperature dependence of 13 C– 18 O bonding preference in the mineral lattice, which is captured in the product CO 2 , there is limited information on the phosphoric acid reaction mechanism and the magnitude of clumped isotopic fractionation (mass 63 in CO 3 2− to mass 47 in CO 2 ) during the acid digestion. Methods 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 47 acid fractionation. Experimental Δ 47 data were acquired by processing the sample CO 2 gas through the dual‐inlet peripheral of a ThermoFinnigan MAT253 isotope ratio mass spectrometer. Results We showed that the acid digestion reaction, which results in the formation of CO 2 enriched with 13 C– 18 O bonds, began with the protonation of calcium carbonate in the presence of water. Our simulations yielded a relationship between the Δ 47 acid fractionation and reaction temperature as Δ 47 = −0.30175 + 0.57700 × (10 5 / T 2 ) – 0.10791 × (10 5 / T 2 ) 2 , with T varying between 298.15 and 383.15 K. Conclusions We propose a reaction mechanism that shows a higher slope (Δ 47 acid fractionation vs. 1/ T 2 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.