Exchange of Dual Clumped Isotopes of CO₂ With the Phosphoric Acid-Water System

The reaction of carbonate materials with phosphoric acid is a standard method for extracting CO₂, not only for the subsequent analysis of the ratios of ¹³C/¹²C and ¹⁸O/¹⁶O but also for the ratios of ¹³C¹⁸O¹⁶O/¹²C¹⁶O¹⁶O and ¹²C¹⁸O¹⁸O/¹²C¹⁶O¹⁶O and the calculation of the Δ₄₇ and Δ₄₈ values. For the determination of Δ₄₇ and Δ₄₈ values, possible exchange between the CO₂ and H₂O in the acid is of particular importance as this influences the precision and accuracy of the measurement.

Stochastic and non-stochastic CO₂ was exposed to "fresh" and "used" phosphoric acid at 90°C. Fresh acid is defined as acid that had not previously reacted with calcium carbonate, whereas "used" acid had previously dissolved a significant amount of carbonate material. The δ¹³C, δ¹⁸O, Δ₄₇, and Δ₄₈ values of the CO₂ were then measured using a Thermo-253 plus using conventional methods, and the rate and the amount of exchange were determined.

When exposed to "fresh" phosphoric acid, ¹³C¹⁸O¹⁶O and ¹²C¹⁸O¹⁸O readily exchanged with the H₂O in the acid, attaining Δ₄₇ and Δ₄₈ values close to those expected as a result of equilibration with H₂O at 90°C. In contrast with "used" acid, the rate of exchange of ¹³C¹⁸O¹⁶O with H₂O in the acid was negligible, whereas the exchange of ¹²C¹⁸O¹⁸O, although reduced compared with "fresh" acid, was still significant.

Such data have implications on the precision and accuracy of Δ₄₇ and Δ₄₈ values in systems in which the acid has been previously used to react in multiple samples, as well as in those in which fresh acid is used for every sample. The use of a sacrificial amount of carbonate prior to the reaction of "real" samples will reduce some of the changes observed in this study.