Development of a quantum cascade laser absorption spectrometer for simultaneous measurement of 13C–18O and 18O–18O clumping in CO2

Rationale Dual clumped isotope paleothermometry determines carbonate formation temperatures by measuring the frequency of 13 C– 18 O (∆ 638 ) and 18 O– 18 O (∆ 828 ) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high‐precision measurements of 18 O– 18 O clumping using current techniques requires large sample sizes and long acquisition times. Methods We developed a mid‐infrared isotope ratio laser spectrometer (IRLS) for simultaneous measurement of the isotopologue ratios ∆ 638 and ∆ 828 in gas‐phase carbon dioxide (CO 2 ) at room temperature. Our IRLS uses a single laser scanning from 2290.7 to 2291.1 cm −1 and a 31 m pathlength optical cell, and it simultaneously measures the five isotopologues required for calculating ∆ 638 and ∆ 828 : 16 O 12 C 16 O, 16 O 13 C 16 O, 16 O 12 C 18 O, 16 O 13 C 18 O, and 18 O 12 C 18 O. In addition, our IRLS can measure 16 O 12 C 17 O, enabling ∆ 17 O analysis. Results At ~20°C and a CO 2 pressure of ~2 Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues 16 O 13 C 18 O and 18 O 12 C 18 O, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for 16 O 12 C 16 O, 16 O 13 C 16 O, and 16 O 12 C 18 O. This yielded precisions of 0.348‰ (∆ 638 ) and 0.302‰ (∆ 828 ) within 25 s. Simulated sample–reference switching highlights the potential of our system and the need for further development. Conclusions We demonstrated simultaneous measurements of ∆ 638 and ∆ 828 in CO 2 to precisions of <0.35‰ within 25 s using a room‐temperature, single‐laser IRLS. Future developments on better resolving 16 O 12 C 16 O and 16 O 13 C 16 O peaks and system temperature control could further improve the measurement precision.