Development of a quantum cascade laser absorption spectrometer for simultaneous measurement of C-O and O-O clumping in CO.

Rationale: Dual clumped isotope paleothermometry determines carbonate formation temperatures by measuring the frequency of C-O (∆) and O-O (∆) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high-precision measurements of O-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 ∆ and ∆ in gas-phase carbon dioxide (CO) at room temperature. Our IRLS uses a single laser scanning from 2290.7 to 2291.1 cm and a 31 m pathlength optical cell, and it simultaneously measures the five isotopologues required for calculating ∆ and ∆: OCO, OCO, OCO, OCO, and OCO. In addition, our IRLS can measure OCO, enabling ∆O analysis.

Results: At ~20°C and a CO pressure of ~2 Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues OCO and OCO, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for OCO, OCO, and OCO. This yielded precisions of 0.348‰ (∆) and 0.302‰ (∆) 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 ∆ and ∆ in CO to precisions of <0.35‰ within 25 s using a room-temperature, single-laser IRLS. Future developments on better resolving OCO and OCO peaks and system temperature control could further improve the measurement precision.