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 ¹³C–¹⁸O (∆₆₃₈) and ¹⁸O–¹⁸O (∆₈₂₈) pairs in carbonates. It resolves isotopic kinetic biases and thus enables more accurate paleotemperature reconstructions. However, high-precision measurements of ¹⁸O–¹⁸O 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 (CO₂) 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: ¹⁶O¹²C¹⁶O, ¹⁶O¹³C¹⁶O, ¹⁶O¹²C¹⁸O, ¹⁶O¹³C¹⁸O, and ¹⁸O¹²C¹⁸O. In addition, our IRLS can measure ¹⁶O¹²C¹⁷O, enabling ∆¹⁷O analysis.

Results

At ~20°C and a CO₂ pressure of ~2 Torr, our IRLS system achieved precisions of 0.128‰ and 0.140‰ within 20 s for abundances of the clumped isotopologues ¹⁶O¹³C¹⁸O and ¹⁸O¹²C¹⁸O, respectively, and precisions of 0.267‰, 0.245‰, and 0.128‰ for ¹⁶O¹²C¹⁶O, ¹⁶O¹³C¹⁶O, and ¹⁶O¹²C¹⁸O. 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 CO₂ to precisions of <0.35‰ within 25 s using a room-temperature, single-laser IRLS. Future developments on better resolving ¹⁶O¹²C¹⁶O and ¹⁶O¹³C¹⁶O peaks and system temperature control could further improve the measurement precision.