Sensitivity of tunable infrared laser spectroscopic measurements of ∆’¹⁷O in CO₂ to analytical conditions

Abstract. Triple oxygen isotope (∆’¹⁷O) measurements of CO₂ are increasingly used in paleoenvironmental and atmospheric sciences, in part due to the emergence of tunable infrared laser direct absorption spectroscopy (TILDAS) as a cost- and time-effective method for quantifying rare isotopologues in CO₂. This study aims to provide users with a clear understanding of how the stability of analytical conditions — such as optical cell temperature, pressure, and CO₂ concentration — affects measurement quality. Using data from two laboratories equipped with TILDAS instruments (University of Göttingen and University of Cape Town), both operating in high-precision dual-inlet mode, we demonstrate how variations in these parameters influence measurement repeatability and long-term stability. The most significant factor affecting short-term repeatability of ∆’¹⁷O is a mismatch in CO₂ concentration between sample and working standard. The resulting scale-offset effect can amount to several ppm per 1 µmol mol mismatch, depending on instrumental parameters. We show that empirical corrections for such offsets, arising from variable pCO₂ of the analyte across measurements, significantly improve reproducibility. In contrast, the dominant influence on long-term stability is drift in optical cell temperature and pressure. In air monitoring studies, unrecognized instrumental drift due to variations in optical cell temperature, pressure, and CO₂ concentrations can be misinterpreted as genuine seasonal variations in ∆’¹⁷O. We conclude with practical recommendations for achieving the highest possible precision with TILDAS, emphasizing that continuous monitoring and reporting of analytical conditions is essential.