Sensitivity of tunable infrared laser spectroscopic measurements of ∆’ ¹⁷ O in CO ₂ 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.