Sensitivity of tunable infrared laser spectroscopic measurements of Δ′ ¹⁷ O in CO ₂ to analytical conditions

Abstract. Triple oxygen isotope (Δ′17O) measurements of CO2 are increasingly used in atmospheric and paleoenvironmental research, supported by the rise of tunable infrared laser direct absorption spectroscopy (TILDAS) as a cost- and time-effective method for quantifying rare isotopologues in CO2. In this study we use data from two TILDAS instruments (University of Göttingen and University of Cape Town) to evaluate how the stability of analytical conditions, such as temperature, optical cell pressure, and the analyte's CO2 concentration influences measurement repeatability. We identify the mismatch in CO2 concentration between sample and working gas as the most significant factor affecting the repeatability of Δ′17O measurements. The resulting scale-offset effect can amount to several ppm per µmol mol−1 mismatch, depending on instrumental configuration. Applying empirical corrections for such offsets significantly improves repeatability. We show that maintaining electronics temperature, cell temperature, and cell pressure stability within ±0.1 K, ±1 mK, and ±10 Pa, respectively, across the cycles of a replicate measurement is sufficient to avoid any resolvable impact on the internal error. Over multi-week timescales, drift in measured isotope ratios is primarily driven by variations in the temperature of electronic components, while fluctuations in cell temperature, pressure, and analyte concentration exert smaller but detectable influences. We conclude with practical recommendations for achieving high precision triple oxygen isotope measurements with TILDAS, emphasizing that continuous monitoring and reporting of analytical conditions is essential.