Optimization of gas chromatography-isotope ratio mass spectrometry oxygen isotope analysis method and recommendations...

Accurate measurement of oxygen isotope composition (δ¹⁸O) in dissolved oxygen (DO) is critical for quantifying oxygen dynamics in aquatic systems, yet traditional methods such as dual-inlet isotope ratio mass spectrometry (DI-IRMS) require labor-intensive offline preparation. The continuous-flow gas chromatography-IRMS (GC-IRMS) technique improves throughput but faces limitations in precision calibration and simultaneously O₂/Ar detection. This study addresses these challenges by optimizing GC-IRMS protocols for gas-phase δ¹⁸O analysis, with a focus on applications in DO studies. A semi-automated O₂-Ar purification system was developed for δ¹⁸O analysis, integrating a modified Precon unit, a GC, and an IRMS. DO was converted to gaseous O₂ via headspace equilibration and calibrated using synthetic air. A total of three injections were sequentially conducted in a single run and our results showed that at peak intensities >6.0 × 10³, δ¹⁸O precision reached <0.15 ‰. This study's breakthrough involves enhancing δ¹⁸O measurement accuracy via Baseline drift correction (BDC) and Argon interference correction (AIC), with Helium (He) blank correction (HBC) additionally required when using ambient air as reference, as well as the acquisition of the O₂/Ar ratio, which facilitates the geochemical application of oxygen isotopes in aquatic sciences. Analysis of East China Sea profiles revealed that the δ¹⁸O corrected using synthetic air as the reference were higher than those corrected based on ambient air, with HBC contributing the most to the final δ¹⁸O, up to 0.3 ‰. Our results indicate that employing a He-mixed air method for daily calibration is crucial for enhancing δ¹⁸O precision in continuous-flow DO isotope analysis.