High-precision δ¹³C-CO₂ analysis from 1 mL of ambient atmospheric air via continuous flow IRMS: from sampling to storage to analysis

Abstract. The carbon isotopic composition (δ¹³C) of atmospheric carbon dioxide (CO₂) is a key tracer for understanding terrestrial carbon dynamics, yet its application in small-volume sampling systems remains constrained by analytical limitations. Here, we present a novel methodology for high-precision δ¹³C analysis of ambient atmospheric CO₂ from 1 mL air samples, tailored to the challenges of growth chamber experiments using microcosm model systems and other volume-limited systems. Our approach emerged from testing the effects of custom vial conditioning, dual-sealing with Terostat^®, ultra-low-temperature storage at -80 °C, and cryogenic pre-concentration coupled to continuous-flow isotope-ratio mass spectrometry (IRMS). We demonstrate that vial conditioning and improved dual sealing are critical to ensure analytical precision. Our combined method achieves a precision of ± 0.1 ‰ on δ¹³C measurements, with negligible isotopic drift for storage durations up to 1-week if ultra-low-temperature storage and zip-lock bags full of CO₂-free air were used. Longer storage times reduces measurement precision, emphasising the importance of short-term preservation. This technique offers a significant advance for carbon stable isotope applications in constrained environments, enabling minimally invasive, high-frequency δ¹³C monitoring with good precision at the millilitre scale.