A new gas chromatograph‐isotope ratio mass spectrometry technique for high‐precision, N₂O‐free analysis of δ¹³C and δ¹⁸O in atmospheric CO₂ from small air samples

A new gas Chromatograph‐isotope ratio mass spectrometry (GC‐IRMS) technique for the first N 2 O‐free, high‐precision (<0.05‰) isotopic analysis of δ 13 C and δ 18 O in atmospheric CO 2 from small air samples has been developed. On‐line GC separation of CO 2 and N 2 O from a whole air sample is combined with IRMS under elevated ion source pressures. A specialized open split interface is an integral part of the inlet system and ensures a continuous flow of either sample gas or pure helium to the IRMS. The analysis, including all flushing, uses a total amount of 45 mL of an air sample collected at ambient pressure. Of this, three 0.5 mL aliquots are injected onto the GC column, each providing ∼0.8 nmol CO 2 in the IRMS source. At this sample size, δ 13 C precision obtained is at the theoretical shot noise limit. For typical ambient air samples collected in the Southern Hemisphere, demonstrated precisions for δ 13 C, δ 18 O, and the CO 2 mixing ratio (all measured simultaneously) are 0.02‰, 0.04‰, and 0.4 ppm, parts per million (ppm) respectively. Since these data are achieved from small air samples without contamination by atmospheric N 2 O or the use of cryogen, the technique will be a valuable tool in global carbon cycle research.