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 N2O-free, high-precision (< 0.05 parts per thousand) isotopic analysis of delta(13)C and delta(18)O. in atmospheric CO2 from small air samples has been developed. On-line GC separation of CO2 and N2O 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 similar to 0.8 nmol CO2 in the IRMS source. At this sample size, delta(13)C precision obtained is at the theoretical shot noise limit. For typical ambient air samples collected in the Southern Hemisphere, demonstrated precisions for delta(13)C, delta(18)O, and the CO2 mixing ratio (all measured simultaneously) are 0.02 parts per thousand, 0.04 parts per thousand, and 0.4 ppm, parts per million (ppm) respectively. Since these data are achieved from small air samples without contamination by atmospheric N2O or the use of cryogen, the technique will be a valuable tool in global carbon cycle research.