Calibration Methodology for the Scripps 13C/12C and 18O/16O stable Isotope program 1992-2018
This report details calibration method for measurements of 13C/12C and 18O/16O ratios of atmospheric CO2 by the Scripps CO2 program from 1992-2018. The method depends principally on repeat analysis of CO2 derived from a suite of high-pressure gas cylinders filled with compressed natural air pumped at La Jolla. The first set of three cylinders were given isotopic assignments in 1994 based on comparisons with material artifacts NBS16, NBS17, and NBS19. Six cylinders subsequently brought into service were assigned values by comparing directly or indirectly with this first set. A tenth cylinder with natural CO2 in air was obtained from MPI Jena. Aliquots of CO2 from these cylinders, which serve as secondary standards, were extracted into heat-sealed glass ampoules (“flame-off tubes”) before introduction into the mass spectrometer. Some of these ampoules have been stored for many years before analysis, allowing long-term isotopic drift of the cylinders to be quantified. All secondary standards contain natural levels of N2O. The method corrects for any detected drift, while also applying corrections for N2O interference, for isobaric interferences (“Craig correction”) and for an inter-lab offset identified in early comparisons with the isotope lab at the University of Groningen. The Jena cylinder was found to be drifting upwards in δ18O at a rate of +0.10 ‰ per decade. Five of the other nine cylinders were found to be drifting downwards in δ18O, δ13C, or both, at rates of up to -0.11‰ per decade. The secondary standards were applied uniformly across a transition to a new mass spectrometer in 2000, thereby establishing continuity across this transition. Results are presented also for instrumental precision based on replicate analyses of standards. Drift-corrected analyses of the Jena cylinder establishes offsets of +0.037 ‰ in δ13C and +0.041 ‰ in δ18O between the Scripps and JRAS isotopic scales (Scripps more positive).