The ITS-90 after definition of neon isotopic reference composition. Extent of the isotopic effect on previous inter-comparison results

Starting from the end of the past century, the importance has been recognized of the effect of isotopic composition on some of the temperature fixed points for the most accurate realizations of the ITS-90. In the original definition of the latter, dating back to 1990, only a generic reference was made to natural composition of the substances used for the realization of the fixed points, except for helium. The definition of a reference isotopic composition for three fixed points, e-H2, Ne and H2O, while eliminating the non-uniqueness of the Scale in this respect, induced detectable differences in the present and future realizations of the Scale, at the highest accuracy level, with respect to the previous realizations, when they affected the results of past key comparisons, namely the K1 and K1.1, and K2 and K2.1 to K2.5 and the related regional and supplementary ones. The paper provides evidence of the extent of this effect by using the results of the relevant key comparisons for Neon archived in the BIPM KCDB, and of other comparisons existing in the literature: 1979-1984, 2007-2012 and 2009-2010 sealed cell comparisons; and discusses the meaning and the outcomes of this evaluation.Comment: 19 pages, 4 figures, 4 tables. BIPM, Doc. CCT/17-1

Using laser absorption spectroscopy for the measurement of d13C, d18O and D17O of atmospheric CO2

A new method to do stable isotope composition measurements on atmospheric CO2 is presented, using laser absorption spectroscopy (LAS). Traditionally measurements are conducted on pure CO2 extracted from air with isotope ratio mass spectrometry (IRMS). With LAS measurements can be conducted directly on dry air, thereby reducing the sample preparation time dramatically and eliminating any inaccuracies related with the CO2 extraction. The 12C16O2, 13C16O2, 12C16O18O and 12C16O17O molecules are measured directly, so no mass interferences occur and this enables us tomeasure the δ17O (next to the δ13C and δ18O), something that is still very complex using IRMS. With the δ17O and δ18O, the Δ17O can be determined, a promising new proxy to quantify the gross primary production of the biosphere. We focus on minimizing the combined uncertainty of the measurements by comparing various calibration methods and study sample stability. The oxygen composition of CO2-in-air stored in glass sample flasks tends to drift over time and we present new insights in the mechanisms behind this process and do recommendations on how to ensure a better stability. Uncertainties of 0.03 ‰ for δ13C and δ18O and 0.05 ‰ for δ17O and Δ17O are reached with the LAS method. Measurement results from the two coastal atmospheric measurement stations over the period 2017-2022 are presented. Results show that LAS measurements capture the seasonal cycles of δ13C and δ18O well. For the Δ17O measurements seasonal cycles cannot be identified, but a significant change of Δ17O values over the years is observed