Determination of the total oxygen isotopic composition of nitrate and the calibration of a Δ17Ο nitrate reference material
A thermal decomposition method was developed and tested for the simultaneous determination of δ18O and δ17Ο in nitrate. The thermal decomposition of AgNO3 allows for the rapid and accurate determination of 18O/16O and 17O/16O isotopic ratios with a precision of ±1.5‰ for δ18O and ±0.11‰ for Δ17Ο (Δ17Ο = δ17Ο − 0.52 × δ18O). The international nitrate isotope reference material IAEA-NO3 yielded a δ18O value of +23.6‰ and Δ17Ο of −0.2‰, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO3 from the Atacama Desert, Chile, was found to have Δ17Ο = 21.56 ± 0.11‰ and δ18O = 54.9 ± 1.5‰, demonstrating a substantial mass-independent isotopic composition consistent with the proposed atmospheric origin of the desert nitrate. It is suggested that this sample (designated USGS-35) can be used to generate other gases (CO2, CO, N2O, O2) with the same Δ17Ο to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.