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.