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 δ 18 O and δ 17 Ο in nitrate. The thermal decomposition of AgNO 3 allows for the rapid and accurate determination of 18 O/ 16 O and 17 O/ 16 O isotopic ratios with a precision of ±1.5‰ for δ 18 O and ±0.11‰ for Δ 17 Ο (Δ 17 Ο = δ 17 Ο − 0.52 × δ 18 O). The international nitrate isotope reference material IAEA-NO3 yielded a δ 18 O value of +23.6‰ and Δ 17 Ο of −0.2‰, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO 3 from the Atacama Desert, Chile, was found to have Δ 17 Ο = 21.56 ± 0.11‰ and δ 18 O = 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 (CO 2 , CO, N 2 O, O 2 ) with the same Δ 17 Ο to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.