Determination of the total oxygen isotopic composition of nitrate and the calibration of a Δ17O nitrate reference material

A thermal decomposition method was developed and tested for the simultaneous determination of delta(18)O and delta(17)O in nitrate. The thermal decomposition of AgNO3 allows for the rapid and accurate determination of O-18/ O-16 and O-17/O-16 isotopic ratios with a precision of +/-1.5parts per thousand for delta(18)O and +/-0.11parts per thousand for Delta(17)O (Delta(17)O = delta(17)O - 0.52 x delta(18)O). The international nitrate isotope reference material IAEA-NO3 yielded a delta(18)O value of +23.6parts per thousand and Delta(17)O of -0.2parts per thousand, consistent with normal terrestrial mass-dependent isotopic ratios. In contrast, a large sample of NaNO3 from the Atacama Desert Chile, was found to have Delta(17)O = 21.56 +/- 0.11parts per thousand and delta(18)O = 54.9 +/- 1.5parts per thousand, 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, O-2) with the same Delta(17)O to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.