Determination of the total oxygen isotopic composition of nitrate and the calibration of a delta 17O nitrate reference material.
A thermal decomposition method was developed and tested for the simultaneous determination of delta 18O and delta 17O 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 per thousand for delta 18O and +/- 0.11 per thousand for delta 17O (delta 17O = delta 17O - 0.52 x delta 18O). The international nitrate isotope reference material IAEA-NO3 yielded a delta 18O value of +23.6 per thousand and delta 17O of -0.2 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 17O = 21.56 +/- 0.11 per thousand and delta 18O = 54.9 +/- 1.5 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, O2) with the same delta 17O to serve as measurement references for a variety of applications involving mass-independent isotopic compositions in environmental studies.
Michalski
Greg
G
gmichalski@ucsd.edu
Savarino
Joel
J
Böhlke
J K
JK
Thiemens
Mark
M