Determination of delta O-18 and delta N-15 in nitrate

The analyses of both O and N isotopic compositions of nitrate have many potential applications in studies of nitrate sources and reactions in hydrology, oceanography, and atmospheric chemistry, but simple and precise methods for these analyses have yet to be developed. Testing of a new method involving reaction of potassium nitrate with catalyzed graphite (C + Pd + Au) at 520 degrees C resulted in quantitative recovery of N and O from nitrate as free CO2, K2CO3, and N-2. The delta(18)O values of nitrate reference materials were obtained by analyzing both the CO2 and K2CO3 from catalyzed graphite combustion. Provisional values of delta(18)O(VSMOW) for the internationally distributed KNO3 reference materials IAEA-N3 and USGS-32 were both equal to +22.7 +/- 0.5 parts per thousand. Because the fraction of free CO2 and the isotopic fractionation factor between CO2 and K2CO3 were constant in the combustion products, the delta(18)O value of KNO3 could be calculated from measurements of the delta(18)O of free CO2. Thus, delta(18)O(KNO3) = a delta(18)O(free CO2) - b, where a and b were equal to 0.9967 and 3.3, respectively, for the specific conditions of the experiments. The catalyzed graphite combustion method can be used to determine delta(18)O of KNO3 from measurements of delta(18)O of free CO2 with reproducibility on the order of +/- 0.2 parts per thousand or better if local reference materials are prepared and analyzed with the samples. Reproducibility of delta(15)N was +/- 0.1 parts per thousand after trace amounts of CO were removed.