Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO3- in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying δ 18O values have not been readily available. To address this problem, we prepared large quantities of two nitrate salts with contrasting O isotopic compositions for distribution as reference materials for O isotope-ratio measurements: USGS34 (KNO3) with low δ18O and USGS35 (NaNO3) with high δ18O and 'mass-independent' δ17O. The procedure used to produce USGS34 involved equilibration of HNO3 with 18O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO3- reference materials with a range of δ18O values and normal (mass-dependent) 18O: 17O:16O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (α) between [NO 3-] and H2O decreases with increasing temperature from 1.0215 at 22°C to 1.0131 at 100°C. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high 17O:18O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO3- isotopic reference materials IAEA-N3 (=IAEA-NO-3) and USGS32, can be used to calibrate local laboratory reference materials for determining offset values, scale factors, and mass-independent effects on N and O isotope-ratio measurements in a wide variety of environmental NO 3- samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of ±0.2-0.3‰, 1σ): IAEA-N3 has δ18O = +25.6‰ and δ17O = +13.2‰ USGS32 has δ 18O = +25.7‰ USGS34 has δ18O = -27. 9‰ and δ17O = -14.8‰ and USGS35 has δ 18O = +57.5‰ and δ17O = +51.5‰.
CITATION STYLE
Böhlke, J. K., Mroczkowski, S. J., & Coplen, T. B. (2003). Oxygen isotopes in nitrate: New reference materials for 18O:17O:16O measurements and observations on nitrate-water equilibration. Rapid Communications in Mass Spectrometry, 17(16), 1835–1846. https://doi.org/10.1002/rcm.1123
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