Oxygen isotopes in nitrate: new reference materials for 18 O: 17 O: 16 O measurements and observations on nitrate‐water equilibration

Despite a rapidly growing literature on analytical methods and field applications of O isotope‐ratio measurements of NO 3 − in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying δ 18 O 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 (KNO 3 ) with low δ 18 O and USGS35 (NaNO 3 ) with high δ 18 O and ‘mass‐independent’ δ 17 O. The procedure used to produce USGS34 involved equilibration of HNO 3 with 18 O‐depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO 3 − reference materials with a range of δ 18 O values and normal (mass‐dependent) 18 O: 17 O: 16 O variation. Preliminary data indicate that the equilibrium O isotope‐fractionation factor ( α ) between [NO 3 − ] and H 2 O 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 17 O: 18 O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO 3 − 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‰, 1 σ ): IAEA‐N3 has δ 18 O = +25.6‰ and δ 17 O = +13.2‰; USGS32 has δ 18 O = +25.7‰; USGS34 has δ 18 O = −27.9‰ and δ 17 O = −14.8‰; and USGS35 has δ 18 O = +57.5‰ and δ 17 O = +51.5‰. Published in 2003 by John Wiley & Sons, Ltd.