S.J. Mroczkowski T.B. Coplen J.K. Böhlke 2003 <p><span>Despite a rapidly growing literature on analytical methods and field applications of O isotope-ratio measurements of NO</span>₃⁻<span> in environmental studies, there is evidence that the reported data may not be comparable because reference materials with widely varying </span><i>δ</i>¹⁸<span>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</span>₃<span>) with low </span><i>δ</i>¹⁸<span>O and USGS35 (NaNO</span>₃<span>) with high </span><i>δ</i>¹⁸<span>O and ‘mass-independent’ </span><i>δ</i>¹⁷<span>O. The procedure used to produce USGS34 involved equilibration of HNO</span>₃<span> with </span>¹⁸<span>O-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO</span>₃⁻<span> reference materials with a range of </span><i>δ</i>¹⁸<span>O values and normal (mass-dependent) </span>¹⁸<span>O:</span>¹⁷<span>O:</span>¹⁶<span>O variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (</span><i>α</i><span>) between [NO</span>₃⁻<span>] and H</span>₂<span>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 </span>¹⁷<span>O:</span>¹⁸<span>O ratio owing to its atmospheric origin. These new reference materials, combined with previously distributed NO</span>₃⁻<span> 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</span>₃⁻<span> samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of ±0.2–0.3‰, 1</span><i>σ</i><span>): IAEA-N3 has </span><i>δ</i>¹⁸<span>O = +25.6‰ and </span><i>δ</i>¹⁷<span>O = +13.2‰; USGS32 has </span><i>δ</i>¹⁸<span>O = +25.7‰; USGS34 has </span><i>δ</i>¹⁸<span>O = −27.9‰ and </span><i>δ</i>¹⁷<span>O = −14.8‰; and USGS35 has </span><i>δ</i>¹⁸<span>O = +57.5‰ and </span><i>δ</i>¹⁷<span>O = +51.5‰. </span></p> application/pdf 10.1002/rcm.1123 en Wiley Oxygen isotopes in nitrate: New reference materials for 18O:17O:16O measurements and observations on nitrate-water equilibration article