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

Despite a rapidly growing literature on analytical methods and field applications of 0 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 6'80 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 delta(18)O and USGS35 (NaNO3) with high delta(18)O and mass-independent' delta(17)O. The procedure used to produce USGS34 involved equilibration of HNO3 with O-18-depleted meteoric water. Nitric acid equilibration is proposed as a simple method for producing laboratory NO3- reference materials with a range of delta(18)O values and normal (massdependent) O-18:O-17:O-16 variation. Preliminary data indicate that the equilibrium O isotope-fractionation factor (alpha) between [NO3-] and H2O decreases with increasing temperature from 1.0215 at 22degreesC to 1.0131 at 100degreesC. USGS35 was purified from the nitrate ore deposits of the Atacama Desert in Chile and has a high 170:18 0 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 NO3 samples. Preliminary analyses yield the following results (normalized with respect to VSMOW and SLAP, with reproducibilities of +/-0.2-0.3parts per thousand, 1alpha): IAEA-N3 has delta(18)O = +25.6parts per thousand and delta(17)O = +13.2parts per thousand.; USGS32 has delta(18)O = +25.7parts per thousand; USGS34 has delta(18)O = -27.9parts per thousand and delta(17)O = -14.8parts per thousand; and USGS35 has delta(18)O = +57.5parts per thousand and delta(17)O = +51.5parts per thousand. Published in 2003 by John Wiley Sons, Ltd.