title: High-precision measurements of δ(17)O and (17) Oexcess of NBS19 and NBS18.
abstract: BACKGROUND
  Measurements of oxygen-17 excess ((17)Oexcess) in carbonates have become of great importance. However, to compare results obtained by different laboratories, it is necessary to normalize them to international standards. With this purpose in mind, we measured accurate and high precision δ(17)O and (17)Oexcess values for NBS19 and NBS18, two international standards, for which δ(18)O values are already widely used as the references for carbonates.
  
  
  METHODS
  The measurements are based on isotopic exchange at steady state between O2 and CO2 over hot platinum sponge at 750°C. Dual-inlet isotope ratio mass spectrometry (IRMS) measurements of the δ(17)O and δ(18)O values of this O2 allow δ(17)O values of CO2 to be obtained with very high precision (0.01 to 0.03‰) and, correspondingly, accurate (17)Oexcess values with a precision of 5 per meg.
  
  
  RESULTS
  We measured, for the first time, the δ(17)O values and (17)Oexcess of CO2 liberated from NBS19 at 25°C (39.196 ± 0.026; 20.276 ± 0.015‰ and -227 ± 4 per meg, respectively) and NBS18 (17.591 ± 0.041‰; 9.253 ± 0.018‰ and 3 ± 5 per meg, respectively). The values are given versus VSMOW.
  
  
  CONCLUSIONS
  Accurate values of δ(17)O and (17)Oexcess of the international standards NBS19-CO2 and NBS18-CO2 are now available. The new values should be used for normalization of measured oxygen isotope ratios of carbonates to allow meaningful comparison of results among different laboratories.
authors.affiliation: Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond J. Safra Campus Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Eugeni
authors.initials: E
authors.lastname: Barkan
authors.affiliation: Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond J. Safra Campus Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Israela
authors.initials: I
authors.lastname: Musan
authors.affiliation: Institute of Earth Sciences, Hebrew University of Jerusalem, Edmond J. Safra Campus Givat Ram, Jerusalem, 91904, Israel.
authors.forename: Boaz
authors.initials: B
authors.lastname: Luz
mesh_terms: D002245:Carbon Dioxide / Q000032:analysis; D002254:Carbonates / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods* / Q000592:standards; D010100:Oxygen / Q000032:analysis*; D010103:Oxygen Isotopes / Q000032:analysis; D012015:Reference Standards
chemical_list: D002254:Carbonates; D010103:Oxygen Isotopes; D002245:Carbon Dioxide; D010100:Oxygen