Measurement of SLAP2 and GISP 17O and proposed VSMOW-SLAP normalization for 17O and 17Oexcess

RATIONALE The absence of an agreed-upon 17O value for the primary reference water SLAP leads to significant discrepancies in the reported values of 17O and the parameter 17Oexcess. The accuracy of 17O and 17Oexcess values is significantly improved if the measurements are normalized using a two-point calibration, following the convention for 2H and 18O values. METHODS New measurements of the 17O values of SLAP2 and GISP are presented and compared with published data. Water samples were fluorinated with CoF3. Helium carried the O2 product to a 5A (4.2 to 4.4 angstrom) molecular sieve trap submerged in liquid nitrogen. The O2 sample was introduced into a dual-inlet ThermoFinnigan MAT 253 isotope ratio mass spectrometer for measurement of m/z 32, 33, and 34. The 18O and 17O values were calculated after 90 comparisons with an O2 reference gas. RESULTS We propose that the accepted 17O value of SLAP be defined in terms of 18O=55.5 parts per thousand and 17Oexcess=0, yielding a 17O value of approximately 29.6968 parts per thousand. Using this definition for SLAP and the recommended normalization procedure, the 17O value of GISP is 13.16 +/- 0.05 parts per thousand and the 17Oexcess value of GISP is 22 +/- 11 per meg. Correcting previous published values of GISP 17O to both VSMOW and SLAP improves the inter-laboratory precision by about 10 per meg. CONCLUSIONS The data generated here and compiled from previous studies provide a substantial volume of evidence to evaluate the various normalization techniques currently used for triple oxygen isotope measurements. We recommend that reported 17O and 17Oexcess values be normalized to the VSMOW-SLAP scale, using a definition of SLAP such that its 17Oexcess is exactly zero. Copyright (c) 2013 John Wiley & Sons, Ltd.