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Vol. 31 No. 4
Jul-August 2009

Making an imPACt | Recent IUPAC technical reports and recommendations that affect the many fields of pure and applied chemistry.
See also www.iupac.org/publications/pac

Comprehensive Inter-Laboratory Calibration of Reference Materials for δ18O Versus VSMOW Using Various On-Line High-Temperature Conversion Techniques
W.A. Brand, T.B. Coplen, A.T. Aerts-Bijma, J.K. Böhlke, M. Gehre, H. Geilmann, M. Gröning, H.G. Jansen, H.A. J. Meijer, S.J. Mroczkowski, H. Qi, K. Soergel, H. Stuart-Williams, S.M. Weise, and R.A. Werner
Rapid Comm. Mass Spectrom, 2009
Vol. 23, Issue 7, pp. 999–1019
doi:10.1002/rcm.3958

Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5 300 measurements using a variety of high-temperature conversion techniques (HTC) in an evaluation sponsored by IUPAC. To aid in the calibration of these reference materials, which span more than 125 per mille, an artificially enriched reference water (δ18O of +78.91 per mille) and two barium sulfates (one depleted and one enriched in 18O) were prepared and calibrated relative to VSMOW2* and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded:

Isotopic reference material 103 δ(18,16O VSMOW-SLAP)
reference value
IAEA-602 benzoic acid +71.28
USGS35 sodium nitrate +56.81
IAEA-NO-3 potassium nitrate +25.32
IAEA-601 benzoic acid +23.14
IAEA-SO-5 barium sulfate +12.13
NBS 127 barium sulfate +8.59
VSMOW water 0
IAEA-600 caffeine –3.48
IAEA-SO-6 barium sulfate –11.35
USGS34 potassium nitrate –27.78
SLAP water –55.5

A primary conclusion of this study is that nitrate samples analyzed for δ(18,16O) should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios [δ(18,16O)] of nitrates, sulfates, or organic material should explicitly state in their reports the δ(18,16O) values of two or more internationally distributed nitrates (USGS34, IAEA-NO-3, and USGS35), sulfates (IAEA-SO-5, IAEA-SO-6, and NBS 127), or organic material (IAEA-601 benzoic acid, IAEA-602 benzoic acid, and IAEA-600 caffeine), as appropriate to the material being analyzed, had these reference materials been analyzed with unknowns. This procedure ensures that readers will be able to normalize the δ(18,16O) values at a later time should it become necessary. The high-temperature reduction technique for analyzing δ(18,16O) and δ(2,1H) is not as widely applicable as the well-established combustion technique for carbon and nitrogen stable isotope determination. To obtain the most reliable stable isotope data, materials should be treated in an identical fashion; within the same sequence of analyses, samples should be compared with working reference materials that are as similar in nature and in isotopic composition as feasible.

*In 2007, VSMOW2 replaced the almost exhausted VSMOW as the primary reference material and anchor to the VSMOW scale (for details, see <http://www-naweb.iaea.org/NAALIHL/> and <http://www-naweb.iaea.org/NAALIHL/docs/ref_mat/InfoSheet-VSMOW2-SLAP2.pdf>). For 18O, VSMOW2 and VSMOW are indistinguishable. The scale itself remains unaltered and keeps its name (“VSMOW”).

www.iupac.org/web/ins/2005-022-1-200


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