Tyler B. Coplen Anita T. Aerts-Bijma John Karl Bohlke Matthias Gehre Heike Geilmann Manfred Groning Henk G. Jansen Harro A. J. Meijer Stanley J. Mroczkowski Haiping Qi Karin Soergel Hilary Stuart-Williams Stephan M. Weise Roland A. Werner Willi A. Brand 2009 <p>Internationally distributed organic and inorganic oxygen isotopic reference materials have been calibrated by six laboratories carrying out more than 5300 measurements using a variety of high-temperature conversion techniques (HTC)&nbsp;in an evaluation sponsored by the International Union of Pure and Applied Chemistry (IUPAC). To aid in the calibration of these reference materials, which span more than 125‰, an artificially enriched reference water (<i>δ</i>¹⁸O of +78.91‰) and two barium sulfates (one depleted and one enriched in ¹⁸O) were prepared and calibrated relative to VSMOW2&nbsp;and SLAP reference waters. These materials were used to calibrate the other isotopic reference materials in this study, which yielded:</p><div id="scrollable-1496341670582" class="table__overflow js-module scrollable" data-module="Table"><table class="table table--article-section mce-item-table" border="0"><thead class="table__header table__header-data--bottom"><tr class="table__row"><th class="table__header-data table-border--bottom ">Reference material</th><th class="table__header-data table-border--bottom "><i>δ</i>¹⁸O and estimated combined uncertainty&nbsp;</th></tr></thead><tbody class="table__body table__data--top"><tr class="table__row"><td class="table__data ">IAEA-602 benzoic acid</td><td class="table__data table__data--center ">+71.28 ± 0.36‰</td></tr><tr class="table__row"><td class="table__data ">USGS35 sodium nitrate</td><td class="table__data table__data--center ">+56.81 ± 0.31‰</td></tr><tr class="table__row"><td class="table__data ">IAEA-NO-3 potassium nitrate</td><td class="table__data table__data--center ">+25.32 ± 0.29‰</td></tr><tr class="table__row"><td class="table__data ">IAEA-601 benzoic acid</td><td class="table__data table__data--center ">+23.14 ± 0.19‰</td></tr><tr class="table__row"><td class="table__data ">IAEA-SO-5 barium sulfate</td><td class="table__data table__data--center ">+12.13 ± 0.33‰</td></tr><tr class="table__row"><td class="table__data ">NBS 127 barium sulfate</td><td class="table__data table__data--center ">+8.59 ± 0.26‰</td></tr><tr class="table__row"><td class="table__data ">VSMOW2 water</td><td class="table__data table__data--center ">0‰</td></tr><tr class="table__row"><td class="table__data ">IAEA-600 caffeine</td><td class="table__data table__data--center ">−3.48 ± 0.53‰</td></tr><tr class="table__row"><td class="table__data ">IAEA-SO-6 barium sulfate</td><td class="table__data table__data--center ">−11.35 ± 0.31‰</td></tr><tr class="table__row"><td class="table__data ">USGS34 potassium nitrate</td><td class="table__data table__data--center ">−27.78 ± 0.37‰</td></tr><tr class="table__row"><td class="table__data ">SLAP water</td><td class="table__data table__data--center ">−55.5‰</td></tr></tbody></table></div><p>The seemingly large estimated combined uncertainties arise from differences in instrumentation and methodology and difficulty in accounting for all measurement bias. They are composed of the 3-fold standard errors directly calculated from the measurements and provision for systematic errors discussed in this paper. A primary conclusion of this study is that nitrate samples analyzed for <i>δ</i>¹⁸O should be analyzed with internationally distributed isotopic nitrates, and likewise for sulfates and organics. Authors reporting relative differences of oxygen-isotope ratios (<i>δ</i>¹⁸O) of nitrates, sulfates, or organic material should explicitly state in their reports the <i>δ</i>¹⁸O 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 <i>δ</i>¹⁸O values at a later time should it become necessary.</p><p>The high-temperature reduction technique for analyzing <i>δ</i>¹⁸O and <i>δ</i>²H 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.</p> application/pdf 10.1002/rcm.3958 en Wiley Comprehensive inter-laboratory calibration of reference materials for δ18O versus VSMOW using various on-line high-temperature conversion techniques article