Project Details Calibration of organic and inorganic oxygen-bearing isotopic reference materials

Project No.:
2005-022-1-200
Start Date:
01 December 2005
End Date:
28 February 2009

Objective

The purpose of this project is to assess the relative amounts of oxygen isotopes in internationally distributed organic and inorganic isotopic reference materials and publish “consensus” values and uncertainties for these materials.

Description

During the past three decades, the determination of the relative amounts of stable isotopes of the light elements (H, C, N, O, and S) has dramatically increased because of expanded use in hydrology, environmental studies, microbiology, forensic investigations, atmospheric investigations, oceanography, etc. In the last 10 years, determination of the relative amounts of the isotopes of oxygen in organic and inorganic solids has increased because of developments in instrumentation. In the last 5 years, several new oxygen isotopic reference materials have been prepared. However, the values of the relative amounts of oxygen isotopes in these new materials and in older materials are not well known. Thus, the problem arises that two isotope laboratories analyzing the same sample may not report the same result within analytical uncertainty because they do not know what values to accept for internationally distributed oxygen isotopic reference materials.

The purpose of this work is to bring together expert analytical laboratories (Jena, Reston, Leipzig, Canberra, and Zurich) to measure the relative amounts of oxygen isotopes in isotopic reference materials. This highly coordinated analytical effort will include inorganic materials, organic materials, atmospheric oxygen, and two water reference materials-the scale for expressing relative amounts of oxygen isotopes in natural materials is anchored by the two isotopic reference materials VSMOW water and SLAP water. Strict analytical protocols will be designed and followed by all laboratories. An initial phase of the project will be to assess oxygen exchangeability of potential materials to eliminate those with exchangeable oxygen. At the conclusion of the analytical effort, task group members will convene for a 3-day workshop to determine “best” values and uncertainties. Some of the materials under consideration for inclusion in this proposed project are IAEA-CH-3 cellulose, IAEA-CH-6 sucrose, IAEA-600 caffeine, IAEA-601 & IAEA-602 benzoic acid, USGS40 & USGS41 L-glutamic acids, USGS32 KNO3, IAEA-NO-3 KNO3, USGS34 KNO3, USGS35 NaNO3, NBS-127 BaSO4, and IAEA-SO-5 & IAEA-SO-6 BaSO4.

Progress

> project announcement published in Chem. Int. Mar-Apr 2006, p. 29

March 2007 – Since this project commenced, thousands of very difficult measurements have been performed in this coordinated analytical measurement program. A meeting in Europe is planned for May 2007 to bring together task group members to determine consensus values for internationally distributed oxygen isotopic reference materials analyzed in this program.

January 2009 – Manuscript prepared for Rapid Communications in Mass Spectrometry accepted. To aid in the calibration of organic and inorganic isotopic reference materials, which span more than 125 ‰, an artificially enriched reference water [δ(18,16O) of +78.91 ‰] and two barium sulfates (one depleted and one enriched in 18O) were prepared and calibrated relative to the VSMOW-SLAP scale. Reference values of internationally distributed isotopic reference materials analyzed in this project are shown below.

 

Isotopic reference material103 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
VSMOW2 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.

Published outcomes:
– ‘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. 23 (2009) 999 [doi:10.1002/rcm.3958]

March 2009 – Project completed