32 No. 5
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Assessment of Stable Isotopic Reference and Inter-Comparison Materials
While many school text books still consider the terrestrial abundances of the stable isotopes of the elements as constants and characteristic for the planet or for the solar system, this is not really true and needs to be refined. Some of the elements contain radiogenic isotopes, and the respective abundance changes can, for instance, be used for dating the age of stones or sediments. In particular, the light life-science elements carbon, nitrogen, oxygen, and hydrogen exhibit variations originating from fractionation processes:
- Upon evaporation, the water isotope 2H is depleted in the gas phase, similarly 18O. Conversely, during condensation, 2H and 18O preferentially accumulate in the liquid. Repeating this process, as is permanently happening in precipitation, can leave a larger trace in form of significant isotope abundance alterations, which form the basis for paleo-climatic investigations using ice cores from Antarctica or Greenland ice sheets.
- During photosynthesis, the light isotope 12C in CO2 is preferred, hence the resulting organic material is depleted in the heavier 13C isotope. Generation of all living matter starts with CO2 from air (or in the oceans). The 13C/12C ratio of CO2 in air has varied very little over billions of years, hence all fossil organic carbon has a lower 13C/12C ratio (roughly 0.0110 instead of 0.0112).
The stable isotope ratio changes due to fractionation processes such as the ones above (there are many more in nature, in particular in living matter, where the reactions mostly are catalyzed and proceed close to the threshold) appear small; yet, they are both, significant and robust. They also can be analyzed reliably with specialized technology that was developed in the early 1950s. We now have more than 2000 stable isotope ratio laboratories world wide, and one of the major challenges is to have any two laboratories produce identical results from the same materials. Fractionation processes in general also happen during sample preparation (all carbon bearing material has to be converted to CO2 for stable isotope measurement). Therefore, best results are obtained from identical chemical compounds or classes of compounds. This contrasts with the fact that international standards often are carbonates, water samples, or a few other compounds. There is a need for a larger chemical variety in stable isotope reference materials. Solving this problem entails accurate calibration of these materials.
Accurate calibration often is a major challenge, in particular for materials that are chemically very different or that have a large deviation in isotopic composition. Therefore, a number of calibration studies are always under way somewhere; the results of which are then published and the results made available (e.g., through NIST or the IAEA in Vienna). What we are missing is an authority that audits the respective analytical results and is capable of separating “the wheat from the chaff.”
The project is aimed at producing an authoritative, citable reference table for Stable Isotope Ratio Reference Materials, with a biennial update cycle.
For more information contact Task Group Chair Willi A. Brand
last modified 8 September 2010.
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