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How well do we know VPDB? Variability of δ 13 C and δ 18 O in CO 2 generated from NBS19‐calcite
Author(s) -
Brand Willi A.,
Huang Lin,
Mukai Hitoshi,
Chivulescu Alina,
Richter Jürgen M.,
Rothe Michael
Publication year - 2009
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.3940
Subject(s) - calcite , chemistry , isotope , homogeneous , stable isotope ratio , isotopes of carbon , isotopes of oxygen , carbon fibers , consistency (knowledge bases) , mineralogy , analytical chemistry (journal) , environmental chemistry , total organic carbon , nuclear chemistry , thermodynamics , physics , materials science , composite number , composite material , geometry , mathematics , quantum mechanics
In order to generate a local daughter scale from the material defining the international δ 13 C and δ 18 O stable isotope ratio scales (NBS19‐calcite),1,2 the carbon and oxygen must be liberated to the gas phase, usually as CO 2 , using acid digestion of the calcite with H 3 PO 4 . It is during this conversion step that systematic errors can occur, giving rise to commonly observed discrepancies in isotopic measurements between different stable isotope laboratories. Scale consistency is of particular importance for air‐CO 2 isotope records where very small differences in isotopic composition have to be reliably compared between different laboratories and quantified over long time periods.3 The information is vital for estimating carbon budgets on regional and global scales and for understanding their variability under the conditions of climate change. Starting from this requirement a number of CO 2 preparations from NBS19 were made at Environment Canada (EC) and analyzed in our laboratories together with Narcis II, a set of well‐characterized CO 2 samples in sealed tubes available from the National Institute for Environmental Studies (NIES).4,5 Narcis II is very homogeneous in δ 13 C and δ 18 O with the isotopic composition close to NBS19‐CO 2 . Among our laboratories the results for δ 13 C agreed to within ±0.004‰. The same level of agreement in δ 13 C was obtained when CO 2 was generated from NBS19‐calcite using different experimental procedures and conditions in the other two laboratories. For δ 18 O, the corresponding data were ±0.011‰ when using NBS19‐CO 2 produced at EC, but discrepancies were enhanced by almost one order of magnitude when NBS19‐CO 2 was prepared by the other laboratories using slightly different reaction conditions (range = 0.13‰). In a second series of experiments, larger amounts of CO 2 prepared from NBS19 at the Max‐Planck‐Institut für Biogeochemie (MPI‐BGC) were analyzed together with Narcis II and then mixed into CO 2 ‐free air. The resulting artificial air samples then were measured by the same three laboratories for the stable isotopic composition of CO 2 using locally established extraction and evaluation procedures. Comparison of the results with the prior CO 2 values and between the laboratories revealed additional systematic differences owing to the local CO 2 extraction processes and standardization procedures. For δ 13 C the results showed a narrow range of discrepancies of about 0.02‰; for δ 18 O cumulative disagreements in the range of 0.1‰ were observed. From these results the following conclusions are inferred: NBS19‐CO 2 is a reliable primary anchor to the VPDB δ 13 C scale. Although prepared by different methods an accuracy of better than ±0.003‰ has been reached. This applies to sample amounts of 5 mg calcite or more. NBS19‐CO 2 can be used as a general anchor to the VPDB δ 18 O scale only for accuracy requirements of ±0.1‰. For a higher scale resolution additional agreements regarding details of the acid digestion reaction will have to be worked out and agreed upon. Narcis II‐CO 2 comprises an ideal set of test samples for the VPDB scale. The δ 13 C value is +1.923 ± 0.003‰ (combined uncertainty); δ 18 O is between −2.50 and −2.65‰ versus VPDB‐CO 2 , with most of the variation in this figure depending on details of the NBS19‐CO 2 preparation used for the calibration. (Ampoule to ampoule homogeneity is better than 0.01‰.) When mixing NBS19‐CO 2 into artificial air and using this to test performance between laboratories, the δ 13 C offsets are small with a remaining discrepancy of only 0.02‰. For δ 18 O, systematic disagreements are considerably larger than those found for the pure CO 2 comparison. Further experimental clarification is required. Artificial air samples such as NBS19‐CO 2 in air can be used as reliable anchors to create a unified stable isotope scale between different laboratories. An adjustment of local scales based on these air standards appears to be necessary for improving data comparability. Copyright © 2009 John Wiley & Sons, Ltd. and Her Majesty the Queen in Right of Canada