Abstract
RationaleThe stable carbon isotopic (δ 13C) reference material (RM) LSVEC Li2CO3 has been found to be unsuitable for δ 13C standardization work because its δ 13C value increases with exposure to atmospheric CO2. A new CaCO3 RM, USGS44, has been prepared to alleviate this situation.MethodsUSGS44 was prepared from 8 kg of Merck high‐purity CaCO3. Two sets of δ 13C values of USGS44 were determined. The first set of values was determined by online combustion, continuous‐flow (CF) isotope‐ratio mass spectrometry (IRMS) of NBS 19 CaCO3 (δ 13CVPDB = +1.95 milliurey (mUr) exactly, where mUr = 0.001 = 1‰), and LSVEC Li2CO3 (δ 13CVPDB = −46.6 mUr exactly), and normalized to the two‐anchor δ 13CVPDB‐LSVEC isotope‐delta scale. The second set of values was obtained by dual‐inlet (DI)‐IRMS of CO2 evolved by reaction of H3PO4 with carbonates, corrected for cross contamination, and normalized to the single‐anchor δ 13CVPDB scale.ResultsUSGS44 is stable and isotopically homogeneous to within 0.02 mUr in 100‐μg amounts. It has a δ 13CVPDB‐LSVEC value of −42.21 ± 0.05 mUr. Single‐anchor δ 13CVPDB values of −42.08 ± 0.01 and −41.99 ± 0.02 mUr were determined by DI‐IRMS with corrections for cross contamination.ConclusionsThe new high‐purity, well‐homogenized calcium carbonate isotopic reference material USGS44 is stable and has a δ 13CVPDB‐LSVEC value of −42.21 ± 0.05 mUr for both EA/IRMS and DI‐IRMS measurements. As a carbonate relatively depleted in 13C, it is intended for daily use as a secondary isotopic reference material to normalize stable carbon isotope delta measurements to the δ 13CVPDB‐LSVEC scale. It is useful in quantifying drift with time, determining mass‐dependent isotopic fractionation (linearity correction), and adjusting isotope‐ratio‐scale contraction. Due to its fine grain size (smaller than 63 μm), it is not suitable as a δ 18O reference material. A δ 13CVPDB‐LSVEC value of −29.99 ± 0.05 mUr was determined for NBS 22 oil.
Highlights
High accuracy measurements of stable carbon isotope ratios (δ13C values) in naturally occurring materials are necessary in an increasing number of fields, including oceanography, atmospheric sciences, biology, paleoclimatology, geology, environmental sciences, food and drug authentication, and forensic applications
Following recommendations of the Commission on Isotopic Abundances and Atomic Weights (CIAAW) in August 2005 at the 43rd General Assembly of International Union of Pure and Applied Chemistry (IUPAC) in Beijing and recommendations of an International Atomic Energy Agency (IAEA) panel, a recommendation evolved that δ13C values of all carbon-bearing materials should be measured and expressed relative to Vienna Peedee Belemnite (VPDB) on a scale normalized by assigning consensus values of −46.6 mUr to LSVEC lithium carbonate and +1.95 mUr to NBS 19 calcium carbonate.[10,11]
The excellent agreement in δ13CVPDB values of USGS44 between the DI-isotope-ratio mass spectrometry (IRMS) and elemental analyzer (EA) measurements suggests that: (a) the EA/IRMS measurements are not faulty and (b) the discrepancy in δ13CVPDB values between the EA and DI-IRMS measurements is caused by scaling due to an incorrect value of −46.6 mUr assigned to LSVEC
Summary
High accuracy measurements of stable carbon isotope ratios (δ13C values) in naturally occurring materials are necessary in an increasing number of fields, including oceanography, atmospheric sciences, biology, paleoclimatology, geology, environmental sciences, food and drug authentication, and forensic applications. In the past several decades, the international isotopic RMs NBS 18, NBS 19, NBS 22, LSVEC, IAEA-CO-1, IAEA-CO-8, IAEA-CO-9, and IAEA603 have been gradually introduced to the isotope community and used for the determination of δ13C values of carbon-bearing materials.[1,2,3] In 1985, the primary recommendation of a Consultants' Group Meeting of the International Atomic Energy Agency (IAEA)[4] was that a new Vienna Peedee Belemnite (VPDB) δ13C scale be established with NBS 19 carbonate assigned the value of +1.95 milliurey (mUr) exactly as its single anchor, where 1 mUr = 0.001 = 1‰.1,5 Implementation of this recommendation improved consistency among δ13C measurements.[6] Recognizing that two-point normalization of the δ2H and δ18O scales substantially improved agreement among laboratories,[7] the IAEA convened a panel in 2004 to review stable carbon isotopic RMs and to recommend a second RM for two-point normalization of the δ13C scale. An IUPAC technical report, which assessed international RMs for isotoperatio measurements, published in 2014 by Brand et al,[1] tabulates a comprehensive list of δ13C values of RMs on the NBS 19-LSVEC scale
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