Measurements of δ( 13 C) determined on CO 2 with an isotope-ratio mass spectro meter (IRMS) must be corrected for the amount of 17 O in the CO 2 . For data consistency, this must be done using identical methods by different laboratories. This report aims at unifying data treatment for CO 2 IRMS by proposing (i) a unified set of numerical values, and (ii) a unified correction algorithm, based on a simple, linear approximation formula. Because the oxygen of natural CO 2 is derived mostly from the global water pool, it is recommended that a value of 0.528 be employed for the factor λ, which relates differences in 17 The equation [δ( 13 C) ≈ 45 δ VPDB-CO2 + 2 17 R/ 13 R ( 45 δ VPDB-CO2 -λ 46 δ VPDB-CO2 )] closely approximates δ( 13 C) values with less than 0.010 ‰ deviation for normal oxygenbearing materials and no more than 0.026 ‰ in extreme cases. Other materials containing oxygen of non-mass-dependent isotope composition require a more specific data treatment. A similar linear approximation is also suggested for δ( 18 O). The linear approximations are easy to implement in a data spreadsheet, and also help in generating a simplified uncertainty budget.

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To calculate delta(13)C from raw CO(2) isotope data, the ion beam ratio of m/z 45 to 44 is corrected for the contribution arising from the contribution of (17)O-bearing molecules. First, a review on the current state of (17)O-corrections for CO(2) mass spectrometry is presented. The three correction algorithms that are generally in use, however, do produce biased delta(13)C values, and the bias is actually larger than the precision of modern isotope ratio mass spectrometers. The origin of this bias is twofold: different values for (17)R(VPDB-CO2) as well as different values for lambda are used in the correction algorithms. Despite both values being of high importance, large discrepancies between the absolute values published for (17)R(VPDB-CO2) appear to be the main reason for the delta(13)C biases. Next, the question of how to choose the value of lambda to best be used is considered. Natural (e.g. tropospheric) CO(2) as well as primary reference materials (PDB and NBS-19), having been in isotope exchange with water, are assumed to lie on the fractionation line for waters. On this ground, lambda = 0.5281 +/- 0.0015, as determined for waters (Meijer and Li, Isot. Environ. Health Stud., 1998; 34: 349-369), is suggested to be a base for the (17)O-correction algorithm. Finally, an approach to determine the absolute value for (17)R(VPDB-CO2), based on data of relative isotope measurements on two CO(2) gases having a large (17)O difference, is discussed and algebraic formulas are considered. Experimental data and new numerical values determined for (17)R(VPDB-CO2) and (17)R(VSMOW) are given in a companion paper.

An international project developed, quality-tested, and determined isotope-δ values of 19 new organic reference materials (RMs) for hydrogen, carbon, and nitrogen stable isotope-ratio measurements, in addition to analyzing pre-existing RMs NBS 22 (oil), IAEA-CH-7 (polyethylene foil), and IAEA-600 (caffeine). These new RMs enable users to normalize measurements of samples to isotope-δ scales. The RMs span a range of δ 2 H VSMOW-SLAP values from-210.8 to +397.0 mUr or ‰, for δ 13 C VPDB-LSVEC from-40.81 to +0.49 mUr, and for δ 15 N Air from-5.21 to +61.53 mUr. Many of the new RMs are amenable to gas and liquid chromatography. The RMs include triads of isotopically contrasting caffeines, C 16 nalkanes, n-C 20-fatty acid methyl esters (FAMEs), glycines, and L-valines, together with polyethylene powder and string, one n-C 17-FAME, a vacuum oil (NBS 22a) to replace NBS 22 oil, and a 2 H-enriched vacuum oil. Eleven laboratories from 7 countries used multiple analytical approaches and instrumentation for 2-point isotopic calibrations against international primary measurement standards. The use of reference waters in silver tubes allowed direct calibration of δ 2 H values of organic materials against isotopic reference waters following the principle of identical treatment. Bayesian statistical analysis yielded the mean values reported here. New RMs are numbered from USGS61 through USGS78, in addition to NBS 22a. Due to exchangeable hydrogen, amino acid RMs currently are recommended only for carbon-and nitrogen-isotope measurements. Some amino acids contain 13 C and carbon-bound organic 2 Henrichments at different molecular sites to provide RMs for potential site-specific isotopic analysis in future studies.

Rationale NBS19 carbonate, a primary reference material (RM) for the Vienna Pee Dee Belemnite (VPDB) scale realisation introduced in 1987, was exhausted in 2009, and no primary RM was available for several years. This study describes the preparation and characterisation of a new RM, IAEA‐603 (Ca‐carbonate, calcite of marble origin), which shall serve as a new primary RM (replacement for NBS19) or primary calibrator aimed at the highest realisation of the VPDB scale for δ13C and δ18O values, including the VPDB‐CO2 δ18O scale. Methods IAEA‐603 preparation and characterisation (value transfer) against NBS19 were performed by addressing the major modern technical requirements for the production and characterisation of RMs (ISO Guide 35). IAEA‐603 was produced in a large quantity, and the first batch was sealed into ampoules (0.5 g) to ensure RM integrity during storage; four other batches were sealed for long‐term storage. The most accurate method of CO2 preparation for isotope mass spectrometry was used, namely carbonate–H3PO4 reaction under controlled conditions. Results The assigned values of δ13C = +2.460 ± 0.010‰ and δ18O = −2.370 ± 0.040‰ (k = 1) are based on a large number of analyses (~10 mg aliquots) performed at IAEA and address all the known uncertainty components. For aliquots down to 120 μg, the δ18O uncertainty remains unchanged but shall be doubled for δ13C. The uncertainty components considered are as follows: (a) material homogeneity (within and between the 5200 ampoules produced), (b) value assignment against NBS19, (c) storage effects and (d) effect of the 17O correction. Conclusions The new primary RM IAEA‐603 replaces NBS19 in its use as the highest calibrator for the VPDB δ13C and δ18O scale, including the VPDB‐CO2 δ18O scale. The use of IAEA‐603 will allow laboratories worldwide to establish consistent realisation of the scales for δ13C and δ18O values and metrological comparability of measurement results for decades. The VPDB scale definition based on NBS19 stays valid.

In a companion paper in this issue we presented a review of the current state of (17)O-corrections for CO(2) mass spectrometry and considered an approach (including algebraic formulae) of how to determine absolute values for (17)R(VPDB-CO2) and (17)R(VSMOW). Here we present the results of experiments conducted to determine these values. Two oxygen gases (one depleted in heavy isotopes and the other isotopically normal oxygen) were analysed to obtain the relative (17)O content. Samples of both gases were converted into CO(2), and the resulting CO(2) samples were analysed as well. Possible experimental and analytical errors are carefully considered and eliminated as far as feasible. Much attention was paid to understanding and dealing with cross-contamination effects occurring in the mass spectrometer. Based on the data obtained, the absolute values are calculated to be: (17)R(VPDB-CO2) = 0.00039511 +/- 0.00000094 and (17)R(VSMOW) = 0.00038672 +/- 0.00000087 (expanded uncertainties). Both values are on the original scale of Craig (Geochim. Cosmochim. Acta 1957; 12: 133-149) with (13)R(VPDB-CO2) = 0.0112372. A (17)O-correction algorithm incorporating the newly determined value for (17)R(VPDB-CO2) and lambda = 0.528 by Meijer and Li (Isot. Environ. Health Stud. 1998; 34: 349-369) is constructed. A computational test is performed to demonstrate the degree of delta(13)C bias relative to the previously known correction algorithms. delta(13)C values produced by the constructed algorithm are in the middle of the values produced by the other algorithms. We refrain, however, from giving any recommendation concerning which (17)O-correction algorithm to use in order to obtain delta(13)C data in the most accurate way. The present work illuminates the need to reconsider recommendations concerning the correction algorithm.

This paper discusses a simple method to determine 17O isotope excess or deficiency ('mass-independent isotopic composition') in CO2 gas. When applying conventional mass spectrometry of CO2 (m/z 44, 45 and 46) to determine the 17O/16O ratio, the 13C/12C ratio has to be established separately. This can be achieved by analysing an aliquot of sample CO2 before and after subjecting it to oxygen isotope exchange with a pool of oxygen with 'normal' 17O/16O ratio, i.e. with Delta17O approximately equal to delta17O-0.516 x delta18O = 0. Cerium oxide has been shown to be practically well suited for the exchange of CO2 oxygen; the reagent is safe and does not produce any contamination. The CO2-CeO2 exchange reaction has 99.8 +/- 0.7% recovery yield. At 650 degrees C this reaction reaches equilibrium in 30 min and, as tested, results in complete oxygen replacement. Delta17O determinations depend on accuracy of CO2 delta measurements: the repeatability of +/-0.015 per thousand (1sigma) in delta(45)R and delta(46)R determination relative to the working reference results in an error of Delta17O as small as +/-0.33 per thousand. Such a precision is sufficient for Delta17O determination in stratospheric CO2. The calculated Delta17O value systematically depends on absolute 17R and 13R ratios in isotopic reference materials, which are presently not yet known with certainty (the 17R value is most important), and may be inadequate for 17O-correction with a = 0.516. Within the present uncertainty, Delta17O determined in 17O-enriched CO2 agrees with the value directly measured in the enriched O2 from which this CO2 was produced. Besides Delta17O determination, investigated CO2-CeO2 equilibration may have several other implications. Fast, complete isotopic exchange of CO2 by reaction with CeO2 may also be employed to get reproducible 17O-correction and, hence, to better monitor small delta13C shifts and to isotopically equilibrate mixtures of CO2 gases.

Using natural gas for fuel releases less carbon dioxide per unit of energy produced than burning oil or coal, but its production and transport are accompanied by emissions of methane, which is a much more potent greenhouse gas than carbon dioxide in the short term. This calls into question whether climate forcing could be reduced by switching from coal and oil to natural gas. We have made measurements in Russia along the world's largest gas-transport system and find that methane leakage is in the region of 1.4%, which is considerably less than expected and comparable to that from systems in the United States. Our calculations indicate that using natural gas in preference to other fossil fuels could be useful in the short term for mitigating climate change.