The Commission on Isotopic Abundances and Atomic Weights (hereafter called the Commission or CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) has provided regular assessments of the standard atomic weights and isotopic compositions of the elements. The first Table of Isotopic Compositions was published by the Commission in 1931, based on the work of Francis W. Aston, and it included only seven elements . Since then, CIAAW has evaluated the isotopic compositions of all elements having a tabulated standard atomic weight by examining the most accurate and precise isotopic-abundance measurements in selected samples through its Subcommittee for Isotopic Abundance Measurements (SIAM), its predecessor Subcommittee on Assessment of Isotopic Composition (SAIC), and by compiling evidence for known variations of the isotopic abundances of the elements in normal materials through its former Subcommittee on Natural Isotopic Fractionation (SNIF). By a “normal material” CIAAW means a material from a terrestrial source that satisfies the following criterion :
“The material is a reasonably possible source for this element or its compounds in commerce, for industry or science; the material is not itself studied for some extraordinary anomaly and its isotopic composition has not been modified significantly in a geologically brief period.”
The results of these investigations are important for a number of reasons. The evaluated best measurements indicate the state of the metrology of isotope-abundance measurements, the best measurements provide benchmark data for isotopic reference materials, and the combination of best measurements and documented variations serve as the basis for the standard atomic weights of the elements.
The current Table of the Isotopic Compositions of the Elements, TICE 2013, was produced by CIAAW to accompany the 2013 Table of Standard Atomic Weights of the Elements (TSAW 2013) . The previous table of the isotopic compositions of the elements, TICE 2009, was published in 2011 , following CIAAW deliberations in 2009. TICE 2013 is intended to include data for normal materials and does not include published values for meteoritic or other extraterrestrial materials. Additional supporting data and background information can be found in the reports of de Laeter et al.  and Coplen et al. [6, 7].
2 Consistency with the atomic weights
Care has been taken to ensure that the representative isotopic abundances and uncertainties for all elements are consistent with the corresponding standard atomic weights and their uncertainties, including the twelve elements whose standard atomic-weight values are given as an interval. Isotope-amount ratios, isotopic abundances (also called isotope-amount fractions and atom fractions ), and the atomic weight of an element are related quantities. For this reason, the information given in the Table of Standard Atomic Weights and in the Table of Isotopic Compositions (Column 9) must be consistent with one another, as stated by CIAAW in 1983 . Consider, for example, the standard atomic weight of copper, Ar(Cu)=63.546(3), and its representative isotopic composition, x(63Cu)=0.6915(15) and x(65Cu)=0.3085(15). The atomic-weight value of 63.546 corresponds to isotopic abundances 0.6915 and 0.3085. Likewise, the atomic-weight value of, for example, 63.546+0.003 corresponds to isotopic abundances 0.6915–0.0015 and 0.3085+0.0015. In practice, however, matching the standard atomic weights, isotopic compositions and their uncertainties with the corresponding isotope amount ratios and their uncertainties is not an easy task considering that, over the last fifty years, the input data of these tables have been evaluated while statistical data treatment and other methods have changed. Nevertheless, care has been taken in this report to ensure that the representative isotopic abundances and their uncertainties for all elements (Column 9) are consistent with the standard atomic weights and their uncertainties.
3 Isotopic-abundance intervals for elements with intervals as standard atomic weights
Since 2009, standard atomic weights of selected elements are expressed using the interval notation to reflect the variability of atomic-weight values in normal materials  and to emphasize that atomic-weight values of these elements are not constants of nature [10, 11]. In keeping with these interval values of standard atomic weights, intervals for representative isotopic abundances of these elements are given in Column 9 of the Table of Isotopic Compositions of the Elements (Table 1). For elements with only two stable isotopes (H, Li, B, C, N, Cl, Br, and Tl), the lower and upper bounds of the isotopic abundances listed in Column 9 will agree with the corresponding bounds of the standard atomic weight to within ±1 in the last digit quoted. For elements with more than two stable isotopes (O, Mg, Si, and S), the reported isotopic abundance variations are calculated from the standard atomic weights using standard mass-dependent fractionation models (see discussion page 2551 of ). The number of decimal digits in the isotopic-abundance intervals is determined in the same fashion as they are determined for the standard atomic weights. In particular, the number of decimal digits in the lower and upper bounds is determined so that the uncertainty in the last quoted digit is less than one.
4 Table of isotopic compositions
The Table of Isotopic Compositions of the Elements (Table 1) consists of nine columns, as explained below.
Column 1: Atomic number (Z) of the element.
Column 2: Symbol of the element.
Column 3: Mass number (A) of each isotope that can be found in normal materials.
Column 4: Interval of isotopic-abundance variation in normal materials. No data are given in this column unless an interval has been reliably established (see, e.g., Coplen et al. [6, 7]). The interval given may not include those of certain exceptional samples, which are indicated with a “g” in Column 5. The isotopic abundance of each stable isotope is given as an isotopic-abundance interval with the symbol [a, b] to denote the set of isotopic-abundance values in normal materials; thus, a≤isotopic abundance≤b. The symbols a and b denote the lower and upper bounds of the interval [a, b], respectively. For 12 elements having atomic-weight values that are intervals (H, Li, B, C, N, O, Mg, Si, S, Cl, Br, and Tl), figures of variations in isotopic abundances and atomic weights for selected materials are presented in TSAW 2013 .
Column 5: Annotations.
g geologically exceptional specimens are known in which the element has an isotopic composition outside the reported interval.
m modified isotopic compositions may be found in commercially available material that has been subjected to an undisclosed or inadvertent isotope fractionation. Substantial deviations from the listed isotopic compositions can occur (refers to Column 9).
r range in isotopic composition of normal terrestrial material prevents more precise values (for Column 9) to be given. The tabulated values should be applicable to any normal material.
Note that the annotations apply to all isotopes of a given element.
Column 6: The best measurement of isotopic abundances from a single terrestrial source. The values are reproduced or recalculated by CIAAW from the original literature and are sometimes adjusted for minor errors or reformatted for easier reading. The uncertainties on the last digits are given in parentheses. As they are not reported in a uniform manner in the literature, numerals 1, 2, or 3 indicate the coverage factor applied to the standard deviation (s), to the standard error (standard deviation of the mean, se), or to the combined uncertainty (uc).
C is appended for a fully calibrated measurement when gravimetrically prepared isotope mixtures have been used to correct the underlying mass spectrometric measurement results for bias.
F is appended for a partially calibrated measurement when calibrated mixtures have been used to correct for measurement bias, but the measurement fails to fulfill all of the requirements of a “C” measurement.
N is appended for a non-calibrated measurement.
Users should be aware that a “best measurement” is not necessarily calibrated, nor is it necessarily free of systematic biases; it is just the best measurement available. Users seeking isotopic composition of unspecified natural sample should refer to Column 9.
Column 7: Reference for the best measurement in Column 6.
Column 8: Material that was used for the best measurement given in Column 6. An asterisk signifies that a substance is a recognized isotopic reference material . The listed materials do not necessarily define the origin of the delta zero scale.
Column 9: Representative isotopic abundances. This column lists the values that, in the opinion of CIAAW, represent the isotopic abundances of chemicals and natural materials that are likely to be encountered in the laboratory. These values are consistent with the standard atomic weights to the stated precision, i.e. to within ±1 in the last digit quoted . For 12 elements (H, Li, B, C, N, O, Mg, Si, S, Cl, Br, and Tl) having interval standard atomic-weight values the isotopic abundance of each stable isotope is given as an interval with the symbol [a, b] to denote the set of isotopic-abundance values in normal materials. The symbols a and b denote the lower and upper bounds of the interval [a, b], respectively and a≤isotopic abundance≤b. For these 12 elements Column 9 entries represent the observed interval of isotope-abundance variation in natural materials; thus, entries in Columns 4 and 9 are identical. For elements with known isotope-abundance variations that do not have interval atomic-weight values, Column 9 values may differ from the values corresponding to the best measurements. The values in parentheses following the isotopic abundances indicate the range of probable isotope-abundance variations among different materials as well as measurement uncertainties. Users should be aware of the following:
Values in Column 9 can be used to determine the average properties of the element in materials of unspecified natural terrestrial origin, but those values may not represent the most abundant materials, and it is possible that no real sample exists having the exact values listed.
When precise work is to be undertaken, such as assessment of isotope-dependent properties, samples with precisely known isotopic abundances (such as those listed in Column 8 or those listed in the SNIF diagrams ) should be used or suitable isotopic analyses should be made.
The Commission uses nuclide masses as published in the Atomic Mass Evaluation 2012 report by Wang et al. . However, the uncertainty of the nuclide masses is not taken as reported. Rather, all uncertainty estimates are expanded by a factor of six in order to conform to the conservative reporting practices of CIAAW.
5 Recommended isotope ratios
In addition to the standard atomic weights and the corresponding representative isotopic composition, CIAAW has recommended standard isotope ratios for selected elements.
Nitrogen.  CIAAW recommends that a standard value of 272.0(3) be employed for N(14N)/N(15N) of N2 in air. This value is in agreement with the isotopic composition for nitrogen given in Column 6.
Argon.  CIAAW recommends that a standard value of 298.56(31) be employed for N(40Ar)/N(36Ar) of argon in air. This value is in agreement with the isotopic composition for argon given in Column 6 (note that the uncertainty quoted here is not an expanded uncertainty, but rather with the coverage factor of k=1).
Uranium.  CIAAW recommends the representative natural terrestrial isotope ratio for uranium, N(238U)/N(235U)=137.8(1). This value is in agreement with the isotopic composition given in Column 9.
6 Sources of isotopic reference materials
Isotope reference materials are produced by National Metrology Institutes of many countries . A selected list of isotopic-reference material vendors is listed below. More information regarding available reference materials is available from the International database for certified reference materials COMAR (www.comar.bam.de/en/)
Federal Institute for Materials Research and Testing
European Reference Materials Program
Isotope Hydrology Laboratory
International Atomic Energy Agency
Institute for Reference Materials and Measurements
Joint Research Centre, European Commission
New Brunswick Laboratory
U.S. Department of Energy
Argonne, Illinois, USA
National Institute of Standards and Technology
U.S. Department of Commerce
Gaithersburg, Maryland, USA
Measurement Science and Standards
National Research Council Canada
Ottawa, Ontario, Canada
Reston Stable Isotope Laboratory
U.S. Geological Survey
Reston, Virginia, USA
7 Membership of sponsoring body
Membership of the IUPAC Inorganic Chemistry Division Committee for the period 2012–2013 was as follows:
President: R. D. Loss (Australia); Secretary: M. Leskelä (Finland); Vice President: J. Reedijk (Netherlands); Titular Members: M. Drábik (Slovakia); N. E. Holden (USA); P. Karen (Norway); S. Mathur (Germany); L. R. Öhrström (Sweden); K. Sakai (Japan); E. Y. Tshuva (Israel); Associate Members: J. Buchweishaija (Tanzania); T. Ding (China); J. Garcia-Martinez (Spain); D. Rabinovich (USA); A. Kilic (Turkey); R.-N. Vannier (France); National Representatives: F. Abdul Aziz (Malaysia); S. Ali (Pakistan); V. Chandrasekhar (India); B. Prugovecki (Croatia); H. E. Toma (Brazil); N. Trendafilova (Bulgaria); S. Youngme (Thailand).
Membership of the IUPAC Commission on Isotopic Abundances and Atomic Weights for the period 2012–2013 was as follows:
Chair: W. A. Brand (Germany); Secretary: J. Meija (Canada); Titular Members: M. Gröning (Austria); T. Hirata (Japan), T. Prohaska (Austria); R. Schönberg (Germany); Associate Members: M. Berglund (Belgium); G. O’Connor née Singleton (USA); M. Wieser (Canada); X.-K. Zhu (China); Ex-officio: R. D. Loss (Australia); National Representatives: T. B. Coplen (USA), P. De Bièvre (Belgium).
Membership of the IUPAC Subcommittee on Isotopic Abundance Measurements for the period 2012–2013 was as follows:
Chair: R. Schönberg (Germany); Secretary: M. Gröning (Austria); Members: M. Berglund (Belgium); J.-K. Böhlke (USA); W. A. Brand (Germany); T. B. Coplen (USA); P. De Bièvre (Belgium); T. Ding (China); T. Hirata (Japan), N. Holden (USA); R. D. Loss (Australia); J. Meija (Canada); T. Prohaska (Austria); G. O’Connor née Singleton (USA); T. Walczyk (Singapore); M. Wieser (Canada); S. Yoneda (Japan); X.-K. Zhu (China).
We thank Prof. J. Stohner (Zürich University of Applied Sciences) and several anonymous reviewers for constructive comments that improved the original manuscript. The financial support given by all coauthor institutions made this report possible. We also wish to gratefully acknowledge the intellectual contributions of past members of SIAM who provided us the predecessor TICE reports. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the Government of Canada, U.S. Government or the International Atomic Energy Agency. The following IUPAC Projects contributed to this Technical Report: 2009-025-1-200, 2009-029-1-200, and 2011-027-1-200.
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Published Online: 2016-02-20
Published in Print: 2016-03-01
Citation Information: Pure and Applied Chemistry, Volume 88, Issue 3, Pages 293–306, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2015-0503.http://creativecommons.org/licenses/by-nc-nd/4.0/.