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Pure and Applied Chemistry

The Scientific Journal of IUPAC

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Volume 90, Issue 7

Issues

Clarification of the term “normal material” used for standard atomic weights (IUPAC Technical Report)

Tyler B. CoplenORCID iD: http://orcid.org/0000-0003-4884-6008 / Norman E. HoldenORCID iD: http://orcid.org/0000-0002-1988-4729 / Michael E. WieserORCID iD: http://orcid.org/0000-0002-1037-0772 / John Karl BöhlkeORCID iD: http://orcid.org/0000-0001-5693-6455
Published Online: 2018-05-25 | DOI: https://doi.org/10.1515/pac-2017-0301

Abstract

The standard atomic weights of the elements apply to normal materials. Since 1984, the Commission on Isotopic Abundances and Atomic Weights (Commission) has defined a normal material as:

“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 term “a geologically brief period” in this definition is confusing, and confusion can be reduced by revising this definition to the following, which was accepted by the Commission on Isotopic Abundances and Atomic Weights at its meeting in Groningen, Netherlands in September 2017:

Normal materials include all substances, except (1) those subjected to substantial deliberate, undisclosed, or inadvertent artificial isotopic modification, (2) extraterrestrial materials, and (3) isotopically anomalous specimens, such as natural nuclear reactor products from Oklo (Gabon) or other unique occurrences.”

Keywords: argon; artificial isotopic separation; atomic weights; isotopes; Oklo natural reactor; strontium

Article note:

Sponsoring body: IUPAC Inorganic Chemistry Division: see more details on page 1224.

1 Introduction

In the last review of the biennial report of the IUPAC Commission on Isotopic Abundances and Atomic Weights (‘Commission’ hereafter), there were concerns that the definition of “normal material” used for standard atomic weights was confusing. The IUPAC Subcommittee on Natural Assessment of Fundamental Understanding of Isotopes (SNAFUI) met 5–6 December 2016, with one of its major objectives the revision of this definition of normal material [1].

2 Normal Material

From its inception in 1919, the International Union of Pure and Applied Chemistry (IUPAC) took over the careful evaluation and dissemination of atomic weights from critically assessed, published information through its Commission on Isotopic Abundances and Atomic Weights. Each standard atomic-weight value reflects the best knowledge of evaluated, published data [2, 3] of normal materials. The implied range of each standard atomic weight is intended to apply to all sources of normal materials of that element. In the Commission’s 1969 report [4], a “normal” material was defined as:

  • “. . . one that contains as a major constituent a specified element with an atomic weight value that does not display a significant difference from the accepted value of that atomic weight because of:

    1. its radiogenic source;

    2. its extraterrestrial origin;

    3. artificial alteration;

    4. mutation; or

    5. a rare geological occurrence in small quantity.”

This definition for normal material in the 1969 Commission report [4] resulted in several decrees:

  • “In assessing variations in isotopic composition, the Commission will continue to disregard other than ‘normal’ materials.”

  • “To arrive at the recommended value for an atomic weight, the Commission will use weighting procedures so that the value will be optimized for materials in world science, chemical technology and trade, rather than represent an estimated geochemical average [of the Earth].”

  • “The Commission must attempt to state the atomic weight values so that they are as precise as possible. At the same time, they must be sufficiently imprecise so that all normal specimens fall within the implied tolerance range. In other words, large quantities of available materials should not lie outside the tolerance range. The difficult judgment has to be made when only a small fraction of normal material falls outside the tolerance range. The Commission has decided in such cases not to discard useful accuracy applicable to the great majority of practical condition, but to exclude from the definition of ‘normal’, geological oddities.”

This definition was updated in the 1971 Table of Standard Atomic Weights (TSAW) [5], where “artificial alteration” was updated to “artificial isotopic fractionation” and “mutation” was updated to “artificial nuclear reaction.” In 1984, the Commission revised the definition of a normal material to a material from a terrestrial source that satisfies the following criteria [2]:

“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.”

Consideration of this definition of normal materials by the Commission has resulted in several changes in standard atomic weights in the last three decades, for example:

  1. The uncertainty value of the standard atomic weight of boron was expanded in 1995 (from 10.811(5) to 10.811(7)) to include boron in sea water because it was decided that boron in sea water was a normal material [6].

  2. In 1995, the Commission decided it could reduce the uncertainty value of the standard atomic-weight of carbon from 12.011(1) to 12.0107(8), noting that specimens outside these limits could be covered by footnote “g” for rare geological occurrences in small quantities [6].

  3. In 1999, the Commission recognized that the standard atomic weight of nitrogen did not include a substantial fraction of naturally occurring materials in the terrestrial environment, and the Commission changed the standard atomic weight from 14.006 74(7) to 14.0067(2) [7].

Although this definition of normal material has served the Commission for the last three decades [8], a revised definition is needed, in part, because the clause “its isotopic composition has not been modified significantly in a geologically brief period” is not clear to many readers. Considering the intended meaning of normal materials since the 1960s, confusion in its definition can be reduced by revising this definition to the following, which was accepted by the Commission at its meeting in Groningen, Netherlands in September 2017:

Normal materials include all substances, except (1) those subjected to substantial deliberate, undisclosed, or inadvertent artificial isotopic modification, (2) extraterrestrial materials, and (3) isotopically anomalous specimens, such as natural nuclear reactor products from Oklo (Gabon) or other unique occurrences.”

This revised definition of normal material no longer includes the exclusion that “its isotopic composition has not been modified significantly in a geologically brief period.” This modification recognizes the fact that much of the atomic-weight variation of some elements is caused by isotopic fractionation processes that operate on many different time scales. This revised definition re-introduces the exclusion of extraterrestrial materials from the determination of standard atomic weights, consistent with the 1969 definition of normal material [4] and with recent Commission practice. In contrast with the 1969 definition and recent practice, this revised definition anticipates that materials with radiogenic or nucleogenic isotopic variability may no longer be excluded in the definition of a normal material, except in very exceptional situations, such as anomalous occurrences of 87Sr in rubidium ores having a 87Sr amount fraction of 1 [9], giving rise to the assignment of footnote “g” to strontium. This observation [9] in 1937 predates by 35 years the discovery of natural nuclear chain reactor products, discovered in 1972 at the Oklo quarry in Gabon, Africa, and first discussed by the Commission in its 1973 report [8, 10]. For example, with this revised definition of normal materials, based on the recent evaluation of argon (Fig. 1) by Böhlke [11], the standard atomic weight of argon could be changed from the current value of 39.948±0.001 [12] to [39.792, 39.963], when expressed as an interval, to include naturally occurring sources having nucleogenic and radiogenic isotopic variation.

Argon atomic weights in normal terrestrial materials compared with the current standard atomic weight [12] (modified from [11]). “F” indicates variation primarily related to isotopic fractionation. “N” indicates variation primarily related to nucleogenic 38Ar and (or) 36Ar production. “R” indicates variation primarily related to radiogenic 40Ar production.
Fig. 1:

Argon atomic weights in normal terrestrial materials compared with the current standard atomic weight [12] (modified from [11]). “F” indicates variation primarily related to isotopic fractionation. “N” indicates variation primarily related to nucleogenic 38Ar and (or) 36Ar production. “R” indicates variation primarily related to radiogenic 40Ar production.

Membership of the sponsoring body

Membership of the IUPAC Inorganic Chemistry Division Committee for the period 2016–2017 was as follows:

President: J. Reedijk (Netherlands); Secretary: M. Leskelä (Finland); Vice President: L. R. Öhrström (Sweden); Past President: R. D. Loss (Australia); Titular Members: L. Armelao (Italy); T. Ding (China); P. Karen (Norway); D. Rabinovich (USA); T. Walczyk (Republic of Singapore); M. E. Wieser (Canada); Associate Members: Y. Abdul Aziz (Malaysia); J. Colón (Puerto Rico); M. Drábik (Slovakia); L. Meesuk (Thailand); K. Sakai (Japan); N. Trendafilova (Bulgaria); National Representatives: J. Darkwa (South Africa); M. Diop (Senegal); J. G. Correia (Portugal); M. Hasegawa (Japan); S. N. Kalmykov (Russia); A. Kiliç (Turkey); P. Knauth (France); G. J. Leigh (United Kingdom); S. Mathur (Germany); K. B. Yoon (South Korea).

Membership of the IUPAC Commission on Isotopic Abundances and Atomic Weights for the period 2016–2017 was as follows:

Chair: J. Meija (Canada); Secretary: T. Prohaska (Austria); Titular Members: M. Gröning (Austria); J. Irrgeher (Germany); J. Vogl (Germany); X.-K. Zhu (China); Associate Members: L. Chesson (USA); H. A. J. Meijer (Netherlands); A. Possolo (USA); Ex-officio member: J. Reedijk (Netherlands).

Membership of the IUPAC Subcommittee on Natural Assessment of Fundamental Understanding of Isotopes for the period 2016–2017 was as follows:

Chair: N. E. Holden (USA); Secretary: T. B. Coplen (USA); Members: J.K. Böhlke (USA); P. De Bièvre (deceased; Belgium); M. Wieser (Canada).

Acknowledgments

Comments by Prof. J. Stohner (Zürich University of Applied Sciences, Winterthur, Switzerland) are greatly appreciated. The comments of the following Commission on Atomic Weights members during its September 2017 meeting in Groningen, Netherlands were helpful: Dr. M. Gröning (Austria), J. Irrgeher (Germany), Dr. J. Meija (Canada), Prof. H. A. J. Meijer (Netherlands), Dr. J. Vogl (Germany), and Dr. X.-K. Zhu (China). Additionally, discussion with Dr. T. Ding (Chinese Academy of Sciences, Beijing, China), Dr. P. Dunn (LGC, Teddington, U.K.), Dr. H. Moossen (Max-Planck-Institute for Biogeochemistry, Jena, Germany), Dr. J. Wang (National Institute of Metrology, China), and Dr. S. Yoneda (National Museum of Nature and Science, Tokyo, Japan) were beneficial. The following IUPAC project contributed to this Technical Report: 2015-030-2-200.

References

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About the article

Received: 2017-03-03

Accepted: 2017-12-20

Published Online: 2018-05-25

Published in Print: 2018-07-26


Citation Information: Pure and Applied Chemistry, Volume 90, Issue 7, Pages 1221–1224, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2017-0301.

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