Atomic weights of the elements. Review 2000 (IUPAC Technical Report)

Abstract

Republication or reproduction of this report or its storage and/or dissemination by electronic means is permitted without the need for formal IUPAC permission on condition that an acknowledgmentAbstract: A consistent set of internationally accepted atomic weights has long been an essential aim of the scientific community because of the relevance of these values to science and technology, as well as to trade and commerce subject to ethical, legal, and international standards. The standard atomic weights of the elements are regularly evaluated, recommended, and published in updated tables by the Commission on Atomic Weights and Isotopic Abundances (CAWIA) of the International Union of Pure and Applied Chemistry (IUPAC). These values are invariably associated with carefully evaluated uncertainties. Atomic weights were originally determined by mass ratio measurements coupled with an understanding of chemical stoichiometry, but are now based almost exclusively on knowledge of the isotopic composition (derived from isotope-abundance ratio measurements) and the atomic masses of the isotopes of the elements. Atomic weights and atomic masses are now scaled to a numerical value of exactly 12 for the mass of the carbon isotope of mass number 12. Technological advances in mass spectrometry and nuclear-reaction energies have enabled atomic masses to be determined with a relative uncertainty of better than 1 × 10 -7 . Isotope abundances for an increasing number of elements can be measured to better than 1 × 10 -3 . The excellent precision of such measurements led to the discovery that many elements, in different specimens, display significant variations in their isotope-abundance ratios, caused by a variety of natural and industrial physicochemical processes. While such variations increasingly place a constraint on the uncertainties with which some standard atomic weights can be stated, they provide numerous opportunities for investigating a range of important phenomena in physical, chemical, cosmological, biological, and industrial processes. This review reflects the current and increasing interest of science in the measured differences between source-specific and even sample-specific atomic weights. These relative comparisons can often be made with a smaller uncertainty than is achieved in the best calibrated "absolute" (= SI-traceable) atomic-weight determinations. Accurate determinations of the atomic weights of certain elements also influence the values of fundamental constants such as the Avogadro, Faraday, and universal gas constants. This review is in two parts: the first summarizes the development of the science of atomic-weight determinations during the 20 th century; the second summarizes the changes and variations that have been recognized in the values and uncertainties of atomic weights, on an element-by-element basis, in the latter part of the 20 th century.J. R. de LAETER et al.

Resolving the Germanium Atomic Weight Disparity Using Multicollector ICPMS

“…22 The logic and validation of this measurement process has been described in detail elsewhere. [24][25][26] As the model has now been sucesfully employed for the analysis of Cu-Zn 26 , Hg-Tl 22 and Pb-Tl 27 , we believe that it can be applicable for high-precision absolute isotope ratio measurements of any other element. Moreover, the universal applicability of this model is also strongly supported by the lack of untestable assumptions in its mathematical framework.…”

“…5,6 Since these mass spectrometric measurements were not calibrated, the Commission on Atomic Weights recommended the 'chemical' value in 1961. 5 The first fully calibrated germanium isotope ratio measurements appeared only a decade ago, in 1999, and the atomic weights derived were discordant:…”

Inductively coupled plasma- and glow discharge plasma-sector field mass spectrometry : Part I. Tutorial: Fundamentals and instrumentation

“…Alternatively, 'natural' standards are used and their isotopic composition taken from the IUPAC tables. 155 It is clear that the latter approach is only sufficient for, e.g., isotope dilution purposes, since the measurement of natural variations of isotopic composition could be affected by a systematic bias or at least an increased uncertainty. In case of isotope dilution mass spectrometry, it is preferable if the isotopic composition of the standard matches the isotopic composition of the blend, which is not always achievable.…”

“…Otherwise, the isotopic composition of elements with little naturally occurring fractionation can be derived from the IUPAC tables, even though a certain possible systematic bias might be taken into account. 155 Several approaches can be used for correcting the investigated isotope pair, depending on whether it is assumed that mass discrimination varies according to a linear, a power-law or an exponential function by taking into account the difference in mass between the two isotopes Dm: 158,159 In Russell's equation 150,151 on the other hand, the mass of the isotopes rather than the mass difference between them is considered as the factor determining the extent of mass discrimination:…”

Calibration of isotope amount ratios by analysis of isotope mixtures

“…That is, the isotope amount ratios measured are not always free from the biases incurred during the measurement process. Even in the twenty-first century, calibrated isotope amount ratio measurements have never been performed for some 20 elements [6]. Hence, the need for calibration of isotope amount ratio measurements remains a state-of-the-art activity.…”