Standard Atomic Weights Research Articles

TSAW—a lifelong challenge or simply an unsolved mystery?

Abstract The table of standard atomic weights (TSAW) provides of the numbers included in all reprints of the periodic tables of the elements, in all textbooks in schools, high schools and universities, found on the internet, and numerous databases [1]. Whenever one needs to relate masses of an element to molar quantities, standard atomic weights come into the play. They are of fundamental importance in science, education, technology, industries, trade and commerce. It is therefore not surprising that the determination of atomic weights has a central role in chemistry. Also, it is not surprising that it needs a whole commission of experts to set-up such table with highest confidence even though at first sight it seems to be a simple compilation of numbers. That commission of experts is the Commission on Isotopic Abundances and Atomic Weights (referred to as the Commission in the text). (For more information about the Commission visit ciaaw.org).

Standard atomic weights of the elements 2021 (IUPAC Technical Report)

Standard atomic weights of the elements 2021 (IUPAC Technical Report)

The Table of Standard Atomic Weights-An exercise in consensus.

The present Table of Standard Atomic Weights (TSAW) of the elements is perhaps one of the most familiar data sets in science. Unlike most parameters in physical science whose values and uncertainties are evaluated using the “Guide to the Expression of Uncertainty in Measurement” (GUM), the majority of standard atomic‐weight values and their uncertainties are consensus values, not GUM‐evaluated values. The Commission on Isotopic Abundances and Atomic Weights of the International Union of Pure and Applied Chemistry (IUPAC) regularly evaluates the literature for new isotopic‐abundance measurements that can lead to revised standard atomic‐weight values, Ar °(E) for element E.The Commission strives to provide utmost clarity in products it disseminates, namely the TSAW and the Table of Isotopic Compositions of the Elements (TICE). In 2016, the Commission recognized that a guideline recommending the expression of uncertainty listed in parentheses following the standard atomic‐weight value, for example, Ar °(Se) = 78.971(8), did not agree with the GUM, which suggests that this parenthetic notation be reserved to express standard uncertainty, not the expanded uncertainty used in the TSAW and TICE. In 2017, to eliminate this noncompliance with the GUM, a new format was adopted in which the uncertainty value is specified by the “±” symbol, for example, Ar°(Se) = 78.971 ± 0.008. To clarify the definition of uncertainty, a new footnote has been added to the TSAW. This footnote emphasizes that an atomic‐weight uncertainty is a consensus (decisional) uncertainty.Not only has the Commission shielded users of the TSAW and TICE from unreliable measurements that appear in the literature as a result of unduly small uncertainties, but the aim of IUPAC has been fulfilled by which any scientist, taking any natural sample from commerce or research, can expect the sample atomic weight to lie within Ar °(E) ± its uncertainty almost all of the time.

Standard atomic weights of the elements 2021 (IUPAC Technical Report)

Abstract Following the reviews of atomic-weight determinations and other cognate data in 2015, 2017, 2019 and 2021, the IUPAC (International Union of Pure and Applied Chemistry) Commission on Isotopic Abundances and Atomic Weights (CIAAW) reports changes of standard atomic weights. The symbol A r°(E) was selected for standard atomic weight of an element to distinguish it from the atomic weight of an element E in a specific substance P, designated A r(E, P). The CIAAW has changed the values of the standard atomic weights of five elements based on recent determinations of terrestrial isotopic abundances: Ar (argon): from 39.948 ± 0.001 to [39.792, 39.963] Hf (hafnium): from 178.49 ± 0.02 to 178.486 ± 0.006 Ir (iridium): from 192.217 ± 0.003 to 192.217 ± 0.002 Pb (lead): from 207.2 ± 0.1 to [206.14, 207.94] Yb (ytterbium): from 173.054 ± 0.005 to 173.045 ± 0.010 The standard atomic weight of argon and lead have changed to an interval to reflect that the natural variation in isotopic composition exceeds the measurement uncertainty of A r(Ar) and A r(Pb) in a specific substance. The standard atomic weights and/or the uncertainties of fourteen elements have been changed based on the Atomic Mass Evaluations 2016 and 2020 accomplished under the auspices of the International Union of Pure and Applied Physics (IUPAP). A r° of Ho, Tb, Tm and Y were changed in 2017 and again updated in 2021: Al (aluminium), 2017: from 26.981 5385 ± 0.000 0007 to 26.981 5384 ± 0.000 0003 Au (gold), 2017: from 196.966 569 ± 0.000 005 to 196.966 570 ± 0.000 004 Co (cobalt), 2017: from 58.933 194 ± 0.000 004 to 58.933 194 ± 0.000 003 F (fluorine), 2021: from 18.998 403 163 ± 0.000 000 006 to 18.998 403 162 ± 0.000 000 005 (Ho (holmium), 2017: from 164.930 33 ± 0.000 02 to 164.930 328 ± 0.000 007) Ho (holmium), 2021: from 164.930 328 ± 0.000 007 to 164.930 329 ± 0.000 005 Mn (manganese), 2017: from 54.938 044 ± 0.000 003 to 54.938 043 ± 0.000 002 Nb (niobium), 2017: from 92.906 37 ± 0.000 02 to 92.906 37 ± 0.000 01 Pa (protactinium), 2017: from 231.035 88 ± 0.000 02 to 231.035 88 ± 0.000 01 Pr (praseodymium), 2017: from 140.907 66 ± 0.000 02 to 140.907 66 ± 0.000 01 Rh (rhodium), 2017: from 102.905 50 ± 0.000 02 to 102.905 49 ± 0.000 02 Sc (scandium), 2021: from 44.955 908 ± 0.000 005 to 44.955 907 ± 0.000 004 (Tb (terbium), 2017: from 158.925 35 ± 0.000 02 to 158.925 354 ± 0.000 008) Tb (terbium), 2021: from 158.925 354 ± 0.000 008 to 158.925 354 ± 0.000 007 (Tm (thulium), 2017: from 168.934 22 ± 0.000 02 to 168.934 218 ± 0.000 006) Tm (thulium), 2021: from 168.934 218 ± 0.000 006 to 168.934 219 ± 0.000 005 (Y (yttrium), 2017: from 88.905 84 ± 0.000 02 to 88.905 84 ± 0.000 01) Y (yttrium), 2021: from 88.905 84 ± 0.000 01 to 88.905 838 ± 0.000 002

Interpretation and use of standard atomic weights (IUPAC Technical Report)

Interpretation and use of standard atomic weights (IUPAC Technical Report)

Standard Atomic Weight of Lead Revised

Abstract Following the recent publication of the IUPAC Technical Report on the variation of lead isotopic composition and atomic weight in terrestrial materials [1], the IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) is recommending changes to the standard atomic weight (i.e. relative atomic mass) of lead:

Interpretation and use of standard atomic weights (IUPAC Technical Report)

Abstract Many calculations for science or trade require the evaluation and propagation of measurement uncertainty. Although relative atomic masses (standard atomic weights) of elements in normal terrestrial materials and chemicals are widely used in science, the uncertainties associated with these values are not well understood. In this technical report, guidelines for the use of standard atomic weights are given. This use involves the derivation of a value and a standard uncertainty from a standard atomic weight, which is explained in accordance with the requirements of the Guide to the Expression of Uncertainty in Measurement. Both the use of standard atomic weights with the law of propagation of uncertainty and the Monte Carlo method are described. Furthermore, methods are provided for calculating uncertainties of relative molecular masses of substances and their mixtures. Methods are also outlined to compute material-specific atomic weights whose associated uncertainty may be smaller than the uncertainty associated with the standard atomic weights.

Determination of the Isotopic Composition of Zirconium Using MC-ICPMS and a Regression Model for Mass Bias Correction.

Measurements of zirconium isotopes provide insights into the formation of planetary bodies, dating nuclear explosions, or maintenance of nuclear reactors. Although many comparative measurements for the isotopic composition of zirconium have been performed using mass spectrometry, there is a lack of zirconium isotopic reference materials and only a single calibrated measurement has been reported to date by total evaporation thermal ionization mass spectrometry (TE-TIMS). In this study, we report an independent measurement of full zirconium isotopic composition in a new zirconium isotopic reference material by multicollector inductively coupled plasma MS using the regression method and two independent certified isotopic reference materials, NIST SRM 984 Rb and NIST SRM 987 Sr. Our results agree well with the recent TE-TIMS measurements and suggest a revision of the standard atomic weight of zirconium.

Variation of lead isotopic composition and atomic weight in terrestrial materials (IUPAC Technical Report)

Abstract The isotopic composition and atomic weight of lead are variable in terrestrial materials because its three heaviest stable isotopes are stable end-products of the radioactive decay of uranium (238U to 206Pb; 235U to 207Pb) and thorium (232Th to 208Pb). The lightest stable isotope, 204Pb, is primordial. These variations in isotope ratios and atomic weights provide useful information in many areas of science, including geochronology, archaeology, environmental studies, and forensic science. While elemental lead can serve as an abundant and homogeneous isotopic reference, deviations from the isotope ratios in other lead occurrences limit the accuracy with which a standard atomic weight can be given for lead. In a comprehensive review of several hundred publications and analyses of more than 8000 samples, published isotope data indicate that the lowest reported lead atomic weight of a normal terrestrial materials is 206.1462 ± 0.0028 (k = 2), determined for a growth of the phosphate mineral monazite around a garnet relic from an Archean high-grade metamorphic terrain in north-western Scotland, which contains mostly 206Pb and almost no 204Pb. The highest published lead atomic weight is 207.9351 ± 0.0005 (k = 2) for monazite from a micro-inclusion in a garnet relic, also from a high-grade metamorphic terrain in north-western Scotland, which contains almost pure radiogenic 208Pb. When expressed as an interval, the lead atomic weight is [206.14, 207.94]. It is proposed that a value of 207.2 be adopted for the single lead atomic-weight value for education, commerce, and industry, corresponding to previously published conventional atomic-weight values.

Determination of the Isotopic Composition of Gadolinium Using Multicollector Inductively Coupled Plasma Mass Spectrometry

In this study, we report independent measurements of all stable isotope ratios of gadolinium. Our study employs multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) with National Research Council Canada (NRC) HALF-1 isotopic hafnium standard as a primary calibrator and surveys four commercial gadolinium materials, including a NRC candidate isotopic reference material, GADS-1. The isotopic composition of gadolinium is determined using the regression model without reliance on conventional normalizing isotope ratios or mass-dependent isotope ratio correction models. In this work, all gadolinium isotope ratios were obtained from 160Gd/158Gd which, in turn, was measured from hafnium 178Hf/177Hf either directly or indirectly through 167Er/166Er. The latter approach was used for the final determination of gadolinium isotopic composition, as it provides smaller combined uncertainty. We report high-precision measurements of the isotopic composition of gadolinium, which support a revised standard atomic weight. Isotope amount ratios of R152/158 = 0.008 20(2)k=1, R154/158 = 0.087 98(12)k=1, R155/158 = 0.596 81(63)k=1, R156/158 = 0.825 08(57)k=1, R157/158 = 0.630 60(22)k=1, and R160/158 = 0.879 10(60)k=1, and the atomic weight of Ar(Gd) = 157.2502(6)k=1 were obtained for gadolinium in GADS-1.

Standard Atomic Weight of Hafnium Revised

Standard Atomic Weight of Hafnium Revised

An Improved Program for the Estimation of Protein Theoretical Relative Molecular Mass and Uncertainty.

Presented here is the National Institute of Standards and Technology (NIST) Mass Calculator, a software tool that provides both a relative molecular mass (Mr) value and an associated uncertainty value for a given protein sequence. The calculation of a protein Mr based on its amino acid residue sequence is performed by many software applications. Unfortunately, many of the results do not agree with each other. Reasons for this lack of consensus may include the use of rounded values, calculation errors, or alternative calculation methods, and it is often not possible to distinguish the source of the differences. Additionally, the result is provided without an estimate of precision. This would imply that the Mr is perfectly known when in fact it is not because the basis for the calculation, either the relative atomic masses or the standard atomic weights, are themselves provided as estimates. Until the release of the NIST Mass Calculator, there were no applications available to provide both an Mr value and an associated uncertainty value. Furthermore, the full disclosure of the computation methods and elemental values allows for independent verification of the results. The uncertainty is obtained via Monte Carlo simulation and is now a useful feature in light of the high resolution capability now available in the mass spectrometry of intact proteins.

IUPAC Periodic Table of the Elements and Isotopes (IPTEI) for the Education Community (IUPAC Technical Report)

AbstractThe IUPAC (International Union of Pure and Applied Chemistry) Periodic Table of the Elements and Isotopes (IPTEI) was created to familiarize students, teachers, and non-professionals with the existence and importance of isotopes of the chemical elements. The IPTEI is modeled on the familiar Periodic Table of the Chemical Elements. The IPTEI is intended to hang on the walls of chemistry laboratories and classrooms. Each cell of the IPTEI provides the chemical name, symbol, atomic number, and standard atomic weight of an element. Color-coded pie charts in each element cell display the stable isotopes and the relatively long-lived radioactive isotopes having characteristic terrestrial isotopic compositions that determine the standard atomic weight of each element. The background color scheme of cells categorizes the 118 elements into four groups: (1) white indicates the element has no standard atomic weight, (2) blue indicates the element has only one isotope that is used to determine its standard atomic weight, which is given as a single value with an uncertainty, (3) yellow indicates the element has two or more isotopes that are used to determine its standard atomic weight, which is given as a single value with an uncertainty, and (4) pink indicates the element has a well-documented variation in its atomic weight, and the standard atomic weight is expressed as an interval. An element-by-element review accompanies the IPTEI and includes a chart of all known stable and radioactive isotopes for each element. Practical applications of isotopic measurements and technologies are included for the following fields: forensic science, geochronology, Earth-system sciences, environmental science, and human health sciences, including medical diagnosis and treatment.

Standard Atomic Weights of 14 Chemical Elements Revised

Abstract The IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW) met under the chairmanship of Dr. Juris Meija, at the University of Groningen, the Netherlands, in September 2017. Following its meeting, the Commission recommended changes to the standard atomic weights of 14 chemical elements.

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

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

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

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.”

Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)

Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)

Interpreting and propagating the uncertainty of the standard atomic weights (IUPAC Technical Report)

Abstract In 2009, the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) introduced the interval notation to express the standard atomic weights of elements whose isotopic composition varies significantly in nature. However, it has become apparent that additional guidance would be helpful on how representative values should be derived from these intervals, and on how the associated uncertainty should be characterized and propagated to cognate quantities, such as relative molecular masses. The assignment of suitable probability distributions to the atomic weight intervals is consistent with the CIAAW’s goal of emphasizing the variability of the atomic weight values in nature. These distributions, however, are not intended to reflect the natural variability of the abundances of the different isotopes in the earth’s crust or in any other environment. Rather, they convey states of knowledge about the elemental composition of “normal” materials generally, or about specific classes of such materials. In the absence of detailed knowledge about the isotopic composition of a material, or when such details may safely be ignored, the probability distribution assigned to the standard atomic weight intervals may be taken as rectangular (or, uniform). This modeling choice is a reasonable and convenient default choice when a representative value of the atomic weight, and associated uncertainty, are needed in calculations involving atomic and relative molecular masses. When information about the provenance of the material, or other information about the isotopic composition needs to be taken into account, then this distribution may be non-uniform. We present several examples of how the probability distribution of an atomic weight or relative molecular mass may be characterized, and also how it may be used to evaluate the associated uncertainty.

Accurate Determination of the Absolute Isotopic Composition and Atomic Weight of Molybdenum by Multiple Collector Inductively Coupled Plasma Mass Spectrometry with a Fully Calibrated Strategy.

A fully calibrated strategy has been investigated for the first time for the accurate determination of absolute isotopic composition and atomic weight of molybdenum using multiple-collector inductively coupled plasma mass spectrometry. The correction for instrumental mass bias was performed using synthetic isotope mixtures, which were gravimetrically prepared with all of the seven high-purity and isotopically enriched molybdenum isotope materials together. Six natural molybdenum materials, including molybdenum standard solution NIST SRM 3134, were accurately measured and yielded the absolute isotopic composition (in atom %, k = 1) of 92Mo-14.690(18), 94Mo-9.173(6), 95Mo-15.865(5), 96Mo-16.666(3), 97Mo-9.588(4), 98Mo-24.307(16), and 100Mo-9.711(13). These isotopic data enable an atomic weight Ar(Mo) of 95.9466(34) (k = 2) to be calculated, which is slightly lower than the current standard atomic weight 95.95(1) and with a much improved uncertainty. The associated uncertainties were evaluated according to the Guide to Expression of Uncertainty in Measurement of ISO/BIPM and Monte Carlo simulation to ensure that all sources of uncertainty were fully accounted for. A particular characteristic of the proposed new approach is that mass bias correction factor K for each isotope ratio of molybdenum can be achieved via fully experimental determination without using the traditional semiempirical correction mathematical models. In addition, the relationship between mass of isotope and bias per mass unit β was investigated based on the thorough measurement data.

Data reduction framework for standard atomic weights and isotopic compositions of the elements

We outline a general framework to compute consensus reference values of standard atomic weights, isotope ratios, and isotopic abundances, and to evaluate associated uncertainties using modern statistical methods for consensus building that can handle mutually inconsistent measurement results. The multivariate meta-regression approach presented here is directly relevant to the work of the IUPAC Commission on Isotopic Abundances and atomic weights (CIAAW), and we illustrate the proposed method in meta-analyses of the isotopic abundances and atomic weights of zinc, platinum, antimony, and iridium.