Chemistry
Isotopic Abundance
Isotopic abundance refers to the relative amount of different isotopes of a chemical element found in a natural sample. It is expressed as a percentage or a fraction and is determined by the mass spectrometry technique. Isotopic abundance is important in various fields such as geology, archaeology, and environmental science for understanding processes such as radioactive decay and chemical reactions.
Written by Perlego with AI-assistance
Related key terms
1 of 5
10 Key excerpts on "Isotopic Abundance"
eBook - ePub- Britannica Educational Publishing, Erik Gregersen(Authors)
- 2010(Publication Date)
- Britannica Educational Publishing(Publisher)
Formally, the phrase elemental abundances usually connotes the amounts of the elements in an object expressed relative to one particular element (or isotope of it) selected as the standard for comparison. Isotopic Abundances refer to the relative proportions of the stable isotopes of each element. They are most often quoted as atom percentages. Since the late 1930s, geochemists, astrophysicists, and nuclear physicists have joined together to try to explain the observed pattern of elemental and Isotopic Abundances. A more or less consistent picture has emerged. Hydrogen, much helium, and some lithium isotopes are thought to have formed at the time of the big bang—the primordial explosion from which the universe is believed to have originated. The rest of the elements come, directly or indirectly, from stars. Cosmic rays produce a sizable proportion of the elements with mass numbers between 5 and 10, which are relatively rare. A substantial body of evidence shows that stars synthesize the heavier elements by nuclear processes collectively termed nucleosynthesis. In the first instance, then, nucleosynthesis determines the pattern of elemental abundances everywhere. The pattern is not immutable, for not all stars are alike and once matter escapes from stars it may undergo various processes of physical and chemical separation. A newly formed small planet, for example, may not exert enough gravitational attraction to capture the light gases hydrogen and helium. Conversely, the processes that change elemental abundances normally alter Isotopic Abundances to a much lesser degree. Thus, virtually all terrestrial and meteoritic iron analyzed to date consists of 5.8 percent 54 Fe, 91.72 percent 56 Fe, 2.2 percent 57 Fe, and 0.28 percent 58 Fe. The relative constancy of the Isotopic Abundances makes it possible to tabulate meaningful average atomic masses for the elements
eBook - PDF- Glenn J. MacPherson(Author)
- 2018(Publication Date)
- De Gruyter(Publisher)
Reviews in Mineralogy & Geochemistry Vol. 68, pp. 15-30, 2008 Copyright © Mineralogical Society of America 3 Abundance, Notation, and Fractionation of Light Stable Isotopes Robert E. Criss Washington University St. Louis, Missouri 63130, U.S.A. Stable isotopes have become an essential tool to characterize and understand terrestrial and extraterrestrial matter. This chapter will briefly review the abundances of important light stable isotopes, demonstrate the link between abundance and atomic weight, introduce the notations and diagrams that are commonly used to report isotopic measurements, describe and partially explain the types of fractionation effects known to occur in nature, and direct the reader to more comprehensive sources of information on each subject. The special techniques needed to make accurate isotopic measurements gave rise to special notation for reporting stable isotope data, and these notations in turn gave rise to special diagrams that emphasize compositional differences and facilitate interpretation. Fundamental definitions are the isotope ratio R, representing the ratio of the abundance of a heavy isotope to that of a lighter, typically much more common isotope, and the isotopic fractionation factor a, representing the quotient RA/RB of the isotope ratios of two substances A and B. Under equilibrium conditions, lna can theoretically vary linearly with 1 IT at low temperatures or with 1 IT 2 at high temperatures, forming the basis for a standard graph. For practical reasons the ratio R is difficult to measure and inconvenient to report, so stable isotope abundances are usually reported as delta values (5-values) that describe their deviations from a defined standard material. Thus, the most important diagram for data interpretation is the 5-5 plot where the 5-values of two coexisting phases are simply plotted against each other.
eBook - ePub- Jose Alonso, Pablo Gonzalez(Authors)
- 2019(Publication Date)
- Royal Society of Chemistry(Publisher)
CHAPTER 5 The Isotope Composition of Natural-Abundance Elements and Molecules5.1 THE ISOTOPE COMPOSITION OF THE ELEMENTS
The basic unit of amount of substance of the International System of units, the “mol”, is defined as 12.000000 g of 12 C. In Chemistry, the mol is the reference for the determination of the atomic weights of the rest of the elements. This also requires the knowledge of the exact mass for each isotope,wi, and its isotope abundance,Ai. The atomic weight, W , is then calculated as:(5.1)The determination of the exact mass of each and all existing isotopes and their isotope abundances was carried out during the 20th century by Mass Spectrometry. A very interesting and detailed historical account of these developments was published in 1992.1 These values were also of extreme importance for the accurate determination of other fundamental constants such as the Avogadro constant, the Faraday constant and the Universal Gas constant. Nowadays, it is the responsibility of the Commission of Isotope Abundances and Atomic Weights of the IUPAC (http://www.ciaaw.org ) to provide accurate information on isotopic compositions and atomic weights on a regular basis. The last update on the isotope composition of the elements2 and on the atomic weight of the elements3 was carried out in 2009 and published in 2011. Both can be downloaded from http://www.iupac.org . Those data are extremely important in Chemistry as they are the main metrological reference for traceability purposes and are employed to calculate the expected isotope distribution of any chemical compound.Concerning the isotopic composition of the elements, a small part of Table 1 in the original publication of Berglund and Wieser is reproduced in Table 5.1 . From the point of view of this book, column 9 holds all the required information: the representative isotope composition of the elements (mole fraction) and their uncertainties. This representative isotope composition corresponds to “the isotopic composition of chemicals and/or natural materials that are likely to be encountered in the laboratory ”. It is also indicated that “These values generally are consistent with the standard atomic weights ”. The numbers in parentheses correspond to expanded uncertainties and are due to “the range of probable isotope-abundance variations among different materials as well as measurement uncertainties
- Clark M. Johnson, Brian L. Beard, Francis Albarède(Authors)
- 2018(Publication Date)
- De Gruyter(Publisher)
Because our discussion here is restricted to essential concepts, we refer the reader who is interested in more depth to the works above. Isotopic AbundanceS AND NOMENCLATURE There are many sources for information on the distribution of the elements and isotopes (e.g., Lide 2003), as well as discussions that are pertinent to stable isotope geochemistry (e.g., Criss 1999). In Chapter 2 Birck (2004) reviews in detail the isotopic distribution and nucleosynthetic origin of many elements that are of geochemical interest. He highlights the fact that isotopic variations for many elements are greatest for extraterrestrial samples, where evidence for a variety of fractionation mechanisms and processes are recorded, including mass-dependent and mass-independent fractionation, radioactive decay, and incomplete mixing of presolar material. Below we briefly review a few general aspects of isotope distribution that are pertinent to isotopic studies that bear on nomenclature, expected ranges in isotopic fractionation, and analytical methodologies. The number of stable isotopes for the naturally occurring elements tends to increase with increasing atomic number, to a maximum of 10 for Sn (Fig. 1). Elements with low atomic numbers tend to have the lowest number of stable isotopes, limiting the possible ways in which isotopic compositions can be reported. Both H and C have only two stable isotopes (Fig. 1), and therefore isotopic compositions are reported using one ratio, D/H and 13 C/ 12 C, respectively. Single ratios can only be used for B andN, and data are reported as U B/ 10 B and 15 N/ 14 N, respectively. Of the non-traditional stable isotope systems discussed in this volume, only three have just two stable isotopes (Li, CI, and Cu; Fig. 1). The choice of isotopic ratios for reporting data increases, of course, for elements with three or more isotopes.
eBook - ePub- Dale A. Schoeller, M. Westerterp, Dale A. Schoeller, M. Westerterp(Authors)
- 2017(Publication Date)
- CRC Press(Publisher)
14 C). For all of the elements given in this table, the light isotope is by far the most abundant. It is importantto note that this table refers to the naturally occurring abundance of each isotope. Molecular trac-ers that have been artificially enriched in heavy isotopes have a long history of use in nutritionalbiochemistry and epidemiology; for example, to measure protein turnover, metabolic substrateutilization, and the rate of energy expenditure by the doubly labeled water technique. However,this chapter focuses on natural abundance stable isotope ratios, and will not discuss further the use of stable isotope tracers.Figure 14.1 Naturally occurring stable isotopes of the light elements common in biological molecules, andtheir global abundances.14.2.2 EXPRESSING STABLE ISOTOPE RATIOS : THE DELTA VALUENatural variations in stable isotope ratios, although very consistent, are also very small: typically occurring at the fourth, fifth, or even sixth decimal place (Schoeller 1999). As relative abundancecan be measured more accurately than absolute abundance, stable isotope ratios in nature are always measured relative to standards and are expressed in units of relative abundance. These delta values (δ)are expressed in units of permil (%c) relative abundance (Macko et al. 1999; Richards et al. 2003;Richards et al. 2005; Schoeninger and Moore, 1992). A value of zero permil indicates that the samplehas the same 13 C:12 C as the reference material; thus, zero is a reference point not unlike temperatureon the Fahrenheit or Celsius scales, rather than a null value. As an example of stable isotope nota-tion, the carbon isotope ratio (13 C/12 C) is expressed as a δ13
eBook - PDFSupernovae and Nucleosynthesis
An Investigation of the History of Matter, from the Big Bang to the Present
- David Arnett(Author)
- 2020(Publication Date)
- Princeton University Press(Publisher)
Abundances of Nuclei We see how we may determine their forms, their distances, their bulk, their motions, but we can never know anything of their chemical or mineralogical structure. . . . We must keep carefully apart the idea of the solar system and that of the universe, and be always assured that our only true interest is with the former. . . . The stars serve us scientifically only as providing positions. —Auguste Comte (1798-1857), Course of Positive Philosophy It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. —Arthur Conan Doyle (1859-1930), A Scandal in Bohemia Before considering theories of how the atomic nuclei were formed in their observed abundances, we must consider the abundance data it-self, and how it is obtained. We encounter two fundamental problems: sampling and accuracy. The most accurate methods of abundance deter-mination involve the analysis of a sample in a laboratory, or a spacecraft. Obviously such samples represent only the most minuscule fragment of an astronomical object; to what extent is it representative? With matter so measured we can only probe a restricted region of space and time. To probe further we must use less accurate methods. One of the tri-umphs of twentieth-century astronomy is the use of stellar spectra to infer abundance information from a much larger region of the universe. These data are fundamental to our study, but suffer from some quite understandable flaws. First, the information so obtained refers almost entirely to the abun-dance of atomic elements, not of isotopes; that is, it tells us only about the number of protons in a nucleus, but not the number of neutrons. While different isotopes of the same element will have almost identical ABUNDANCES OF NUCLEI 5 atomic properties, they will have drastically different nuclear properties.
eBook - PDFRadiogenic Isotope Geochemistry
A Guide for Industry Professionals
- Bruce F. Schaefer(Author)
- 2016(Publication Date)
- Cambridge University Press(Publisher)
In contrast, 87 Sr, despite being stable, is constantly being formed due to the radioactive decay of an isotope of Rb, namely 87 Rb. Therefore the relative proportion of 87 Sr with respect to 86 Sr will change over time. Such changes in the abundances of certain isotopes form the basis of one of the most powerful tools in geochemistry and is at the heart of geochronology. This will be investigated further in Chapter 5, but it is useful to note at this point that the approach of measuring relative ratios of the isotopes, rather than their absolute abundances (i.e. reporting the 87 Sr/ 86 Sr ratio rather than the actual number of 87 Sr and 88 Sr atoms present) is central to both the measurement of isotopes and their interpretation. The reasons for this are both instrumental and mathematical. 8 Isotopes and geochemistry 2 Processes At this point, with an appreciation of what an isotope actually is with respect to a chemical element and/or an ion, it is useful to investigate the processes that can cause variations in the relative proportions of the isotopes of an element. At heart, this is the key to the isotope geochemistry, as it is the changes in isotopic ratios that inform geoscientists of rates and process. 2.1 Fractionation: chemical vs isotopic In essence, isotope geochemistry is an understanding of fractionation, a term that is used in a range of applications and situations. Fractionation is used in several confusing contexts in geochemistry, and it is important to distinguish between chemical fractionation and isotopic fractionation. Although it is often clear from the context, chemical fractionation is the process by which a mixture is separated into smaller quantities of differing compositions. That is, changing the chemical composition through successive operations (e.g. crystallisation, boiling, precipitation), each of which removes one or other of the constituents.
eBook - PDF- M.R. Carter, E.G. Gregorich, M.R. Carter, E.G. Gregorich(Authors)
- 2007(Publication Date)
- CRC Press(Publisher)
Chapter 54 Stable Isotopes in Soil and Environmental Research B.H. Ellert and L. Rock Agriculture and Agri-Food Canada Lethbridge, Alberta, Canada 54.1 INTRODUCTION Isotopes refer to elements with nuclei having the same number of protons, but differing numbers of neutrons, so that the masses of contrasting isotopes differ by one to a few neutrons. Isotopes of a specific element have the same chemical properties because they have the same number of electrons. Owing to their mass differences, however, isotopes of an element undergo chemical, biological, and physical reactions at slightly and consistently different rates, leading to isotopic fractionation whenever reactants are not exhausted. As a result, natural variations in Isotopic Abundance provide powerful insight into element dynamics, but fractionation by intertwined transformations can also complicate interpretations. Isotopes of an element may be stable or radioactive. Radioactive isotopes emit radiation as they undergo radioactive decay and are transformed to new elements. One stable isotope accounts for a majority of those in most elements, and at natural abundance radioactive isotopes are far less plentiful than even the rare stable isotopes. Special reagents containing elements that are artificially enriched in one or more radioactive or stable isotopes are used in manipulative tracer studies. The distinctive isotopic composition of tracers (usually enriched in the normally rare isotope) enables element transfers and transformations to be followed in systems where otherwise it would be difficult or impossible. Isotopic techniques to study element transfer and transformation in plants and soils were among the earliest peaceful uses of nuclear technology after 1945.
- R.E. Zeebe, D. Wolf-Gladrow(Authors)
- 2001(Publication Date)
- Elsevier Science(Publisher)
In 1947, Urey published a paper on the thermodynamic properties of isotopic substances (Urey, 1947). This pro- vided the basis for the utilization of stable isotopes in modern disciplines such as stable isotope geochemistry, isotope geology, biogeochemistry, pale- 142 Chapter 3. Stable Isotope Fractionation oceanography and others. For instance, the analysis of the ratio of stable oxygen isotopes in calcium carbonate, secreted by organisms like belemnites, mollusca, and foraminifera and buried in deep-sea sediments, has permitted the reconstruction of paleotemperatures for the last 150 million years or so (McCrea, 1950; Epstein et al., 1953; Emiliani, 1966). This chapter is dedicated primarily to the stable isotopes of the elements of the carbonate system, and thus focuses on carbon, oxygen, and boron. Overviews on these and other elements can be found in textbooks by Faure (1986), Bowen (1988), Mook (1994), Clark and Fritz (1997), Hoefs (1997), and Criss (1999). 3.1 Notation, abundances, standards The nuclei of isotopes of a certain element all contain the same number of protons (Z) but different numbers of neutrons (N). The notation AE is used, where E is the element and A - Z+N is the mass number that equals the sum of protons and neutrons in the nucleus. Often the subscript Z is omitted as in the discussion of isotopes in this book (e.g. 12C - 12C). An important observation is that most of the stable nuclides have even numbers of protons and neutrons, i.e. they are more abundant than nuclides that have, e.g. even numbers of protons and odd numbers of neutrons (Ta- ble 3.1.1). In addition, the ratio of the natural abundances of isotopes of the same element often follow the rule that the even-even nucleus is most abundant (Table 3.1.2). For example, the ratio of abundances of the stable carbon isotopes 13C (even-odd) to ~2C (even-even) is about 1:99.
eBook - PDFIsotopic Tracers in Biology
An Introduction to Tracer Methodology
- Martin D. Kamen, Louis F. Fieser, Mary Fieser(Authors)
- 2013(Publication Date)
- Academic Press(Publisher)
A . INTRODUCTION The determination of the labeled content of tracers enriched with stable isotopes differs from the assay of radioactive tracers in that stable isotopes used as tracers are present in amounts which are appreciable on a percent-age basis. The actual weight of the tracer element is significantly greater than that of the unlabeled element with its normal complement of isotopes. Hence, it is usual to determine the isotopic composition of stable tracer by measuring differences in weight. Because these methods are not so sensitive as those associated with measurements of radioactivity, tracer procedures involving stable isotopes are usually limited to experiments in which dilution of the labeling isotope is not so great as that permissible in experiments with radioactive tracers. In a given element, the normal abundance of the labeling isotope will be Ao per cent, that is, in every 100 atoms there will be A 0 atoms of the labeling isotope and (100 — A 0 ) atoms of the other isotopes. If the element is en-riched in the labeling isotope to the value of A per cent, and is used in a tracer experiment, it will be diluted with the normal mixture of isotopes, say X times. This means that for every 100 atoms of the labeling mixture, there will be 100X atoms containing A atoms of labeling isotope from the labeling mixture and (X — l)Ao atoms of labeling isotope from the diluting material. The total number of labeling isotope atoms is A + A 0 (X — 1), and the atom per cent of the labeling isotope is (A + XA 0 — A 0 )/X or Ao +(A — A 0 )/X. The difference between this value and the normal abundance, A 0 , is defined as the atom per cent excess. This difference is (A — Ao)/X. But (A — Ao) was the atom per cent excess of the original labeled element.
Index pages curate the most relevant extracts from our library of academic textbooks. They’ve been created using an in-house natural language model (NLM), each adding context and meaning to key research topics.
