Examples of Isotopes and Their Uses

Isotopes

Isotopes

Jonathan D. Bethard , in Research Methods in Human Skeletal Biology, 2013

Stable Isotopes and Mass Spectrometry

In typical projects that involve analysis of stable isotopes, the step following collagen and biological apatite extraction utilizes instrumentation called isotope ratio mass spectrometers (IRMS). The IRMS typically have four components: a combustion chamber, an ion source, a mass analyzer, and series of ion detectors. These components act to detect subtle differences in various isotopes, for example ¹³C and ¹²C, by initially combusting the sample and then automatically transferring the converted gas into the mass spectrometer for analysis. Upon entering the mass spectrometer, the gas is ionized so that it can be directed into the mass analyzer where it is measured against known standards in the ion detectors (sometimes called Faraday collectors). Interested skeletal biologists are encouraged to consult other sources for more detailed descriptions of the process of analyzing samples via IRMS (i.e., Barrie and Prosser, 1996; Katzenberg, 2008; Brown and Brown, 2011). Moreover, to fully understand the complexities of the instrumentation, and to perhaps gain experience operating this type of laboratory equipment, individuals are encouraged to seek out specialists to gain practical skills in this area, even if this means temporarily relocating, taking an additional course outside of the traditional skeletal biology curriculum, or volunteering as a laboratory assistant at any IRMS core facility.

In addition to understanding the basic principles of the IRMS, skeletal biologists should have some awareness about how stable isotope ratios are calculated on the mass spectrometer. As mentioned above, the mass spectrometer analyzes both the sample and a known standard. The standards are regulated on an international level by such agencies as the United States National Institute of Standards and Technology (NIST), sometimes referred to as the National Bureau of Standards (NBS), and by the International Atomic Energy Agency (IAEA). A list of common standards can be found by accessing the following website: http://nucleus.iaea.org/rpst/ReferenceProducts/ReferenceMaterials/index.htm. Ultimately, any IRMS core facility will be well versed in NIST/IAEA standards and interested researchers should familiarize themselves with the protocols of their particular laboratory. As research projects progress beyond the analytical stage to scientific presentations and publications, explicitly discussing which standards were used in any stable isotope project is compulsory.

Understanding that stable isotope analyses require comparison of isotopic values in unknown samples to reference standards clarifies how results from the mass spectrometer are calculated and reported. Taking carbon as an example, the following notation is used:

${\delta }^{13}\text{C}=({\text{R}}_{\text{sample}}/{\text{R}}_{\text{standard}}-1)\times 1000\mathrm{\%}$

where R is the ratio of the heavier to the lighter isotope (e.g., ¹³C/¹²C) and the δ (delta) notation expresses this ratio as parts per thousand (‰, per mil). For all stable carbon isotopes, the original standard material was a sample of marine limestone called the Peedee belemnite (PDB). Though PDB has been exhausted in its original form, other standards whose δ¹³C values have been calibrated against it are readily available (Hoefs, 2009). A positive δ¹³C value indicates that ¹³C is enriched compared to the standard and depleted for ¹²C, or as is the case with human tissues, negative δ¹³C values are depleted for ¹³C and enriched in ¹²C (Brown and Brown, 2011).

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Isotope Ratio Studies Using Mass Spectrometry☆

M.E. Wieser , W.A. Brand , in Encyclopedia of Spectroscopy and Spectrometry (Third Edition), 2017

Abstract

Isotope ratio studies are employed in a variety of multidisciplinary research projects encompassing chemistry, physics, biology, medicine, geology, archaeology and environmental technology. This article describes mechanisms responsible for isotope abundance variations, the essential components of an isotope ratio mass spectrometer (with particular reference to electron impact ionization) and isotope abundance variations of H, C, N, O and S. The emergence of multiple collector inductively coupled plasma mass spectrometry is described including the application to laser ablation studies of U-Pb isotopes in zircons. Finally, the development of isotope ratio monitoring techniques to extract isotope information from transient signals is presented.

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Isotope Ratio Studies Using Mass Spectrometry*

Michael E. Wieser , Willi A. Brand , in Encyclopedia of Spectroscopy and Spectrometry (Second Edition), 1999

Introduction

Isotopes of an element contain the same number of protons but different numbers of neutrons. Whereas the former means that isotopically different compounds undergo the same reactions, the latter means that they differ in mass (e.g. ¹² ₆C and ¹³ ₆C are 12 and 13 atomic mass units, respectively). As a result of their different masses, isotopes of an element participate in chemical, biological and physical processes at different rates. Hence, the isotope composition of the element in a given compound depends on the history and origins of the sample. Isotope abundance data can provide information concerning the source of a material or processes responsible for its synthesis and conversion. Variations in the isotope abundance ratios of many elements of biogeochemical importance are subtle, but significant. To resolve these differences, isotope ratio mass spectrometers must accurately measure variations of 20 to 50 parts per million. In the case of carbon, the average ¹³C/¹²C isotope abundance ratio of 0.011   200 ranges over ±0.000   450 with biogeochemically important variations of ±0.000   000   5. Such requirements have resulted in a branch of mass spectrometry that has developed its own specialized instrumentation and analytical methods.

Isotope ratio studies are employed in a variety of multidisciplinary research projects encompassing chemistry, physics, biology, medicine, geology, archaeology and environmental technology. This article will describe mechanisms responsible for isotope abundance variations, the essential components of the isotope ratio mass spectrometer (with particular reference to electron impact ionization) and isotope abundance variations of H, C, N, O and S. Finally, the development of recent isotope ratio monitoring techniques to extract isotope information from transient signals is presented.

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Stable Isotopes in Microbial Ecology

M.C. Redmond , D.L. Valentine , in Encyclopedia of Microbiology (Third Edition), 2009

Isotope Effects and Fractionation

The isotopes of an element react nearly identically in chemical reactions, but the mass differences between isotopes can lead to small differences in binding energies and reaction rates. These differences give rise to isotope effects, which can lead to fractionation in isotopic composition between compounds. There are two major types of isotope effects: equilibrium and kinetic. Equilibrium isotope effects occur when there is a difference in isotopic composition between compounds at chemical equilibrium; the forward and backward reaction rates are equal for each isotope, but the heavier isotope tends to concentrate where the chemical bonds are strongest. Kinetic isotope effects occur in nonequilibrium systems when one isotope, usually the lighter one, reacts more rapidly than the other. Many biological reactions show a kinetic isotope effect. As a reaction proceeds in a closed system, where no material enters or leaves, the reaction products will contain a higher fraction of the light isotope than the starting material. This causes the remaining reactants to become enriched in the heavy isotope. Some isotope effects, such as those associated with dissimilatory sulfate reduction ( ³⁴S/³²S) and methanogenesis (¹³C/¹²C), seem to result from a combination of kinetic and equilibrium factors.

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Chemistry/Trace/Drugs of Abuse

N NicDaéid , H. Buchannan , in Encyclopedia of Forensic Sciences (Second Edition), 2013

Isotope Ratio Mass Spectrometry

In recent years, considerable attention has been paid to the applications of IRMS in forensic science, including the analysis of drugs of abuse.

Isotopes of an element have different mass numbers (i.e., the same number of protons but a different number of neutrons in their nuclei). The lighter isotope (lower mass number) usually dominates in natural abundance, with one or more heavier isotopes existing in less than a few percent. The dominant isotope is often referred to as the major isotope, with the heavier isotope(s) called the minor isotope(s). Normally, IRMS analysis of drugs of abuse involves the determination of the relative isotope abundances of hydrogen ( ¹H and ²H), carbon (¹²C and ¹³C), nitrogen (¹⁴N and N¹⁵), and oxygen (¹⁶O, ¹⁷O, and ¹⁸O). The terrestrial isotope ratios were determined when the earth was formed and are, for the most part, fixed. Compartmentally (e.g., between species or climatic regions), isotope ratios vary constantly as a result of biological, biochemical, chemical, and physical processes. Any such process which changes the relative abundances of an element's isotopes is called isotopic fractionation. Two different types of effects result in isotopic fractionation: kinetic isotope effects (KIEs) and thermodynamic isotope effects (TIEs).

KIEs, or mass discriminating effects, are due to differences in reaction rates as a result of different bond strengths between heavier and lighter elements. Statistical models predict that lighter isotopes form weaker bonds; therefore, lighter isotopes are more reactive and will be concentrated in the products, while the heavier isotopes will be concentrated in the reactants. When this occurs, the effect is said to be normal. If, on the other hand, the products are enriched with the heavier isotopes and the reactants with the lighter ones, the effect is termed inverse.

Isotope fractionation is associated with differences in melting point, boiling point, vapor pressure, IR absorption, and other physicochemical properties. In contrast with KIEs, TIEs are evident in processes in which chemical bonds are neither broken nor formed.

IRMS measures natural-abundance isotope ratios relative to a standard; it does not measure absolute natural abundance. Data are generally quoted as delta values, δ, which can be calculated by the following equation:

${\text{δ}}^{13}\text{C}=1000\left(\frac{R_{\text{samp}}}{R_{\text{std}}}-1\right)\text{,}$

where R _samp is the ratio of the number of atoms of the heavy isotope to the number of atoms of the light isotope of an element and R _std is the equivalent ratio corresponding to the standard. Because differences in isotope abundance ratios between the sample and the standard are typically only 0.001–0.05%, δ values include multiplication by 1000 for ease of discussion and are therefore quoted 'per mill' (‰). A negative δ value indicates the sample is enriched in the light isotope relative to the standard, and a positive δ value indicates the sample is enriched in the heavy isotope relative to the standard.

Natural abundances of isotopes vary constantly despite fixed whole earth isotope abundances. Given that the synthesis of most illicit substances starts with a natural product (e.g., safrole from the sassafras tree in the case of 3,4-methylenedioxymethylamphetamine (MDMA), coca leaf in the case of cocaine, and opium from poppies in the case of heroin), the isotopic fractionation occurring in these plants may play an important role in determining the final isotopic composition of a specific product.

Analysis of a plant's natural carbon isotope abundance can allow determination of the pathway by which the plant fixes (or assimilates) atmospheric CO₂ into carbohydrate. For example, plants that assimilate CO₂ by the Calvin cycle, that is, they convert CO₂ into carbohydrates via the three-carbon-chain molecule phosphoglyceric acid, are called C₃ plants. These plants generally have δ¹³C values of −34‰ to −24‰. Plants that assimilate CO₂ by the Hatch–Slack cycle, that is, they convert CO₂ into carbohydrates via the four-chain molecule oxaloacetic acid, are called C₄ plants. Plants with this type generally have δ¹³C values of −16‰ to −9‰. Humidity, temperature, isotopic composition of the soil, and isotopic composition of CO₂ also affect these δ¹³C values.

Hydrogen and oxygen isotope abundances of terrestrial waters are affected by evaporation. For example, citrus trees growing in subtropical climates may be subjected to extensive evaporation which causes ²H enrichment in the cellular water of the fruit. Variation in nitrogen isotope abundance is caused by many chemical and physical processes, resulting in variable isotope ratios of common materials.

The IRMS system can be interfaced with different sample preparation techniques, which allow either bulk stable isotope analysis (BSIA) or compound-specific isotope analysis (CSIA) to be undertaken. BSIA measures the isotope ratios for the sample as a whole. If carbon is the target element for analysis, then the ¹³C/¹²C ratio returned by the instrument will represent all of the carbon atoms in every compound present in the sample. CSIA measures the isotope ratio for one compound in the sample. The separation of the sample can be achieved by the coupling of the IRMS to a GC instrument. In this case, the sample is dissolved in an organic solvent and injected onto the first GC column. Baseline separation of the peaks is essential, as accurate determination of isotope ratios cannot be achieved from a partial GC peak.

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Isotope Labeling of Biomolecules - Labeling Methods

Chinmayi Prasanna , ... Hanudatta S. Atreya , in Methods in Enzymology, 2015

2.3 Isotope Scrambling

Isotope scrambling implies misincorporation of ¹⁴N/¹³ C isotopes to undesired/nontargeted amino acids ( Muchmore, McIntosh, Russell, Anderson, & Dahlquist, 1989). Scrambling leads to unlabeling of one or more amino acids apart from our choice of amino acid (Muchmore et al., 1989). This is mostly due to bacterial transaminases, which are responsible for amine group transfer between different amino acids, particularly α-amino group in the amino acid metabolism pathway (Shortle, 1994; Waugh, 1996). Isotope scrambling can lead to cross/wrong labeling. The ¹⁴N isotope scrambling is more prominent than ¹³C isotope scrambling. A detailed analysis of ¹⁴N and ¹²CO scrambling using E. coli BL21 (DE3) as a standard system for heterologous protein expression has been discussed (Bellstedt et al., 2013). Table 1 summaries the extent of ¹⁴N isotope scrambling in selective unlabeling.

Table 1. Isotope Scrambling in Selective Unlabeling

Selective Unlabeled Amino Acid Residue	14N Isotope Scrambling a
Alanine	Tryptophan
Arginine	–
Asparagine	–
Aspartic acid	Uniform scrambling
Cysteine	–
Glutamine	–
Glutamic acid	Uniform scrambling
Glycine	Serine, cysteine
Histidine	–
Isoleucine	Leucine, valine
Leucine	Valine, isoleucine
Lysine	–
Methionine	–
Phenylalanine	Tyrosine
Proline	Not applicable
Serine	Glycine, cysteine
Threonine	Glycine
Tryptophan	Uniform scrambling
Tyrosine	Phenylalanine
Valine	Leucine, isoleucine

a

¹⁴N scrambling based on analysis of ¹H, ¹⁵N-HSQC peak intensities for GB1 (Bellstedt et al., 2013), APTX (Bellstedt et al., 2013), and ubiquitin (Krishnarjuna et al., 2011). Uniform scrambling implies uniform decrease in peak intensities of all other amino acids.

Isotope scrambling can be reduced to a certain extent by any of the following methods: (i) use of auxotrophic strains of bacteria (Waugh, 1996), (ii) use of inhibitors of specific amino acid synthesis pathways (Kim, Perez, Ferguson, & Campbell, 1990), (iii) use of unlabeled precursors of amino acids (Rasia, Brutscher, & Plevin, 2012), and (iv) optimal choice and selection of amino acids (Krishnarjuna et al., 2011). Using auxotrophic strains of bacteria for overexpression of proteins with selective unlabeling solves much of the problem of isotope scrambling, as these strains lack certain transaminases which bring about isotope scrambling. This comes however at the cost of undermining the overall cell growth and protein yields. To recompense, larger culture volume and longer growth periods are required to achieve the same protein yield. When a mixture of amino acids is required to be unlabeled in the same sample, constructing strains with multiple auxotrophic markers to avoid isotope scrambling seems impractical. Such strains lacking multiple transaminases may have poor viability.

Immediate metabolic precursor of an amino acid in its biosynthetic pathway can be used for unlabeling instead of the amino acid itself. This is based on the premise that the precursor is directly converted to its amino acid, that is, the final amino acid isotope composition directly relies on the isotope composition of its metabolic precursor. This is useful for site-specific selective unlabeling (Rasia et al., 2012). Since the α-amino group is added at the end of the metabolic pathway by aminotransferases for a few amino acids (e.g., Ile, Leu), adding unlabeled precursors for these residues in a uniform ¹³C, ¹⁵N-labeled medium would result in the ¹²C-labeled amino acid with ¹⁵N-labeled at the α-amino position. Table 2 gives a list of metabolic precursors as suitable candidates for unlabeling as proposed by Rasia et al.

Table 2. Metabolic Precursors of Amino Acids Suitable for Selective Unlabeling (Rasia et al., 2012)

Precursors	Amino Acid
α-Ketobutyrate	Isoleucine
α-Ketoisovalerate	Leucine, valine
Phenyl pyruvate	Phenylalanine
4-Hydroxy phenylpyruvate	Tyrosine

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MASS SPECTROMETRY | Principles and Instrumentation

F.A. Mellon , in Encyclopedia of Food Sciences and Nutrition (Second Edition), 2003

Isotope ratio mass spectrometers

Although isotope ratios may be measured successfully using conventional mass spectrometers, some applications much require higher precision measurements than can be attained by using scanning instruments. In such cases, specialized high precision isotope ratio mass spectrometers are available. An example of this type of instrument is the gas isotope ratio mass spectrometer (GIRMS), used to determine the isotope ratios of gases such as CO₂, N₂, SO₂, and H₂ to a very high degree of precision and accuracy.

The analyte is combusted or reduced to generate the gases that are ionized in a gas tight EI source. A magnetic sector separates the ion beam, with the difference (from a conventional scanning instrument) that each mass channel is collected simultaneously, i.e., the mass spectrometer uses a multicollector ion detection system. This yields high-precision isotope ratios because any fluctuations in ion beam intensity occur simultaneously at each detector. A diagram of a GIRMS instrument is shown in Figure 6.

Figure 6. Schematic diagram of a gas isotope ratio mass spectrometer. From Mellon F, Self R and Startin JR (2000) Mass Spectrometry of Natural Substances in Food. London: The Royal Society of Chemistry with permission.

The main uses of GIRMS in food and nutrition science are in determining the authenticity of foods and in human studies of nutrient metabolism. High-precision multicollector mass spectrometers are also available for use with TIMS or ICP-MS.

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Lymphedema

Catharine L. McGuinness MS, FRCS , Kevin G. Burnand MBBS, MS, FRCS , in Comprehensive Vascular and Endovascular Surgery (Second Edition), 2009

Isotope Lymphography

Isotope lymphography has replaced contrast lymphangiography as the primary diagnostic technique. Rhenium sulfur colloid is specifically taken up by lymphatics and allows the presence of lymphedema to be confirmed by a simple outpatient investigation with a reasonable degree of accuracy. Normally, 0.3% of the injected dose arrives in the groin within 30 minutes, and more than 0.6% arrives within 1 hour. An excessive uptake occurs in patients with venous edema, often above 3% at 30 minutes, and this test can therefore distinguish between venous and lymphatic edema, although a "grey area" of overlap exists. γ-Camera pictures provide information that the isotope is reaching the lymph nodes of the groin, and delayed images may show a failure of progression, indicating proximal obstruction. This should be confirmed by contrast lymphography, as should any equivocal findings. Isotope lymphography is a moderately sensitive test for lymphedema but may mistakenly classify some normal legs as lymphedematous. It often correctly identifies patients who are suitable for lymphatic bypass surgery. ⁴⁸

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Geochemistry

T.A. Mather , in Reference Module in Earth Systems and Environmental Sciences, 2013

Isotope Geochemistry

Isotope geochemistry is the study of the relative and absolute concentrations of the elements and their isotopes in samples from the Earth and solar system. Such measurements offer a powerful tool to interrogate a range of scientific problems from the origins of the terrestrial planets, to past climate change, to igneous processes and the source of elements in a variety of geological reservoirs. Broadly it can be divided into the study of radiogenic isotopes and stable isotopes. As well as radioactive decay, a variety of processes (both physical and chemical and both kinetic and equilibrium) have the potential to fractionate an element's isotopes meaning that elements development different isotopic signatures in different geological and indeed cosmological contexts. Thus the general tool of isotope geochemistry is useful in a variety of ways. For example, the use of the known decay rates of radiogenic isotopes within geochronological techniques is well known and widespread in the Earth and Environmental sciences for determining the age and/or stratigraphic position of samples or the rate of processes. Variations in the ratios of stable isotopes within stratigraphic columns can also be used, commonly coupled with magnetostratigraphic and biostratigraphic information, to correlate different stratigraphic horizons. Stable isotopes as tracers of global cycles have also made important contribution to understanding processes such as the rock and water cycles and changes in the Earth system over geological time ( Figure 1 ) with implications for future anthropogenic climatic and environmental impacts ( Figure 2 ).

Figure 1. The Cenozoic secular change record for the ¹⁸O/¹⁶O ratios of carbonates shows the change in bottom water temperature recorded by benthonic foraminifera in the Tertiary. As Australia mores northward, Antarctica becomes more isolated at the south pole. As the Atlantic grows to become a major ocean, the longitudinal distribution of continents coupled with the isolation of Antarctica results in a reorganization of global ocean circulation. The Pleistocene planktonic foram record shows the signal induced by Milankovitch cycles in a combined ice volume-temperature signal for the last 700   000 years.

Reproduced from Figure 14 of Gregory, R.T. (2003). Stable isotopes as tracers of global cycle. In Encyclopedia of physical science and technology (3rd edn.), pp. 695–714.

Figure 2. Data on the δ¹³C value of atmospheric carbon dioxide for the decade starting in 1978. This record, along with the exponential growth of atmospheric carbon dioxide (inset), indicate that the carbon dioxide increase is driven, in large part, by the burning of fossil fuel and deforestation for alternative land use. Typical organic matter and fossil fuel have strongly negative δ¹³C values so that their combustion pushes the atmosphere towards more negative values (up on the figure scale).

Reproduced from Figure 14 of Gregory, R.T. (2003). Stable isotopes as tracers of global cycle. In Encyclopedia of physical science and technology (3rd edn.), pp 695–714.

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Gamma imaging

Penelope Allisy-Roberts OBE FIPEM FInstP , Jerry Williams MSc FIPEM , in Farr's Physics for Medical Imaging (Second Edition), 2008

Isotopes

Isotopes of an element are nuclides that have the same number of protons (atomic number), position in the periodic table, and chemical and metabolic properties but a different number of neutrons, mass number (protons plus neutrons), density and other physical properties.

The nuclei of all carbon atoms contain six protons. Ninety-nine percent of stable carbon nuclei are carbon-12, with six neutrons, while 1% are carbon-13, with seven neutrons. Carbon-11 (¹¹C), with only five neutrons, has a neutron deficit, while carbon-14 (¹⁴C), with eight neutrons, has a neutron excess; both are artificially produced, unstable and radioactive. Note that all four nuclides are isotopes of carbon.

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Examples of Isotopes and Their Uses

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