Isotope abundances variations

Coplen TB, et al. (2002) Isotope abundance variations of selected elements. Pure Appl Chem 74 1987-2017... 

Oxygen isotope abundance variations in meteorites are very useful in elucidating chemical and physical processes that occurred during the formation of the solar system (Clayton, 1993). On Earth, the mean abundances of the three stable isotopes are 99.76%, 0.039%, and... 

Fig. 6.2.7. Sulfur isotope abundance variations in nature (evaporitic curve after Kaplan, 1975).

The isotopic mass tags may be introduced into the proteins or peptides at different stages of the sample preparation by metabolic (in vivo) or chemical (in vitro) means. The preferred isotopic labels are and N, as they do not induce changes in the LC retention times of the labeled species when compared to the retention of the same peptide with natural isotopic abundances. Variations in LC retention times can occur when deuterium labeling is used, making the interpretation of data difficult. 

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. 

To understand how mass differences give rise to isotope abundance variations, it is useful to consider chemical bonds as harmonic oscillators with fundamental vibrational frequencies inversely proportional to the square root of the molecular masses. The zero point energy of a molecule is defined as the finite energy (above hy where h is Planck s constant and V is the frequency of the oscillation) possessed by that molecule at 0 K. Isotope substitution changes the mass of the molecule and results in a shift in the zero point energy. A molecule containing a... 

The transmutation of elements due to radioactive decay is another important cause of isotope abundance variations. Over time, the numbers of radioactive parent isotopes will decrease as they decay into the daughter products. The abundance determination of radiogenic daughter isotopes is applied extensively for age determinations of geological and organic materials. The ages of minerals and rocks ( can be calculated from the measurement of the number of daughter nuclides formed in a mineral that decayed under closed conditions (D ) and the parent atoms N) where the half-life of the parent (probability of a decay, X) is known (Eqn [2]). 

Hydrogen exhibits the largest natural isotope abundance variations due to the relatively large mass difference between its two stable isotopes JH and (also written commonly as H and D, respectively). H is approximately 99.985% and D 0.015% naturally abundant. The three isotopes of oxygen O and are -99.763%, -0.0375% and -0.1995% abundant, respectively. Examples of isotope abundance variations measured for H and O are summarized in Figures 6 and 7. 

Carbon has two stable isotopes and C) of -98.89% and -1.11% abundance, respectively. Carbon compounds are exchanged between the oceans, atmosphere, biosphere and lithosphere. Significant isotope ratio variations exist within these groups due to both kinetic and equilibrium isotope effects. Representative carbon isotope abundance variations are shown in Figure 9. 

Nitrogen is present primarily as N2 gas in the atmosphere and dissolved N2 in oceans. In terrestrial systems, nitrogen is found in minor amounts bound to H, C and O in both reduced and oxidized states. Nitrogen has two stable isotopes N and /N of -99.64% and -0.36% abundance, respectively. Isotope abundance variations measured for nitrogen are summarized in Figure 10. 

Sulfur has four stable isotopes , gS, j S, which are -95.02%, -0.75%, -4.21% and 0.02% abundant, respectively. Sulfur is ubiquitous in the global environment and major sulfur reservoirs include sulfate in the oceans, evaporites, sulfide ore deposits, and organic sulfur compounds. Sulfur isotope abundance variations are shown in Figure 11. 

Interferences related to environmental samples have been well documented. While they may limit the ultimate detection capability of the technique, accuracy problems can be corrected, to a degree, using interference correction equations. Method 200.8 provides recommended equations that function well for common interferences, e.g. chloride-based interferences on vanadium and arsenic. Table 9.5 gives the equations listed in 200.8. The lead (Pb) equation is not a correction for interferences, but corrects for potential isotopic abundance variations that can be found in lead from natural sources. 

Corrects for natural lead isotopic abundance variations... 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

Cannes, L.Z., Martinez del Rio, C., Koch, P. (1998). Natural abundance variations in stable isotopes and their potential uses in animal physiological ecology. Comparative Biochemistry and Physiology - Part A Molecular Integrative Physiology, Vol. 119, No. 3, pp. 725-737. (http //dx.doi.org/10.1016/S1095-6433(98)01016-2)... 

Different isotopes differ in their atomic masses. The intensities of the signals from different isotopic ions allow isotopic abundances to be determined with high accuracy. Mass spectrometry reveals that the isotopic abundances in elemental samples from different sources have slightly different values. Isotopic ratios vary because isotopes with different masses have slightly different properties for example, they move at slightly different speeds. These differences have tiny effects at the level of parts per ten thousand (0.0001). The effects are too small to appear as variations In the elemental molar masses. Nevertheless, high-precision mass spectrometry can measure relative abundances of isotopes to around 1 part in 100,000. 

The use of natural abundance variations in stable isotopes as tracers relies on the fractionations that occur during chemical, physical and biological processes (Ambrose 1993). Differences in fractionation during these processes lead to distinct isotopic signatures for biological materials, such as in foods exploited by humans in antiquity. 

The conclusions of Hurt s study of year-by-year oxygen isotope ratios in 72 years of S. gigantea are thus supportive of the conclusions of the CIAP study [49] that solar variations influence the abundances of many kinds of chemical species in the stratosphere, and therefore influence the.amount of solar energy they absorb and re-radiate to earth, and therefore influence the surface temperature of the earth and especially the surface temperatures of the oceans. It is the surface temperature of the oceans which produces the phenomena we have discussed the isotope ratio variations in rain and hence in tree rings, the isotope ratio variations in the Greenland ice cap, in the organic carbon and uranium concentrations in sea cores, and furthermore variations of the sea surface temperature produces variations in the carbon-14 to carbon-12 ratio fractionation at the sea air interface and hence in the carbon-14 content of atmospheric carbon dioxide and hence in the carbon-14 content of tree rings. 

Because the natural variations in stable isotope abundances are usually very small (see above), and since routine measurements are usually made in an isotope ratio mass spectrometer which compares the relative intensities of the mass resolved beams of the sample with those of some standard material (Section 7.2.2), it is standard practice to report abundance ratios using the dimensionless 8-value notation. 

Baur ME, Hayes JM, Studley SA, Walter MR (1985) Millimeter-scale variations of stable isotope abundances in carbonates from banded iron-formations in the Hamersley Group of Western Australia. Econ Geol 80 270-282... 

The masses of isotopes can be measured with accuracies better than parts per billion (ppb), e.g., m40Ar = 39.9623831235 0.000000005 u. Unfortunately, determinations of abundance ratios are less accurate, causing errors of several parts per million (ppm) in relative atomic mass. The real limiting factor, however, comes from the variation of isotopic abundances from natural samples, e.g., in case of lead which is the final product of radioactive decay of uranium, the atomic weight varies by 500 ppm depending on the Pb/U ratios in the lead ore. [8]... 

Note The calculation of relative molecular mass, Mr, of organic molecules exceeding 2000 u is significantly influenced by the basis it is performed on. Both the atomic weights of the constituent elements and the natural variations in isotopic abundance contribute to the differences between monoisotopic- and relative atomic mass-based values. In addition, they tend to characteristically differ between major classes of biomolecules. This is primarily because of molar carbon content, e.g., the difference between polypeptides and nucleic acids is about 4 u at Mr = 25,000 u. Considering terrestrial sources alone, variations in the isotopic abundance of carbon lead to differences of about 10-25 ppm in Mr which is significant with respect to mass measurement accuracy in the region up to several 10 u. [41]... 

Radioactive decay is one process that produces variations in isotope abundance. The second cause for differences in isotope abundance is isotope fractionation, caused by small chemical and physical differences between the isotopes of an element. It is exclusively this important process that will be discussed in the following chapters. 

From the measured 63Cu/65Cu isotope ratio the isotope abundances of 63Cu and 65Cu are then calculated in a natural sample as roughly 69.2% and 30.8%, respectively. Small deviations from the IUPAC table value10 could be evidence of fine isotope variation in nature. 

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