Isotopic composition, of elements

Here, the main theory of so-called gas phase IRMS, which is directed toward determination of variations in stable isotope compositions of elements (e.g., C, H, N, O, S, Cl) by analyzing the gases (i. e., C02, H2, N2,02, S02, etc.) are reviewed. Lichtfouse and Budzinski [621] have presented a brief description of the techniques and an account of their applications in organic geochemistry. Although the most demanding applications of IRMS involve determinations of... 

In principle, double-spike techniques represent the most suitable approach to determine the isotope composition of elements with four isotopes or more (Fe, Zn). In most cases, these techniques involve the addition of an isotope which is usually minor in natural samples, such as Zn or Fe, implying that the risk introduced by memory effects on these spike isotopes must be carefully weighed against the added gain in precision from using the double spike. Such a risk is clearly more present with MC-ICP-MS than with TIMS. 

Why can one assume that the isotopic compositions of elements on the Earth are representative of the bulk solar system This was not obvious in the beginning. Isotope-ratio measurements of meteorites made in the late 1950s appeared to show significant variations. However, within a few years, it was shown that those variations were due almost entirely to problems with the measurements, not to real variations in the samples. By the early 1960s enough measurements had been made to convince most scientists that in the material to which they had access, terrestrial rocks and meteorites, each element always had the same isotopic composition (except for small fractionations of stable isotopes and shifts... 

Isotope abundances which are free from all sources of bias are defined as absolute isotope abundances. The absolute isotope composition of elements can be measured by MC-TIMS and MC-ICP-MS via gravimetric synthetic mixtures or standard solutions from highly enriched isotopes, as demonstrated for neodymium,11 erbium13 and samarium,11 13 99 respectively. 

Biological samples are potentially excellent records of their environment and are being examined by inorganic and biomolecular mass spectrometry to an increasing extent. Biomaterials such as carbonate shells, fish otoliths and fish scales or tree rings that increase their mass regularly on an annual basis may record changes in their environment via the concentrations of metals or non-metals and the isotopic composition of elements.2... 

Bird, M. I., and Grocke, D. R. (1997). Determination of the abundance and carbon isotope composition of elemental carbon in sediments. Geochim. Cosmochim. Acta 61, 3413-3423. 

In a coal desulfurization study, Narayan et al. (50) were able to extract an appreciable amount of elemental sulfur (36% of total sulfur) with perchloroethylene at 120 °C from weathered coal, but not from fresh coal. Hackley et al. (51) determined the isotopic composition of elemental sulfur extracted by perchloroethylene and obtained results consistent with the interpretation that the elemental sulfur originates from the oxidation of pyrite. 

For example, the isotopic composition and the atomic weight of neodyminm," dysprosium and erbium have been determined using synthetic mixtnres prepared gravimetrically from highly enriched isotopes of neodymium in the form of oxides of weU defined pnrity by TIMS. No natnral isotopic variation was found in terrestrial neodymium, dysprosium or erbium samples. These isotopic compositions of Dy and Er measnred by TIMS are accepted as the best measurements from a single terrestrial source as noted in the table of isotopic composition of elements, 2001. °... 

Radioactivity is a part of nature—in the process of element formation by nuclear reactions taking place in stars, both stable and radioactive isotopes of elements are formed. The isotopic composition of elements is characterized by properties of nuclear reactions that led to the formation of the elements. Elemental composition of the planet Earth, thought to be about 4.5x 10 years old, although not yet in chemical equilibrium, reflects the composition of the material from which it was formed. Therefore, a number of radionuclides occur in nature, having long half-lives (longer than the age of Earth). In addition there are natural processes which continuously produce new radioisotopes. Recently, human activities have also contributed to the increased concentration of some of the radionuclides. 

This is another method to find the molecular formula of the analyte. A low-resolution mass spectrometer will suffice. As explained below, the principle is based upon the isotopic composition of elements. 

The stable isotopic compositions of elements having low atomic numbers (e. g. H, C, N, O, S) vary considerably in nature as a consequence of the fact that certain thermodynamic properties of molecules depend on the masses of the atoms of which they are composed. The partitioning of isotopes between two substances or two phases of the same substance with different isotope ratios is called isotopic fractionation. In general, isotopic fractionation occurs during several kinds of physical processes and chemical reactions ... 

Sometimes, cosmic-ray-induced isotope shifts are also a nuisance, because they may compromise the precise determinations of isotopic compositions of elements of interest. This is often a major problem in the case of trapped noble gases (Ott 2002, this volume), but may also require attention when e.g., small excesses or deficits of daughter isotopes of an extinct radionuclide present in the early solar system are to be determined (e g., Leya et al. 2000c). 

Field methods are based on tracer dispersion study directly in the natural geologic medium (Figure 3.29). As tracers are utilized either components of the already existing pollution or special components injected with water. Such special components maybe 1) natural components (Br,I or CF) of assigned concentrations 2) artificial compounds of assigned isotope composition of elements (<5D,, H, etc.) 3) radioactive isotopes,... 

Isotopic abundances are said to be anomalous if the measured isotopic ratios cannot be related to the terrestrial (and by inference solar) isotopic composition of elements through a mass fractionation relationship resulting from a natural physiochemical process, and/or the mass spectrometer measurement itself. The only variations in the isotope abundances in terrestrial materials occur by radioactive decay and by the physiochemical processes mentioned above. Thus terrestrial samples serve as a good example of homogenized Solar System material against which anomalous isotopic material can be compared. 

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