Isotope Ratio Measurements and their Application

An outstanding feature of inorganic mass spectrometry is its determination of precise and accurate isotopic abundances and isotope ratios. Isotopes of the same element (of the same number of protons or atomic number of element, Z) are, by definition, nuclides with different mass m and mass number A (A = Z + N) due to the different number of neutrons (N) in the nucleus. Isotope analyses are of special interest for characterizing the composition of samples with respect to stable and unstable isotopes in quite different concentration ranges - from the analysis of matrix elements down to the trace and ultratrace concentration level.1-9 Of 1700 isotopes, nearly 16 % (264 isotopes) are stable. The chemical elements Tc, Pm, Th, U and the transuranic elements do not possess stable isotopes. 

Measurement of the isotope abundances of a chemical element is based on the fact that the sum of all abundances of isotopes with the same Z is 100 %. For example, copper possesses two stable isotopes with m/z = 63 and 65. If the isotope ratio 63Cu/65Cu has been determined, e.g., by mass spectrometry, then the isotope abundance of 65Cu ( ) can be obtained by  

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. 

Inorganic Mass Spectrometry Principles and Applications 1. S. Becker 2007 John Wiley Sons, Ltd 

Furthermore, isotope analysis is relevant for determining the atomic weight (Ar(E)) of elements. The Ar(E) is the average of all masses of all naturally occurring stable isotopes (taking into account the abundances of isotopes) of a chemical element (see Appendix I10). By consideration of the masses of isotopes (/ ,) and the known relative abundances of all stable isotopes (Xi) with i = 1 to n of a selected chemical element, the average atomic weight (Ar(E)) of this element can be calculated  

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Selenium (masses 74, 76, 77, 78, 80, and 82 Table 1) and chromium (masses 50, 52, 53 54 Table 1) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. 

For measurements of isotope ratios or isotope abundances, any of the mass spectrometers discussed in the previous chapters, such as SSMS, LIMS, GDMS56 and LA-ICP-MS,6 are of benefit for the direct isotope analysis of solid samples. SSMS and LIMS are rarely applied in isotope analysis due to their relatively low precision. Several applications of the isotope dilution technique as a calibration strategy in SSMS, mostly on geological samples, are known.57-59 GDMS has been mostly applied in multi-element trace analysis and depth profiling and plays only a minor role... 

Elements in nature come in forms called isotopes that differ only in the number of their neutrons. Most isotopes are stable and can be distinguished from their counterparts simply by their masses. Remarkably, isotopes are associated with a few simple and mass-dependent traits that result in a wide range of useful isotopic signals in natural processes. Coupled with the invention of the isotope ratio mass spectrometer in 1940s (McKinney et al., 1950 Nier, 1947) stable isotope signals provide the basis for application of stable isotopes to environmental sciences. Stable isotopes are denoted by their atomic mass such as and for the two stable isotopes of carbon, and 0, and for the stable isotopes of oxygen. Because the heavy isotope is normally rare (e.g., -1.1% for i c, 0.2% for 0, and 0.04% for O), routine measurements of the absolute isotopic concentrations is difficult and not reliable. Alternatively, the ratio, R, of the rare to the abundant isotopes is measured, such as... 

Murozumi et al. (9) determined the concentration of Pb in Greenland and Antarctic snow and ice by Isotope Dilution Mass Spectrometry (IDMS) (27). They added pure Pb tracer to their samples before chemically processing them. Both the mass of tracer and the water sample were measured. After thoroughly mixing the tracer with the sample Pb was isolated in a chemically pure form and the isotopic ratios were measured in a mass spectrometer. Simple comparison of the ° Pb abundance with the abundance of other isotopes in the mass spectrum allowed the amount of Pb in the sample to be determined. The technique is not limited to Pb, but is applicable to any element with two or more stable or... 

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