Stable isotopes mass independent fractionation

Equations (8) and (10) are applicable to stable isotope systems where isotopic fractionation occurs through mass-dependent processes which comprise the majority of cases described in this volume. These relations may also be used to identify mass-independent fractionation processes, as discussed in Chapter 2 (Birck 2004). Mass-dependent fractionation laws other than those given above distinguish equilibrium from kinetic fractionation effects, and these are discussed in detail in Chapters 3 and 6 (Schauble 2004 Yormg and Galy 2004). Note that distinction between different mass-dependent fractionation laws will generally require very... 

The origin of mass-dependent fractionation (MDF) in isotope systems lies in the mass dependence of the molecular properties (e.g., zero-point energy) and physical processes (e.g., evaporation) affecting the compound. If a compound comprised of atoms with 3 or more stable isotopes, such as oxygen or sulfur, deviates from a mass-dependent relationship, the compound is said to exhibit mass-independent fractionation (MIF). MIF signatures are not affected by mass-dependent processes, and so are excellent tracers of the small number of mass-independent processes that exist in nature. 

An exciting new development in the field of stable isotope geochemistry has been the recent recognition of the importance of mass-independent fractionation in the sulfur isotope... 

Oxygen consists of three stable isotopes 0, " O and " O. For most applications only the ratio between the more abundant Lso-topes, 0 and O, is measured. If all fractionation processes in the environmental system were purely mass-dependent, measurements of 0/ 0, would be redundant, as they could be predicted from the 0/ 0 ratio. However, it has recently been ob.served that photochemical exchange between O2, Oi, and CO, in the stratosphere involves mass-independent fractionation among the oxygen isotopes (1990 Thiemens et al., 1993a, b Thiemens, 1999). Thereby O2 becomes anomalously depleted, while CO2 becomes anomalously enriched. Because of this, measurements of 0/ 0 in atmospheric O2 and/or CO2 provide an independent piece of information. Conveniently, one may define an O anomaly tracer (A 0) (Thiemens et n/., 1995b)... 

Epov, V.N., Malinovsky, D., Sonke, J.E., Vanhaecke, F., Begue, D., and Donard, O.F.X. (2011) Modem mass spectrometry for studying mass-independent fractionation of heavy stable isotopes in environmental and biological sciences. J. Anal. At. Spectrom., 26, 1142—1156. 

Stable isotope analysis of Earth, Moon, and meteorite samples provides important information concerning the origin of the solar system. 8lsO values of terrestrial and lunar materials support the old idea that earth and moon are closely related. On the other hand three isotope plots for oxygen fractionation in certain meteoric inclusions are anomalous. They show unexpected isotope fractionations which are approximately mass independent. This observation, difficult to understand and initially thought to have important cosmological implications, has been resolved in a series of careful experimental and theoretical studies of isotope fractionation in unimolecular kinetic processes. This important geochemical problem is treated in some detail in Chapter 14. 

The major analytical complication in Mo isotope analysis is precise correction for isotope fractionation during Mo purification and mass spectrometric analysis. This subject is reviewed in general by Albarede and Beard (2004), and is discussed here in particular reference to Mo. It is important to recognize that this challenge is fundamentally dififerent in mass dependent stable isotope studies as compared to investigations of mass-independent Mo isotope variations produced by nucleosynthesis. The latter have received attention in recent years for high-precision determination of Mo isotope composition (e.g., Dauphas et al. 2002a,b Yin et al. 2002), but are not relevant here. 

In cosmochemistry, we use stable-isotope fractionations to study evaporation and condensation in the solar nebula, aqueous processes on asteroids, and even ion-molecule reactions to form organic molecules in interstellar clouds. The oxygen isotopes also show large mass-independent shifts that may be related either to chemical or physical processes or to incomplete mixing of the products of nucleosynthesis. These topics will be covered in detail in later chapters. 

Sulfur isotope compositions in sulfonic acids show a mass-independent enrichment in 33S. Determining fractionations that are not controlled by mass is made possible because sulfur has so many stable isotopes. This feature has been attributed to ultraviolet irradiation of carbon disulfide in space, prior to its reaction to make sulfonic acid. 

Small enrichments of stable isotopes can be used for detailed studies of specific processes such as denitrification. If these processes are important in the flux of material in the ecosystem from an isotopic mass-balance standpoint, then process rates and associated isotopic fractionations must be measured independently. Alternatively, sufficient isotope must be added to enrich the pool of interest to a level at which the magnitude of fractionation effects is negligible. For example, an enrichment of ammonium by 1 order... 

As discussed in Chapter 2, many elements have multiple stable isotopes of varying abundance. The abundances for stable isotopes of the elements are listed in Appendix 2. The ratio of two stable isotopes of a given element can be determined by independently measuring their ion currents.The isotope ratio is then computed by dividing one m/z isotope ion current by the other m/z isotope ion current. If no isobaric mass spectral interferences are present, and the sample has a natural isotopic abundance for the element in question (i.e., no fractionation or radiogenic processes have affected the abundances), the calculated ratio should be close to the theoretical value computed from the Appendix 2 table entries. This ability to perform isotope ratio measurements provides an opportunity to carry out isotope dilution analysis. 

As an illustration of the way to disentangle the events that had taken place 2 billion years ago, the measurement of the neodymium isotopes (142-146,148, and 150) wiU be presented. It is important to know that neodymium remained in the rocks without migrating. In addition, it is advantageous that the content of natural neodymium in the minerals is small. This content could be determined since Nd is practically not formed in nuclear fission. As it results from the systematics of fractional fission yields (see Chap. 4 in Vol. 1), the (independent, direct) yield of Nd is very small. An alternative formation by the P decay of more neutron-rich nuclides of the same mass (i.e., isobars) is not possible because the direct precursor Ce is stable. Consequently, the content of the mineral in Nd reflects the amount of natural neodymium not produced by fission. Using the known isotopic composition of natural neodymium, the contributions of natural origin of all neodymium isotopes could be subtracted. This is illustrated in Table 57.1 (Columns 1-3). 

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