Mass-independent isotope fractionation

Isotope Effects in Unimolecular Processes Mass Independent Isotope Fractionation and the Ozone Problem... 

Mass Independent Isotope Fractionation in the Laboratory, the Stratosphere, and the Troposphere... 

Theory of Mass Independent Isotope Fractionation of Ozone... 

The work of Thiemens, Mauersberger, and their collaborators has now definitively established a pervasive mass independent isotope fractionation for oxygen containing molecules found in the atmosphere, upper atmosphere, and in space. These results, initially surprising because they are inconsistent with ordinary equilibrium... 

Mass-independent isotopic fractionations are widespread in the earth s atmosphere and have been observed in O3, CO2, N2O, and CO, which are all linked to reactions involving stratospheric ozone (Thiemens 1999). For oxygen, this is a characteristic marker in the atmosphere (see Sect. 3.9). These processes probably also play a role in the atmosphere of Mars and in the pre-solar nebula (Thiemens 1999). Oxygen isotope measurements in meteorites demonstrate that the effect is of significant importance in the formation of the solar system (Clayton et al. 1973a) (Sect. 3.1). 

Explain how mass-dependent and mass-independent isotopic fractionation of oxygen isotopes interact to produce the range of compositions that we observe in cosmochemical materials. 

The principal focus of the present article is on the "mass-independent isotope fractional effect" (MIF) found in atmospheric and laboratory produced ozone. When this MIF occurs, a plot of the positive or negative "enrichment" in samples versus that of in those same samples has a slope of approximately unity, rather than its typical value of about 0.52. The 0.52 is the value expected using conventional transition state theory when nuclear tunneling effects are absent. For an isotope Q, 5Q is defined in per mil as 1000 [(Q/0)/(Q/0)std - 1]/ where Q/O is the ratio of Q to in the sample and std refers to its value in some standard sample, standard mean ocean water. An example of a three-isotope plot showing a slope of 0.52 is given in Figure 2.1. 

Understanding the mass-independent isotope fractionation effect for ozone in the laboratory [5] and stratosphere [9] poses interesting challenges. These chal-... 

The mass-independent isotope fractionation in the gas-phase synthesis of O3 from O2 produces a slope-1 line on a three-isotope plot, with ozone being depleted in and residual oxygen... 

MASS-INDEPENDENT ISOTOPIC FRACTIONATION PROCESSES IN THE EARLY SOLAR SYSTEM 170... 

When the mass-independent isotopic fractionation chemical process was first discovered by Thiemens and Heidenreich (1983), there existed no physical-chemical mechanism that accounted for the ozone observations. In this paper, a mechanism based upon optical self-shielding was proposed. Although this mechanism may not account for the experimental results, there are potential cosmochemical environments where self-shielding may be operative, as discussed in this paper. These potential applications will be discussed in detail in a later section of this chapter. 

While additional theoretical formalisms for the isotopic fractionation event are still needed, the mass-independent isotopic fractionation process has provided a new and definitive mechanism by which an extraordinary range of environmental... 

Nuclear volume effects, sometimes referred to as nuclear field shift effects, are believed to be one cause of mass-independent isotope fractionation [46]. Nuclei of isotopes differ from one another only in their number of neutrons. Self-evidently, this provides the isotopes with a different mass, but this may also give rise to differences in the size and shape of the nuclei among the isotopes. The nuclei of nuclides with an odd number of neutrons are often smaller than they should be based on the mass difference relative to those of the neighboring nuclides with an even number of neutrons [47]. These differences in nuclear shape and size, and thus charge density, affect the interaction between the nucleus and the surrounding electron cloud. The resulting difference between the isotopes in terms of density and shape of the electron cloud results in slight differences in the efficiency with which they participate in chemical reactions [48]. 

Malinovsky, D. and Vanhaecke, F. (2011) Mass-independent isotope fractionation of heavy elements measured by MC-ICPMS a unique probe in environmental sciences. Anal. Bioanal. Chem., 400 (6), 1619-1624. 

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