Isotope Fractionation Effects

Except for elements with one or more radiogenic isotopes, the isotopic composition of an element in Nature can be considered constant for most applications of stable isotope techniques. However, they are not invariable (see also Chapter 1). Stable isotopes of an element differ slightly in mass, which may cause changes in the element s isotopic composition if a chemical or physical transfer process is sensitive to the isotopes masses and if the transfer is incomplete. For reasons of mass balance, the isotopic abundances cannot be changed if all matter is transferred from a source to a target compartment. 

Physical transfer processes include evaporation and precipitation, and also membrane diffusion as a basic mechanism of element transfer in the human body. A difference in diffusion rates between isotopes inevitably results in their fractionation over the course of element transport (kinetic isotope fractionation effect). When an element undergoes a chemical reaction, this can also be described as an element transfer process from one element species into another. Again, if reaction rates between isotopologs differ, this gives rise to a difference in isotopic 

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In this way, it is possible to reach an extremely high selective sensitivity down to 1 part in 1015, which in 14C dating corresponds to being able to date samples about 50 000 years old. Moreover, modern systems can measure isotopic ratios in modern carbon, both C/ C and C/ C, with an ultimate precision as good as 2%o and l%o, respectively. The former value corresponds to determining the conventional radiocarbon age with an absolute error, smaller than in the past, better than 20 years, while the l%o precision for the 13C/12C allows an adequate correction for isotopic fractionation effects. Even in routine measurements, at least in the case of historical samples, a precision of 5%o in the 14C/12C measured value is standard, corresponding to an uncertainty in the radiocarbon age of 40 years.[27]... 

In contrast, Meckenstock et al. [280] reported larger isotopic enrichments in residual toluene, 3-6%o and up to 10%o during anaerobic and aerobic biodegradation experiments, respectively. These results indicated that isotopic fractionation effects may be different for different compounds, terminal electron-accepting processes (TEAP), degradative metabolic pathways, or microbial populations. 

Some aspects of this phenomenon are still controversial, because diffusion-induced isotopic fractionation effects may be erroneously ascribed to differential reaction kinetics (see section 11.6.2), and vice versa. 

Isotope (H (deuterium), discovered by Urey et al. (1932), is usually denoted by symbol D. The large relative mass difference between H and D induces significant fractionation ascribable to equilibrium, kinetic, and diffusional effects. The main difference in the calculation of equilibrium isotopic fractionation effects in hydrogen molecules with respect to oxygen arises from the fact that the rotational partition function of hydrogen is nonclassical. Rotational contributions to the isotopic fractionation do not cancel out at high T, as in the classical approximation, and must be accounted for in the estimates of the partition function ratio /. 

Another oxygen isotope fractionation effect is documented in CO2 samples collected between 26 and 35 km altitude, which show a mass - independent enrichment in both 0 and 0 of up to about 15%c above tropospheric values (Thiemens et al. 1995). The enrichment of stratospheric CO2 relative to tropospheric CO2 should make it possible to study mixing processes across the tropopause. 

Compound specific stable isotope analysis using gas chromatography combined with an isotope ratio mass spectrometer - GC-IRMS (see also Chapter 7) - is now a mature analytical technique in environmental science and technology, especially in the area of contaminant source attribution and in assessing the biodegradation of contaminants.108 Several studies have focused on 13C/12C, 180/160 and 170/160 isotope ratio measurements for volatile organic and metalor-ganic compounds to study isotope fractionation effects and to identify contamination in the environment.109... 

Carbon isotope fractionation effects of individual compounds were observed in living organisms and also as a result of enzymatic isotope effects and reaction kinetics in biological systems. Such fractionation effects have to be examined by isotope ratio mass spectrometry in order to understand specific processes in life sciences or in environment.75... 

This method of establishing retention time is valid only if isotopic fractionation effects during nitrogen transformations are unimportant. For example, nitrification results in a 15N enrichment of the residual NH4 + pool... 

Figure 2.16 Illustration of isotopic fractionation effects in diffusion. The model is that 132Xe and 134Xe are initially uniformly distributed throughout spheres in the ratio 134Xe/132Xe = 0.382 and then allowed to escape by diffusion with the boundary condition that the concentration vanishes on the surface. The figure shows the instantaneous composition of the released gas at various stages, assuming that the diffusion coefficients varies as m 112. The single-component locus is for all spheres having the same radius the mixed-component locus is for distribution of sizes. Reproduced from Funk, Podosek, and Rowe (1967).

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-... 

Before closing this section we should mention several relatively recent papers that provide additional details relevant to the overview given above. The effects of evaporation, back-reaction, diffusion, and dust enrichment on isotopic fractionation in forsterite have been discussed in great detail by Tsuchiyama et al. (1999) and Nagahara and Ozawa (2000) and extended to multicomponent systems in Ozawa and Nagahara (2001). Richter et al. (2002) combined theoretical and experimental approaches to study elemental and isotopic fractionation effects due to evaporation from CMAS liquids and included consideration of the effects of temperature, gas composition, and diffusion in both the residue and in the surrounding gas. 

This chapter focuses upon some recent observations of mass-independent isotopic processes in nature. As discussed by Thiemens et al. (2001) and Thiemens (2002), there exist other mass-independent isotope effects in nature that derive from non-ozone reactions. For example, CO2 photolysis produces a large mass-independent isotope effect that, in part, may account for observations in the SNC (martian) meteorites and the synthesis of their secondary minerals. UV photolysis of SO2 produces new isotopic fractional effect. An accompanying mass-independent isotopic composition determines the evolution of oxygen in the Earth s earliest atmosphere. 

Glueckauf has derived Eq. (17) for the e-values of alkali metals from equations which contain the correlation between the activity coefficient and the molality of the solution and the ionic radius of the dissolved ion. Equation (17) results in a direct correlation between the isotopic fractionation effect e and the ionic radius difference of the isotopes ... 

Because isotopic fractionation effects are in the magnitude of some few per thousands, they can only be analyzed very inaccurately (see Chap. 3) in one equilibrium stage. Therefore, in many experiments a higher total separation effect is established by multiplication processes. From these results the elementary fractionation effect e can be calculated under certain presuppositions. In principle, a multiplication of the elementary fractionation effect is possible with multi-stage batch experiments (cascade) or with chromatographic systems. 

In these experiments the calcium distribution ratio between the Dowex 50 resin and the solution was established to be about ten by using the corresponding amounts of exchanger and CaCL salt. With such a distribution ratio, the isotopic fractionation effect is almost completely shifted into the solution, whereas the initial isotope ratio is present in the resin phase (see Chap. 2,4). Under this condition and under... 

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