Isotopic fractionation causes

Morasch B, HH Richnow, A Vieth, B Schink, RU Meckenstock (2004) Stable isotope fractionation caused by glycyl radical enzymes during bacterial degradation of aromatic compounds. Appl Environ Microbiol 70 2935-2940. 

Table 3. Kinetic isotopic fractionation caused by Se transformations.

Radioactive decay is one process that produces variations in isotope abundance. The second cause for differences in isotope abundance is isotope fractionation, caused by small chemical and physical differences between the isotopes of an element. It is exclusively this important process that will be discussed in the following chapters. 

To improve detection of isotope composition in samples with concentrations < 5 mg other techniques have been employed. For example, headspace solid-phase microextraction (HS-SPME) has determined 5 C [37,41] and 5 H [37] of MTBE in aqueous samples, reaching up to one order of magnitude lower detection limits (11 pg L ) for carbon mode [41]. For hydrogen isotope analysis, concentrations down to 1 mg L have been measured [37]. The small isotopic fractionations caused by these extraction techniques were evaluated by Zwank et al. [42] and were negligible and highly reproducible. 

Special terms are also used to characterize the amplitude of isotopic fractionation caused by a given process. Isotope fractionation results from both equilibrium processes ( equilibrium fractionation ) and unidirectional reactions ( kinetic fractionation ). Nitrogen isotope variations in the ocean are typically dominated by kinetic fractionation associated with the conversions of N from one form to another. The kinetic isotope effect, , of a given reaction is defined by the ratio of rates with which the two N isotopes are converted from reactant to product ... 

K. Hirao. Ligand effect on uranium isotope fractionations caused by nuclear volume effects An ab initio relativistic molecular orbital study. /. Chem. Phys.,... 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

A third source of error is associated with the fragmentation pattern caused by dissociation of the molecular ions formed in the source region of the spectrometer. Under severe conditions these processes may proceed with substantial isotopic fractionation, and this obscures the measurements of isotopic composition at the collector. To some extent careful standardization of the instrumental conditions may ensure that errors from fragmentation are systematic, and thus cancel (at least to some extent). Alternatively, softer ionization methods can be used to prevent most or all of the fragmentation. The bottom spectrum in Fig. 7.7 illustrates this approach it shows the mass spectrum of chlorobenzene obtained by photoionization. Only the parent molecular ions are observed. It should be kept in mind, however, that softer ionization usually yields smaller ion currents and consequently statistical counting errors increase. 

Jackson SE, Gunther D (2003) The nature and sources of laser induced isotopic fractionation in laser ablation-multicollector-inductively coupled plasma-mass spectrometry. J Anal At Spectrom 18 205-212 Jiang S-J, Houk RS, Stevens MA (1988) Alleviation of overlap interferences for determination of potassium isotope ratios by Inductively-Coupled Plasma Mass Spectrometry. Anal Chem 60 1217-1220 Lam JWH, Horlick G (1990) A comparison of argon and mixed gas plasmas for inductively coupled plasma-mass spectrometry. Spectrochim Acta Part B 45 1313-1325 Langmuir I, Kingdon KH(1925) Thermionic effects caused by vapours of alkali metals. Phil Trans R Soc A107 61-79... 

Figure 1. Schematic representation of the calcium mass spectrum in (a) natural materials, (b) a Ca- Ca tracer solution used for separating natural mass dependent isotopic fractionation from mass discrimination caused by thermal ionization, and (c) a typical mixture of natiwal calcium and tocer calcium used for analysis. The tracer solution has roughly equal amounts of Ca and Ca. In (c) the relative isotopic abundances are shown with an expanded scale. Note that in the mixed sample, masses 42 and 48 are predominantly from the tracer solution, and masses 40 and 44 are almost entirely from natural calcium. This situation enables the instrumental fractionation to be gauged from the Ca/ Ca ratio, and the natural fractionation to be gauged from the sample Ca/ Ca ratio.

This range of isotopic fractionation (5.5%o to 9.1%o, excluding the early time points from Herbel et al. 2000) overlaps strongly with the range observed for abiotic reduction (10%o to 13%o Table 3). This suggests a fundamental difference between Se(lV) reduction and Se(Vl) reduction, in which microbial Se isotope fractionation is much smaller than that caused by abiotic reduction. 

It is possible that a branching reaction could cause isotopic fractionation of Se removed from a Se(0) precipitate, but no evidence exists for this at present. Elemental sulfur can be converted to sulfate and sulfide through a branching reaction mediated by disproportionating bacteria, and the produced sulfate s ratio is shifted +17%o to +31%o relative to the... 

Ordinary diffusion can cause significant isotope fractionations. In general, light isotopes are more mobile and hence diffusion can lead to a separation of light from heavy isotopes. For gases, the ratio of diffusion coefficients is equivalent to the inverse square root of their masses. Consider the isotopic molecules of carbon in CO2 with masses and C 0 0 having molecular weights of 44 and 45. 

Mass balance effects can cause isotope fractionations because modal proportions of substances can change during a chemical reaction. They are especially important for elements in situations where these coexist in molecules of reduced and oxidized compounds. Conservation of mass in an n component system can be described by... 

O Neil and Truesdell (1991) have introduced the concept of structure-making and structure-breaking solutes structure makers yield more positive isotope fractionations relative to pure water whereas structure breakers produce negative isotope fractionations. Any solute that results in a positive isotope fractionation is one that causes the solution to be more structured as is the case for ice structure, when compared to solutes that lead to less structured forms, in which cation - H2O bonds are weaker than H2O - H2O bonds. 

---