Kinetic isotope fractionation

Mariotti, A., Germon, J.C., Hubert, P., Kaiser, R, Letolle, R., Tardieux, A. and Tardieux, P. 1981 Experimental determination of nitrogen kinetic isotope fractionation some principles illustration for the denitrification and nitrification processes. Plant and Soil 62 413-430. 

The products of Rayleigh fractionation are effectively isolated from isotopic exchange with the rest of the system immediately upon formation. If the process occurs slowly, such that each increment of product B forms in isotopic equilibrium with the reactant A prior to isolation of B from the system, then would be an equilibrium isotope fractionation factor. However, if the process of formation of B is rapid, incremental formation of B may be out of isotopic equilibrium withH. In this case, would be a kinetic isotope fractionation factor, which may be a function of reaction rates or other system-specihc conditions. 

MASS-DEPENDENCE OF EQUILIBRnJM AND KINETIC ISOTOPE FRACTIONATIONS... 

Kinetic isotope fractionation obeys a different fractionation law. Young et al. (2002b) showed recently that kinetic processes, again written here in terms of the two Mg isotope fractionation factors, obey the relation... 

Table 3. Kinetic isotopic fractionation caused by Se transformations.

Figure 7. Measured and corrected A Fe Fe(ni)-Hem.tite values ( and O, respectively) relative to average hematite precipitation rate for Experiments 5, 7, and 8 of Skulan et al. (2002). The A Fe jj(ni).Hem.tite values are defined as those measured at the termination of the experiments the corrected A Fe i,e(ni).Hem.tite values reflect the estimated correction required to remove any residual kinetic isotope fractionation that was produced early in experiments that was not completely removed hy dissolution and re-precipitation over the long term. Extrapolation of the corrected A Fe jj(ni).Hem.tite values to zero precipitation rates yields an estimate for the equilihrium Fe(III),q-hematite fractionation, A Fci,e(in).hem.tite,...

Figure 10. Comparison of isotopic fractionations determined between Fe(II)aq and Fe carbonates relative to mole fraction of Fe from predictions based on spectroscopic data (Polyakov and Mineev 2000 Schauble et al. 2001), natural samples (Johnson et al. 2003), DIR (Johnson et al. 2004a), and abiotic formation of siderite under equilibrium conditions (Wiesli et al. 2004). Fe(II)aq exists as the hexaquo complex in the study of Wiesli et al. (2004) hexaquo Fe(II) is assumed for the other studies. Total cations normalized to unity, so that end-member siderite is plotted at Xpe = 1.0. Error bars shown reflect reported uncertainties analytical errors for data reported by Johnson et al. (2004a) and Wiesli et al. (2004) are smaller than the size of the symbol. Fractionations measured on bulk carbonate produced by DIR are interpreted to reflect kinetic isotope fractionations, whereas those estimated from partial dissolutions are interpreted to lie closer to those of equilibrium values because they reflect the outer layers of the crystals. Also shown are data for a Ca-bearing DIR experiment, where the bulk solid has a composition of q)proximately Cao.i5Feo.85C03, high-Ca and low-Ca refer to the range measured during partial dissolution studies (Johnson et al. 2004a). Adapted from Johnson et al. (2004a).

Magnetite that has 5 Fe values near zero might reflect formation during slow (equilibrium) reduction rates, whereas the lowest 5 Fe values might reflect rapid Fe(III) reduction and kinetic isotope fractionation (Fig. 19), or multiple reduction cycles. The correlation between Magnetite-siderite 8 Fe of magnetite observed in BIFs therefore probably reflects changes in... 

The theory of kinetic isotope fractionations has been discussed by Bigeleisen and Wolfsberg (1958), Melander(1960), and Melander and Saunders (1980). Knowledge of kinetic isotope effects is very important, because it can provide information about details of reaction pathways. 

During microbial action, kinetic isotope fractionations on the organic material by methanogenic bacteria result in methane that is highly depleted in typically with 5 C-values between -110 and -50%c (Schoell 1984, 1988 Rice and Claypool 1981 Whiticar et al. 1986). In marine sediments, methane formed by CO2 reduction is often more depleted in than methane formed by acetate fermentation in freshwater sediments. Thus, typical ranges for marine sediments are between -110 and -60%c, while those for methane from freshwater sediments are from -65 to -50%c (Whiticar et al. 1986 Whiticar 1999). 

Forward and backward reaction rates are not identical in systems that are not at isotopic equilibrium. Reactions may, in fact, be unidirectional if reaction products become physically isolated from the reactants. The reaction rates depend on the ratios of the masses of the isotopes and their vibrational energies hence, such reactions result in kinetic isotope fractionations. The magnitude of a kinetic isotope fractionation depends on the reaction pathway and the relative energies of the bonds being severed or formed by the reaction. The kinetic fractionation factor is typically larger than the equilibrium fractionation factor for the same reaction. As a rule, bonds of light isotopes are broken more easily than equivalent bonds of heavier isotopes. Hence, light isotopes react faster than heavy isotopes. 

Isotope effect studies of the mechanism of hydration of alkynes with formic acid as water donor485b,485c leading to ketones (equation 234a) have been undertaken recently4854 by observing the kinetic isotope fractionation of 13C in the course of carbon monoxide... 

Mariotti A. Germon J. C. Leclerc A. Catroux G. Letolle R. Experimental determination of kinetic isotope fractionation of nitrogen isotopes during denitrification. Anal. Chem. Symp. Ser. 1982, 11, 459 464. 

Fig. 11. The Fischer-Tropsch reaction shows a kinetic isotope fractionation between organic and carbonate carbon of the same sign and magnitude as that in meteorites (black bars on ordinate). Observed fractionations in Cl and C2 chondrites correspond to temperatures of about 360 K and 400 K (Uncet, 1972)...

Two stable isotopes of nitrogen exist, (99.634%) and (0.366%). Because of this mass difference the heavier isotope reacts at a sHghtly slower rate, producing a kinetic isotope fractionation in many biological reactions. The increase in the in the reaction products is typically expressed as the parts per... 

Kinetic isotope fractionation reflects the difference in reaction rate for molecules containing the two isotopes of nitrogen, which in turn reflects the different bond strengths and mobilities of these isotopic species. The degree of isotopic fractionation associated with a reaction is commonly expressed as a dimensionless... 

Mariotti, A., Germon, J. C., LeClerc, A., Catroux, G., and LetoUe, R. (1982). Experimental determination of kinetic isotopic fractionation of nitrogen isotopes during denitrification. In Stable Isotopes (Schmidt, Porstel, and Heinzinger, eds.). Elsevier, Amsterdam, pp. 459—464. 

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