Trace isotope fractionation effects

Siebert et cd. [12] and Barling and Anbar [13] suggested that the equUibrium isotope fractionation effect during adsorption of Mo on birnessite could result either from equUibration of two chemical species of Mo in solution, followed by adsorption of just one of them, or from fractionation during the adsorption reaction itself, between one aqueous species and one adsorbed species not present in solution. In order to address this, Tossell [21] used quantum mechanical calculations to explore which trace species of aqueous Mo might fractionate from the... 

Lee and Bethke (1996) presented an alternative technique, also based on mass balance equations, in which the reaction modeler can segregate minerals from isotopic exchange. By segregating the minerals, the model traces the effects of the isotope fractionation that would result from dissolution and precipitation reactions alone. Not unexpectedly, segregated models differ broadly in their results from reaction models that assume isotopic equilibrium. 

If diffusivity is extracted from the profile of the isotopic fraction of an element, it may differ significantly from, and often greater than, the effective binary diffusivity obtained from the concentration profile of the trace or minor element. 

Based on these observations, the diffusivity extracted from isotopic fraction profiles is usually regarded to be similar to intrinsic diffusivity or self-diffusivity even in the presence of major element concentration gradients. That is, the multicomponent effect does not affect the length of isotopic fraction profiles (but it affects the isotopic fractions and the interface position). On the other hand, the diffusion of a trace or minor element is dominated by multicomponent effect in the presence of major element concentration gradients. 

Figure 3-24 Calculated diffusion-couple profiles for trace element diffusion and isotopic diffusion in the presence of major element concentration gradients using the approximate approach of activity-based effective binary treatment. The vertical dot-dashed line indicates the interface. The solid curve is the Nd trace element diffusion profile (concentration indicated on the left-hand y-axis), which is nonmonotonic with a pair of maximum and minimum, indicating uphill diffusion. The dashed curve is the Nd isotopic fraction profile. Note that the midisotopic fraction is not at the interface.

The isotope ratio traces of the GC peaks exhibit a typical S shape. The heavier isotopic species of a compound are eluted more rapidly than the light species. Similar effects can be observed for all chromatographic processes, whereas the size of the isotope fractionation and the elution order of the isotopomers de-... 

When stable isotopes are used, for example, as tracers for in vivo metabolic studies, typically the tracer flux is being measured and inferences are made about the flux of the material being traced ( tracee ). When there is no isotope fractionation, the flux of the heavy tracer is equal to the flux of the lighter major isotope and hence the tracee flux. However, when there is isotope fractionation the fluxes of the heavy tracer and tracee are not equal and thus the flux derived from the kinetic analysis of the tracer is not equal to that of the tracee. For example, is fractionated between water and carbon dioxide (a = 1.041 at 25°C) thus the rate of removal of by CO2 is 4.1% greater than the rate of removal. Because comprises 99.8% of the oxygen pool, the rate of CO2 production is essentially equal to the rate of removal by CO2, which will be less by 4.1% than that measured from 0. Without taking this correction into account, the calculated tracee flux will be in error. On the other hand, the isotope effect becomes negligible in the distribution of the isotopes of heavier elements therefore, no corrections are necessary in the tracer experiments. 

In a simple chemical apparatus, isotopic fractionation occurs by two mechanisms. The first is isotopic disequilibrium, in which the mass difference between the isotopes causes a significant difference between the equilibrium constants or rates of reactions in which the two isotopic species are involved. The magnitude of the isotopic fractionation is greater for traced elements in which there is a large fractional difference between the mass number of the tracer and that of the traced element this effect is considered neghgible for isotopes of aft elements with atomic numbers >10 (Duncan and Cook 1968). 

Dichlorodibenzo-p-dioxin was prepared from isotopic potassium 2,4-dichlorophenate uniformly labeled with Ullman conditions gave a 20.5% yield. Small amounts of dichlorophenoxy chlorophenol were removed from the product by extraction with sodium hydroxide before purification by fractional sublimation and recrystallization from anisole. Chlorination of 2,7-dichlorodibenzo-p-dioxin in chloroform solution containing trace amounts of FeCls and 12 yielded a mixture of tri-, tetra-, and pentachloro substitution products. Purification by digestion in boiling chloroform, fractional sublimation, and recrystallization from anisole was effective in refining this product to 92% 2,3,7,8-tetrachloro isomer, which also contained 7% of the tri- and 1% of the penta-substituted dibenzo-p-dioxin. Mass spectroscopy was used exclusively to monitor the quality of the products during the synthesis. 

DePaolo, D. J. (1981). Trace-element and isotopic effects of combined wallrock assimilation and fractional crystallization. Earth Planet. Sci. Letters, 53, 189-202. 

A good example of translational fractionation is one-way diffusion through an orifice that is smaller than the mean-free path of the gas. Related, but somewhat more complex velocity-dependent fractionations occur during diffusion through a host gas, liquid, or solid. In these fractionations the isotopic masses in the translational fractionation factor are often replaced by some kind of effective reduced mass. For instance, in diffusion of a trace gas JiR through a medium, Y, consisting of molecules with mass ttiy. 

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