Isotopic fractionation elements/compounds

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... 

In recent years, tremendous progress has been achieved in the analysis of the isotope composition of important trace compounds in the atmosphere. The major elements - nitrogen, oxygen, carbon - continually break apart and recombine in a multitude of photochemical reactions, which have the potential to produce isotope fractionations (Kaye 1987). Isotope analysis is increasingly employed in studies of the cycles of atmospheric trace gases e.g., CH4 and N2O, which can give insights into sources and sinks and transport processes of these compounds. The rationale is that various sources have characteristic isotope ratios and that sink processes are accompanied by isotope fractionation. 

Different isotopes of the same element differ slightly in chemical and physical properties because of their mass differences. For elements with low atomic masses, these mass differences are large enough for many physical, chemical, and biological reactions to fractionate or change the relative proportions of different isotopes of the same element in various compounds. Thus, a particular water or mineral may have a unique isotopic composition (ratio of the isotopes of an element) that indicates its source or the process that formed it. Two different processes—equilibrium and kinetic isotope effects—cause isotope fractionation. 

In this chapter we will examine the basic chemical concepts of coprecipitation and solid solutions, and the partition coefficients of different elements and compounds in major sedimentary carbonate minerals will be presented. A brief summary of information on oxygen and carbon isotope fractionation in carbonate minerals will also be presented. A major portion of this chapter is devoted to... 

Sulphur isotopes (32,33,34,36S) fractionate strongly in the earth s crust because (1) the element occurs in different oxidation states with differential preference for heavy isotopes, (2) the existence of volatile and easily soluble compounds favors kinetic separations, and (3) it is involved in biogenic cycles where the oxidation state is easily changed and kinetic processes are important. From theoretical calculations of Bigeleisen (1961) and data on the isotopic properties of sulphur compounds by Sakai (1957, 1968), the amount of S isotope fractionation and its temperature dependence is known. The information on experimental inorganic isotope fractionation in coexisting sulphide minerals which occur naturally was summarized by Thode (1970), who also discussed the application of S isotopes from sulphides for geo thermometry (cf. also Sakai, 1971). Analytical work on all types of sulphur compounds which occur in nature has been reviewed by Nielsen (1973). 

Unlike the reactions of GEM in solution, experimental data on the gas-phase reactions of elemental mercury with some atmospheric oxidants are limited due to challenges including complexity of reactions, the low concentrations of species at atmospheric conditions, the low volatility of products, sensitivity to temperature and pressure, and the strong effects of water vapour and surface on kinetics. The possible effects and distribution of mercury isotope fractionation have not been analysed in any of the studies. The isotopes dilute the signal and mean that with current mass spectrometry techniques, ambient RGM compounds can not be identified. The possibility of theoretically predicting the thermochemistry of mercury-containing species of atmospheric interest is important and is complementary to laboratory and field studies. 

Chemical state of carbon in carbonaceous chondrites agrees with that predicted from their formation conditions (indicated by boxes), as inferred from isotopic fractionation of O and C, or abundanc es of volatile metals (Table 1 and Fig. 11 Onuma et al., 1972, 1974 Anders et al., 1976). Cl and C2 chondrites, having formed between 360 and 400 K, contain mainly organic compounds with only traces of carbynes (Whittaker et al., 1980). C3 chondrites contain mainly elemental carbon, which, at least in the case of Allende, is present as carbynes rather than graphite... 

A summary of the available results on the extent of isotope fractionation during sulfide oxidation is summarized in Table 3. The phototrophic oxidations of sulfide to elemental sulfur and of elemental sulfur to sulfate yield only small or negligible fractionations. Small fractionations also accompany the non-phototrophic, biologically-mediated, oxidation of sulfide to elemental sulfur, as well as the oxidation of sulfur intermediate compounds to sulfate (Table 3). However, significant depletion of sulfate in... 

Radiochemical analysis relies on the assumption that different isotopes of the same element exhibit the same properties in any macroscopic physical or chemical process, and that radioactive labeling does not influence the other properties of a chemical species. This is generally the case, with deviations below 1% (with exception of hydrogen isotopes) owing to isotopic fractionation or radiation effects. For analytical purposes, the radiotracer and the analyte must be present in the same chemical form. This is usually easy to achieve, but specialized preparative techniques may be necessary for radioactive labeling of more complex organic compounds. 

Spectral interferences due to the occurrence of isobaric, polyatomic, and doubly charged ions, signal suppression, increased drift effects, and so on, are typically encountered when the target element or compound is not purified and not separated from its matrix. Several strategies for achieving proper separation have been developed. It must be emphasized that reliable isotope ratio data cannot be achieved with an MC-ICP-MS instrument without chemical analyte-matrix separation. Preferably, analyte isolation via a chromatographic process has to be accomplished with quantitative analyte recovery, as isotope fractionation on the chromatographic column has been reported. 

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

Elements such as C, N, O, S, and Cl that are components of many organic compounds exist naturally as mixtures of stable isotopes. The ratios of these in a compound reflect the different rates of reaction at isotopically labeled positions, and therefore reflect the fractionation—biotic or abiotic—by which it was synthesized or to which the compound has been subjected. Techniques have been developed whereby the ratios C/ C (5 C), (5 N), (5 0),... 

In these types of isotopic analysis, the same compound as that to be measured is used (element or molecule) where one of the atoms in it has been replaced by a radioisotope to allow radioactivity measurements. A small, precisely known quantity of the labelled compound, called the tracer, is added to the sample and, after homogenisation, an aliquot of the spiked sample is isolated by a fractionation technique such as recrystallisation or chromatography. The specific activity of the tracer is measured before and after fractionation. 

---