Isotopes fractionation

The complex biochemical paths in organisms offer many ways to fractionate isotopes by both kinetic and equilibrium processes. It is therefore expected, and is observed, that the different carbon, hydrogen, oxygen and nitrogen atoms in organic residues show compound and atom-site specific isotope fractionations... 

Kinetic isotope effects during microbial processes. Micro-organisms have long been known to fractionate isotopes during their sulfur metabolism, particularly during dissimilatory sulfate reduction, which produces the largest fractionations in the sulfur cycle... 

Maximum fractional Isotopic Abundances Observed for the Photosensitized... 

The fact that different isotopes of an element do not have the same physical-chemical properties means that kinetic and isotope exchange processes can lead to variations in isotopic composition. This phenomenon is usually referred to as isotope fractionation. Isotope fractionation, in most cases, leads to only small differences in isotopic composition. Consequently, isotope ratios are generally reported in terms of parts per thousand (per mil, o/00) differences. While it is possible to report isotope compositions in absolute terms, it has been found most convenient to report them relative to a standard with a composition typical of common natural materials. These two considerations result in the commonly used "8" notation ... 

Some plants employ a photosynthetic pathway creating at first a three-carbon phosphoglyceric acid (C3 or Calvin-Benson photosynthesis). These plants fractionate isotopes more intensely, and so have more negative values (-33%o to —22%o PDB) than plants which use a photosynthetic pathway creating at first a four-carbon malic and aspartic acid (C4 or Hatch-Slack photosynthesis -16%o to -9%o PDB). Crassulacean acid metabolism (CAM) is yet another photosynthetic pathway, which creates organic matter of intermediate isotopic composition (-35%o to -ll%o PDB). Methanogenic microbes are even more extreme in their fractionation of the light isotope (5 C down to -110%o and typically -60%o PDB ... 

Figure 6 Distribution of reported organic matter turnover under the various TEAPs in pristine surface water sediments and groundwater aquifers (a) and landfill-leachate contaminated groundwater (b). The respiration rates in pristine groundwater ((a) middle and right panels) were modeled using total electron acceptor species concentration (geochemical) and carbon isotope fractionation (isotopic constraints) (after Murphy and Schramke, 1998 and...

MDF follows specific rules based on the relative atomic mass differences between the fractionating isotopes, and as a result produces highly correlated relationships between isotope ratios for that element (Chapter 2, Section 2.3.3.2 Chapater 5, Section 5.3.1.3). MDF is expressed as a deviation from the appropriate standard as a lower-case delta-value in parts per thousand, for example, <5180 %o, del oxygen eighteen - which is computed from the expression... 

Oxygen isotope fractionations. Isotopic fractionation is generally thought to be independent of pressure. However, in specific cases, pressure can lead to significant changes in the magnitude of fractionation factors. From classical thermodynamics, the effect of pressure on an equilibrium constant is given by ... 

Gunning and his coworkers (429a) have in fact succeeded in enriching I he reaction product of ° Hg preferentially excited with a lamp containing " Hg operated in a microwave discharge. Upon illumination of mixtures Ilf natural mercury and HCl by the ° Hg lamp, the maximum fractional isotopic abundance of ° HgCl (calomel) obtained is 0.45 with intermittent... 

Atomic weight = [(isotope mass) X (fractional isotope abundance)]... 

The isotopes are assumed to have fractional isotope abundances of 0.4782 and 0.5218, respectively, based on 1961 values (Cameron and Wichers, 1962) which were currently at the time of the heat capacity measurements of Krusius et al. (1974) determined in the nuclear heat capacity region and the Mbssbauer effect measurements by Hiifner and Wemick (1968) (Table 98). 

It is possible to rearrange this equation to deduce absolute ratios in terms of at.% and fractional isotope abundance. 

An estimate by Loewenstein et al. [370] of the fractional isotope effect, i.e. the ratio of the isotope effect and the total shielding, was mentioned in Section 6.1 as well as its application [245] to obtain absolute shieldings of ions in solution. 

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