Isotope fractionation

As isotope is defined as the shift of the frequency of the isotopes of an element caused by physical / chemical processes. Fractionation is thermodynamic and therefore temperature dependent.

overview

The fractionation factor is calculated as the ratio of the isotopes of two samples Rx and Ry, for example for oxygen isotopes in water: ${\displaystyle \alpha }$

${\displaystyle \alpha =R_{x}/R_{y}=(^{18}O/^{16}O)_{\text{Wasser}}/(^{18}O/^{16}O)_{\text{Wasserdampf}}}$

There are other values ​​that are used as a measure of the degree of equilibrium: the separation or isotope difference ( ), the enrichment factor (E), and the value . All of these values ​​indicate the ratio of isotope concentrations. All values ​​can be used for minor deviations from the reference. However, the separation and the enrichment factor are approximations, which means that if there are large deviations from the reference, the other values ​​should be used. ${\displaystyle \delta }$ ${\displaystyle 10^{3}\ln(\alpha )}$

A distinction is made between mass-dependent and mass-independent isotope fractionation.

In the case of mass-dependent isotope fractionation, the change in the frequency ratio of two isotopes of the same element depends (in a first approximation linearly) on the mass difference of the isotopes. This is e.g. B. the case with kinematic processes such as diffusion . For example, lighter isotopes diffuse faster through a porous membrane out of a given volume than heavier isotopes of the same element. In a three-isotope diagram , samples that have a common origin, but have undergone different degrees of mass-dependent isotope fractionation, lie on a fractionation line .

In the rarer mass-independent isotope fractionation, the frequency of one or more isotopes of an element can be changed independently of the mass compared to the other isotopes. Mass-independent isotope fractionation occurs in some photochemical processes in the upper atmosphere. MH Thiemens and IE Heidenreich have demonstrated mass-independent isotope fractionation in ozone formation .

Applications

Isotope fractionation is technically important. B. in uranium enrichment . The investigation of isotope fractionation also plays a role in forensics , for example when investigating the origin and purity of food ingredients or doping control in sport.

The proof of isotope fractionation is also used in the environmental sector in the analysis of the origin (search for the cause) and the degradation processes ( natural attenuation ) in contaminated groundwater. In the geosciences, Si isotope fractionation is still used as a tool for the reconstruction of biogeochemical material cycles in the history of the earth. This method belongs to the so-called “non-traditional” stable isotope systems (Mg, Ca, Si, Ti, Cr, Fe, Cu, Zn, Mo, Tl).

Isotope examinations such as Δ18O or Δ13C are among the most important methods of paleoclimatology . With them z. B. Reconstruct average temperatures, but also detect extensive carbon inputs into the environment that were of organic origin.

In archeology , too, methods have been developed that use isotope studies to make statements about origin (or migration and mobility) with oxygen isotope analysis and / or strontium isotope analysis and nutrition with carbon isotope analysis ( C4 plant n-based or C3 plant n-based nutrition, influence of lacustrine or marine nutrition) and / or nitrogen isotope analysis (tracking the trophic level: herbi -, omni - and carnivore , as well as marine nutrition).

literature

• Microbiological NA investigation methods: technical basis for the application of methods for recording the natural microbial degradation of pollutants in the aquifer . DECHEMA, Frankfurt a. M. 2007, ISBN 978-3-89746-086-7 , Chapter A7: Substrate-specific isotope fractionation. ( PDF ( Memento of December 17, 2008 in the Internet Archive )).
• Environmental Protection Agency (EPA): A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). ( Memento of April 2, 2010 in the Internet Archive ), Report A / 600 / R-08/148, December 2008 (pdf; 3.7 MB)

Web links

• Environmental monitoring with isotope analyzes

Individual evidence

1. ^ Ian Douglas Clark and Peter Fritz: Environmental isotopes in hydrogeology . CRC Press, 1997, ISBN 978-1-56670-249-2 , pp. 31 .
2. ^ JE Heidenreich III, MH Thiemens: A non-mass-dependent isotope effect in the production of ozone from molecular oxygen. Journal of Chemical Physics, Vol. 78, No. 2, pp. 892-895, 1983, doi: 10.1063 / 1.444791 .
3. ↑ A Guide for Assessing Biodegradation and Source Identification of Organic Groundwater Contaminants Using Compound Specific Isotope Analysis (CSIA). ( Memento of April 2, 2010 in the Internet Archive ) Environmental Protection Agency (EPA), Report A / 600 / R-08/148, December 2008. p. 1.
4. ↑ Isotope fractionation. Helmholtz Center for Environmental Research , accessed on August 19, 2017 . 
5. ^ Claudia Gerling: A Multi-Isotopic Approach to the Reconstruction of Prehistoric Mobility and Economic Patterns in the West Eurasian Steppes 3500 to 300 BC. (PDF) Gerd Graßhoff and Michael Meyer, communicated by Wolfram Schier, August 22, 2014, accessed on February 11, 2017 (English). 
6. ^ HP Schwarcz, MJ Schoeninger: Stable Isotopes of Carbon and Nitrogen as Tracers for Paleo-Diet Reconstruction . In: Handbook of Environmental Isotope Geochemistry (=  Advances in Isotope Geochemistry ). Springer Berlin Heidelberg, 2012, ISBN 978-3-642-10636-1 , p. 725-742 , doi : 10.1007 / 978-3-642-10637-8_34 .