Isotope fractionation

Magnetic sector mass spectrometer used in isotope ratio analysis, throughthermal ionization.

Isotope fractionationdescribesfractionationprocesses that affect the relative abundance of isotopes, phenomena which are taken advantage of inisotope geochemistryand other fields. Normally, the focus is onstable isotopesof the same element. Isotopic fractionation can be measured byisotope analysis,usingisotope-ratio mass spectrometryorcavity ring-down spectroscopyto measureratios of isotopes,an important tool to understand geochemical and biological systems. For example,biochemicalprocesses cause changes in ratios of stablecarbon isotopesincorporated into biomass.

Definition

Stable isotopes partitioning between two substancesAandBcan be expressed by the use of the isotopic fractionation factor (alpha):

α_A-B= R_A/R_B

whereRis the ratio of the heavy to light isotope (e.g.,²H/¹H or¹⁸O/¹⁶O). Values for alpha tend to be very close to 1.

Types

There are four types of isotope fractionation (of which the first two are normally most important):equilibrium fractionation,kinetic fractionation,mass-independent fractionation(or non-mass-dependent fractionation), andtransient kinetic isotope fractionation.

Example

Isotope fractionation occurs during aphase transition,when the ratio of light to heavy isotopes in the involved molecules changes. Whenwater vaporcondenses (anequilibrium fractionation), the heavier water isotopes (¹⁸O and²H) become enriched in the liquid phase while the lighter isotopes (¹⁶O and¹H) tend toward the vapor phase.

See also

• Isotope separation

Literature

• Faure G., Mensing T.M. (2004),Isotopes: Principles and Applications(John Wiley & Sons).
• Hoefs J., 2004.Stable Isotope Geochemistry(Springer Verlag).
• Sharp Z., 2006.Principles of Stable Isotope Geochemistry(Prentice Hall).