Kinetic fractionation is an
isotopic fractionation process that separates stable
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
s from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isotope reactions. All organisms preferentially use lighter isotopic species, because "energy costs" are lower, resulting in a significant fractionation between the substrate (heavier) and the biologically mediated product (lighter). As an example, photosynthesis preferentially takes up the light isotope of carbon
12C during assimilation of an atmospheric CO
2 molecule. This kinetic isotope fractionation explains why plant material (and thus fossil fuels, which are derived from plants) is typically depleted in
13C by 25 per mil (2.5 per cent) relative to most inorganic carbon on Earth.
A naturally occurring example of non-biological kinetic fractionation occurs during the evaporation of
seawater
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has appro ...
to form
cloud
In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may ...
s under conditions in which some part of the transport is unidirectional, such as evaporation into very dry air . In this instance, isotopically lighter
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
molecules (i.e., those with
16 O) will evaporate slightly more easily than will the isotopically heavier water molecules with
18 O, and this difference will be greater than it would be if the evaporation was taking place under equilibrium conditions (with bidirectional transport).
During this process the oxygen isotopes are
fractionated
Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in whi ...
: the clouds become enriched with
16O, and the seawater becomes enriched in
18O. Whereas equilibrium fractionation makes the vapor about 10 per mil (1%) depleted in
18O relative to the liquid water, kinetic fractionation enhances this fractionation and often makes vapor that is about 15 per mil (1.5%) depleted. Condensation occurs almost exclusively by equilibrium processes, and so it enriches cloud droplets somewhat less than evaporation depletes the vapor. This explains part of the reason why rainwater is observed to be isotopically lighter than seawater.
The isotope of hydrogen in water, deuterium, is much less sensitive to kinetic fractionation than oxygen isotopes, relative to the very large equilibrium fractionation of deuterium. For this reason kinetic fractionation does not deplete deuterium nearly as much, in a relative sense, as
18O. This gives rise to an excess of deuterium in vapor and rainfall, relative to seawater. The value of this "deuterium excess", as it is called, is about +10 per mil (1%) in most meteoric waters and its non-zero value is a direct manifestation of kinetic isotope fractionation.
A generalized treatment of kinetic isotopic effects is via the GEBIK and GEBIF equations describing
transient kinetic isotope effects.
[Maggi F. and W. J. Riley, (2010), Mathematical treatment of isotopologue and isotopomer speciation and fractionation in biochemical kinetics, Geochim. Cosmochim. Acta, ]
Other types of fractionation
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Equilibrium fractionation
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Mass-independent fractionation
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Transient kinetic isotope fractionation
See also
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Isotopic enrichment
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Isotopic ratio
In physics, natural abundance (NA) refers to the abundance of isotopes of a chemical element as naturally found on a planet. The relative atomic mass (a weighted average, weighted by mole-fraction abundance figures) of these isotopes is the at ...
*
Kinetic isotope effect
In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for ...
References
Fractionation
Environmental isotopes
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