Isotopic Fractionation

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Isotopic Fractionation
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Reasons for Isotopic Fractionation During Natural
Processes

• Vibrational frequencies of light isotopes gt heavy
  isotopes
• Leads to kinetic isotope effects
• K.E.mv2/2
• Bond strength of heavy isotopes are greater
• Leads to equilibrium isotope effects

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Attributes of a good stable isotope for
biogeochemical study

• Most of the attention in the past 50 years has
  been on H,C,N,O,S the elements of life
• There are good reasons why these elements have
  been so useful
• Low atomic mass (gas source)
• Mass difference between rare and abundant isotope
  is great (low MRP)
• Form covalent-like bonds (respond to bond
  differences)
• Exist in multiple oxidation states
• Rare isotope is relatively abundant

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Isotope Effects (how isotopes fractionate)The
heavy isotopes concentrate in the compound in
which the element is most strongly held

• Kinetic isotope effects
• Occur with fast, incomplete, or unidirectional
  processes
• Biology
• Translational velocities important
• Equilibrium isotope effects
• Preferential enrichment of one isotope ina site
  relative to another
• Minerals vs. water or gas
• Bond energy important

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Isotope Exchange Reactions

• H2O HD HDO H2
• equilbrium constant is
• KHDOH2/H2OHD
• fractionation factor is
• RH2/RH2O
• And
• ?(K/K8)1/n , where nnumber of atoms exchanged
  in exchange reaction

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Theory of isotope exchange

• Ureys theory for isotope partion functions (Q)
  of a diatomic molecule (1) and its isotopomer (2)
  is

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Zero Point Energy

• The energy a molecule contains in its lowest
  vibrational level at absolute 0 (K)
• ZPE declines as mass increases, and bond strength
  increases

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Vibrational Frequencies Decrease with Increasing
Mass
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Temperature Dependence of Isotope Fractionation

• For a given reaction, depends only on T, not P
  etc.
• In general,
• ln ? 1/T or 1/T2
• And, as T?8
• K ? K 8
• ?1
• 1000ln ??0
• Crossovers sometimes occur as well..

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