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There are some ways this is an easy question and there are some ways it is
not.

It is often a perfectly useful approximation to hold that hydrostatic
pressure does not alter the distribution of stable isotopes in a system at
equilibrium.  This is not strictly true; one can write Gibbs free energy in
terms of pressure and volume.  If the molar volumes of two isotopologues
are not equal, neither are their Gibbs free energies.  In most cases, the
change in molar volume that comes with isotopic substitution is so small
that it is safely ignored.  Hydrogen is almost sui generis in that the
molar volume difference is not trivial with substitution, so it's not a
safe bet to assume there's no difference.

If you are concerned about kinetic phenomena, there are circumstances where
there is unquestionably a pressure effect.  This is most obvious in
biochemistry, where the catalytic capability of enzymes is a function of
hydrostatic pressure.  I haven't spent any real time thinking about whether
fractional crystallization causes pressure-dependent isotopic
fractionation, but I would not be surprised in the least if so.  Someone
here probably knows that kind of thing.

-patrick



Indiana University
Department of Earth and Atmospheric Sciences
Bloomington IN 47405

On Fri, Sep 14, 2018 at 12:58 PM, Lukas Flierl <[log in to unmask]> wrote:

> Dear all,
>
> Does anybody know some literatur about the pressure dependence of isotope
> ratios?
> I have found some about He or Ar and some about CO2 from the 1970's.
>
> Thanks and cheers,
>
> Lukas
>
>
> -----------------------------------------------------------------
> Lukas Flierl
> Physikalisch-Technische Bundesanstalt PTB
> Metrology in Chemistry, 3.11
> Bundesallee 100
> <https://maps.google.com/?q=Bundesallee+100+%0D%0A38116+Braunschweig,+Germany&entry=gmail&source=g>
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> Tel. +49-531-592-3318
> e-mail: [log in to unmask]
> web: www.ptb.de