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



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Lukas Flierl
Physikalisch-Technische Bundesanstalt PTB
Metrology in Chemistry, 3.11
Bundesallee 100
38116 Braunschweig, Germany
Tel. +49-531-592-3318
e-mail: [log in to unmask]
web:
www.ptb.de