In several sources I have read that heavier isotopologues eluate earlier than lighter ones. As explanation vapor isotope effects are given. But this is something I do not understand. I tought in a GC column substances are separated by their different vapor pressures and the longer a substance stays in the gas phase the earlier it eluates. Heavy isotopologues have have a smaller vapour pressure and should therefore eluate later. What I am missing?

  • $\begingroup$ You may miss the shift of the distribution constant between phases. $\endgroup$
    – Poutnik
    Commented May 12, 2021 at 7:45
  • $\begingroup$ Could you please explain this a little bit more. $\endgroup$
    – Zorg
    Commented May 12, 2021 at 7:53
  • $\begingroup$ Imagine there is a polar and nonpolar compound with the same saturated vapour pressure, having nonpolar SF. Which would have the longer elution time ? Now generalize for compounds with different distribution constant between MF/SF, or the equivalent Henry constant. $\endgroup$
    – Poutnik
    Commented May 12, 2021 at 8:37
  • $\begingroup$ Isn't the boiling point what matters. $\endgroup$
    – Zorg
    Commented May 12, 2021 at 8:46
  • $\begingroup$ Think longer.... Imagine you have initially 2 containers with the same initial analyte vapour pressure. Then you insert there the SF. Both analytes have different KD. Will they have the same final vapour pressure ? $\endgroup$
    – Poutnik
    Commented May 12, 2021 at 8:51


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