Thermodynamic Pitfall(s)

[Ashraf_Khan]

Is there any theromdynamic pitfall(s) in gas chromatography ?

[tom jupille]

There is lots of thermodynamics, but what do you mean by "pitfalls"?

[Ashraf_Khan]

Thanks.

The word pitfall(s) speaks for itself. By thermodynamic pitfall(s) I meant whether there is any ambiguity when thermodynamic parameters, such as reference state, equilibrium constant, partition coefficient et al., are applied to chromatographic processes.

[tom jupille]

Sorry, if the word "pitfalls" really spoke for itself, I wouldn't have inquired .

One can understand and predict retention on the basis of system thermodynamics. This is extremely valuable on a qualitative (approximate) level, but becomes problematic if you want to make accurate predictions (there are typically multiple interacting processes at work and there is a slight theoretical complication in that there is no true equilibrium because one of the phases is moving).

Peak broadening , on the other hand, is controlled largely by system kinetics.

[Ashraf_Khan]

Thanks for your input (Sorry, the word pitfalls was ambigious itself). I understand your point regarding the retention values.

Are the 'chromatographic' partition coefficient and the 'thermodynamic' partition coefficient same thing ? The former is based on the solute concentration, whereas the latter based on the solute activity (which takes into account the stoichiometry, intermolecular interactions et al.).

[tom jupille]

Are the 'chromatographic' partition coefficient and the 'thermodynamic' partition coefficient same thing ?

Strictly speaking, the answer is "no". My recollection from thermo class (many years ago) is that activity and concentration approach identity at infinite dilution. That said, the analyte concentration is usually sufficiently low that the "infinite dilution" assumption works well enough for practical purposes (i.e., the distribution isotherms are reasonably linear).

We're approaching the limits of my knowledge here, so if any one else wants to chime in and/or correct me, please jump in!