Chromatography Forum

Hello all,


I had a general question about quantitating diastereomers and the EE ratio by LC/MS-MS. The main question is this:

Can the diastereomeric ratio for a chemical be determined in LC/MS-MS based on the relative response of the diastereomers, without tuning each compound beforehand and generating a standard curve? I'm not interested in absolute amounts, just a relative ratio.

I suppose my initial guess is "It depends"

If I had a compound that had two adjacent enantiomeric carbons, and one had an electron withdrawing group and one had an electron donating group, then the orientation about these would make a large difference, since you would be changing the electronic nature of the molecule.

But if you had a compound that had two enantiomeric carbons that were far removed from each other and had, say, alkyl groups on them, I would think that the electronic nature of these and the ionization efficiency would be nearly the same for all of the molecules.

Has anyone had any experience with this or come across any journal articles related to this? I'm not able to find any so any comments on this would be greatly appreciated. Thanks,

Your predication is reasonable, that's why we have a method called "pesudo isotope dilution". But keep in mind that ESI is not stable as EI, so if you want confiendence in the "ratio", you still need make sure their responses/concentration are close.