Effect of buffer cations

[Malte]

Hi,
I would be happy to learn something about the effect of different buffer cations in the mobile phase on the characteristics of RP-HPLC (C18)-runs. Bckground: I want to do prep- RP-HPLC of a very short RNA-oligo (3 bases), and for the precipitation afterwards, I need the buffer contain lithium instead of sodium or TEAA. Elution will be done in approx. 20% ACN.
Thank you

[Chris Pohl]

Malte,

For neutral compounds there is only a very minor effect of the cation on retention. However, for anionic species such as the compounds you mention, the cation may have a significant effect. In general, the most hydrated cations such as lithium will produce the most "hydrophilic salt" and hence induce the lowest level of reversed phase retention. Poorly hydrated cations such as cesium will produce a relatively "hydrophobic salt" resulting in increased retention. I would expect you to see somewhat less retention for lithium salts of your RNA-oligos but I doubt it will be a very dramatic effect unless you're separation is isocratic. You should be able to compensate easily by just decreasing the amount of solvent.