[bashert]

I’m working on the separation of two polar compounds. I’ve been reading up on reversed phase chromatography and just trying to understand one aspect. In an online guide from Waters I read (https://www.waters.com/webassets/cms/library/docs/wa20756.pdf):

“To retain polar compounds on this non polar surface we reduce the amount of organic in the mobile phase (i.e., 100% water).”

I’m confused because by doing that you’re making the mobile phase even more polar….wouldn’t that make it a better solvent for a polar molecule!? Then why would that cause a polar molecule to bind the nonpolar stationary phase?? What am I missing?

Thanks!

[Babcock_Hall]

One way to approach this question is to think of chromatography as a competition between the solvent and the analyte for binding to the stationary phase.  In reverse phase chromatography water (being the most polar commonly used solvent) is less able to compete against the analyte than acetonitrile is.  Therefore, the analyte stays stuck to the stationary phase most of the time.  I realize that this model ignores the interaction between the solvent and the analyte, but it works for many situations in both reverse phase and normal phase chromatography, although I can think of one situation that it does not explain well.  If someone with more knowledge of chromatographic theory wanted to offer further thoughts, that would be great.