Chromatography Forum

Maybe a stupid question but I'm a little bit confused about RP liquid chromatographty. I understand why polar components have less retention than non-polar components in reversed phase. I also understand that you can reduce the retention time of non-polar components by adding more organic modifier. But why do the retention times of polar components also decrease when you add more organic modifier, or the other way around: why are polar components more retained if you add more water? I would expect the polar components to elute even earlier when you add more water to the mobile fase as water is more polar than e.g. acetonitrile

"polar" is a relative term. To grossly oversimplify:

Arguably, the greatest contribution to retention in reversed-phase is hydrophobicity: the hydrogen bonding between water molecules tending to pull them back together and "extrude" the solute onto the stationary phase. Very "non-polar" (aka "hydrophobic", aka "lipophilic", aka "greasy") compounds are more susceptible to this -- and therefore strongly retained -- because they don't hydrogen bond very well themselves. Very "polar" (aka "hydrophilic") compounds are less susceptible -- and therefore more weakly retained -- because they can, to some extent, hydrogen bond with the water.

There is no such thing as a "polar" compound in an absolute sense; it's all a matter of degree.

Retention is effected by the stationary phase, if a polar substance still has a slight "affinity" for this stationary phase it can be retained unless it is completely displaced by an organic component in the mobile phase which may completely dominate the adsorption on the stationary phase via sheer numbers and/or high affinity. Maybe it helps to write some equilibrium equations of analyte distribution between the phases.

Tom, if one looks at dipole moments of molecules then there is certainly absolute or no polarity according to the molecule. The whole thing is complicated by the polarizability of some molecules.

Thank you for the interesting replies! I think I understand now