Help with chromatographic seperation?

Question

I don't understand the functions of both stationary and mobile phases when it comes to chromatographic separation

I would really appreciate some help on this one

Answer 1

OK, the theory is simple enough.  You assume that the materials being separated have varying affinity for the stationary phase.  You use the mobile phase to push things along.  However if you looked at the "front" of the mobile phase, the only thing that would be there would be a substance that has 0.0 affinity for the stationary phase.

As the mobile phase carries the sample, you get a gazillion instantaneous micro-domains of solvent/substance equilibria.  The rate of solvation and deposition is influenced by the affinity of the dissolved substance for the stationary phase.  The more affinity the substance has for the stationary phase, the longer it will stay attached.

Thus you get a set of sample components that separate from each other based on the attraction they have for the stationary phase.

Sometimes this is purely physical - size-based differentiation because the stationary phase is very small and is slow to pass larger molecules.  Sometimes it is chemical, caused by (perhaps) electron cloud interactions slowing down the re-dissolving of the substance into the mobile phase.  There are reasons why you choose a particular solvent and mobile phase based on your "guess" as to the presence of some differential affinity for the sample components.  That choice is more art than science sometimes.