Explanation on Chromatography?

[Lepton]

I've been studying analytical chemistry for the past few days, and I don't really understand the basis on how general chromatography works. I think I've worked up a basic understanding of it, but I'm not sure if I'm completely right or not. Assume that this is column chromatography and the sample is a solvent.

 

From what I understand

It's a way to separate compounds by using a mobile phase and a stationary phase. The mobile phase is what the compound is dissolved into. Once the mobile phase has the compound, it carries it through the column, which has a thin layering of the stationary phase around it. As the mobile phase passes through the column, parts of the compound are adsorbed onto the stationary phase, effectively separating it.

 

Is this correct?

[hypervalent_iodine]

I've been studying analytical chemistry for the past few days, and I don't really understand the basis on how general chromatography works. I think I've worked up a basic understanding of it, but I'm not sure if I'm completely right or not. Assume that this is column chromatography and the sample is a solvent.

 

From what I understand

It's a way to separate compounds by using a mobile phase and a stationary phase. The mobile phase is what the compound is dissolved into. Once the mobile phase has the compound, it carries it through the column, which has a thin layering of the stationary phase around it. As the mobile phase passes through the column, parts of the compound are adsorbed onto the stationary phase, effectively separating it.

 

Is this correct?

 

Not really. The silica on the TLC plate or in a silica flash column forms a matrix in which things can bind reversibly. As you pass through a solution of a compound or compounds in solvent, you get competitive binding into these pockets, with the more polar compounds binding better than less / non-polar compounds. You end up with a situation where non-polar compounds move further up the plate than those that are polar, as the polar compounds are constantly getting stuck (for lack of a better word) to the silica as they move. This is a good overview of the concepts.

[CharonY]

One should probably add that not all chromatographic interactions are based on polarity. The general principle is that the analytes are retained by the stationary phase (which can be due to ionic interactions, hydrophobic interaction, size etc.). One can model that by modeling it as a series of discrete equilibrium processes in which the analyte is either in in the mobile phase or at the stationary phase. I.e. one could imagine it as a series of partitioning processes in which a given concentration of the analyte is distributed between the two phases. The equilibrium constant of this interaction, called partition coefficient in this case, is a measure of how well an analyte gets retained.

 

In chromatographic columns (as opposed to TLC) the constant flow of mobile phase will eventually elute the analyte from the column, and the time necessary (retention time) for a given analyte to come off the column depends on the partition coefficient of an analyte for a given phase mixture.