Principle involved in Reverse phase chromatography

[saipraveen7]

Dear all,
In Reverse phase chromatography the silica in the column is bonded with C18 which is asolid. Why are we then calling it as partition while terming the principle involved in the separation. How the analyte molecules partitions between stationary phase and mobile phase?

[M Farooq]

Dear all,
In Reverse phase chromatography the silica in the column is bonded with C18 which is asolid. Why are we then calling it as partition while terming the principle involved in the separation. How the analyte molecules partitions between stationary phase and mobile phase?

The term partition in chromatography comes from the older terminology when silica particles were coated with liquid adsorbents which were not soluble in the mobile phase. Partitioning was the easiest idea to explain the retention of a molecule. Imagine that you injected 100 molecules into the column. Say, 75 of them "stick or partition" to the liquid stationary phase and 25 remain in the mobile phase.
Modern chromatography is a surface science which makes it even more difficult to judge what is happening in reality. Imagine a silica surface onto which you have plenty of brush like C18 chains extending from the surface. There are plenty of interactions which retard the movement of a molecule in a column. See "Hydrophobic Subtraction Model" by Snyder and others.

[lmh]

I think partition is still a helpful term. It points at the mechanism: the analyte can be "stuck to" the mobile phase or the stationary phase. If it spends, on average, 30% of its time stuck to the mobile phase and 70% to the stationary, it moves at 30% of the linear velocity of the solvent. It may not be partitioning in the strict sense of physically being confined to one of two separate phases, as happens in a two-phase partition in a glass flask, but it is still partitioning in the sense of "spending time associated with one of two phases".

What the physical nature of the association actually looks like, in the column, is a different matter, and probably pretty complex. One can worry too much. Knowing the ways in which it can interact (hydrophobic, ionic etc.) is enough to make some educated conclusions about how to modify a method.

I don't think this is unique to reverse phase, or affected by the fact that the C18 chains are bonded to silica. This doesn't make the material any less stationary. The interaction of a pancake with Teflon in a non-stick frying-pan is unaffected by whether the frying-pan underneath is made of aluminium, copper or steel, and when we toss the pancake we hope for 100% partitioning into air. If the non-stick surface isn't continuous because someone's scratched the pan, then we can get interactions, and a percentage of pancakes will fail to partition correctly! So the partitioning model still holds, it's just that there can be multiple modes of interaction between solid-phase and analyte, and the solid-phase can consist of multiple sorts of surface, all bonded together as one solid unit.