[curiousgeorge1234]

what does relative affinity mean?

[curiousgeorge1234]

To clarify, in chromatography when asked what the relative affinity is to the resin - I don't understand what they are looking for.

[mike]

Do you know any theory on how chromatography works? Let me know what you know first so I can explain this one.

[curiousgeorge1234]

I'm a bit confused on how chromatography works.  Here's what I 'think' happens.

You have a tube (ion-exchange) with a resin in it.  You put a solution in the tube (ie. cation exchange column). The positive ions adsorb to the resin taking the place of the H+ ions in the resin, and the H+ ions are eluted (come out of the column).  The ratio of the eluted H+ and the positive ions adsorbed to the resin is 1:1.

That's what I think happens, but then I get confused.  If you're separating ions, ie. a K+, then if the H+ ions come out of the column how are you separating the K+ ion?  Meaning, how do you test the solution that comes from the column to have a separated K+ ion? 

Also, does this mean that there is a relative affinity for the K+ (I'm just using this ion to make explanation simpler) to the resin?  I read relative affinity as 'relative attraction'.  Because the K+ adsorbs to the resin there is an attraction, thus a relative affinity?

I'm not getting the theory entirely so I'm a bit lost.  Thanks for any help you can give. 

[mike]

In general chromatography separates compounds by their "relative affinity" to the column. This means that due to the different properties of different compounds they will interact with the column differently. In other words compound A has different properties to compound B and therefore compund A will intereact differently to the column relative to compound B.

Chromatography generally involves a stationary phase and a mobile phase. The stationary phase could be a solid (like silica or alumina for example) and the mobile phase could be a liquid or a gas for example. Compound A and compound B will be carried by the mobile phase through the stationary phase. If compound A interacts strongly with the stationary phase (for example it binds strongly to an active site on the solid) it will travel slowly through the column relative to compound B which may not interact as strongly.

It is important to remember that there is a dynamic process occuring in that compound A and B are continuously intereacting with the stationary phase and the mobile phase (to put it another way the compounds may be attaching and unattaching from the solid phase all the time). The longer a compound spends on the stationary phase the slower it will travel through the column. The longer it spends in the mobile phase the quicker it will travel through the column.

Back to you question about relative affinity. Compound A and B are only going to separate if there properties on that particular column are different enough that one intereacts with the stationary phase more often than the other. In other words one compound has more of an affinity for the stationary phase relative to the other compound.