Chromatography Forum

Through previous posts I learned that the pH at which a pairing reagent is used matters. I wonder if this is true for all pairing reagents.

The pH counts primarily with repect to the charge of your analytes. If your analytes are not charged, the can't interact with the ion-pair reagent.
The TBA will be charged all the time. However, alkylsulfonates can loose their charge if the pH is too acidic.

I think the main concern about the pH is to get your analytes charged up

I understend that TBA will be charged at any pH, but that this is not true for alkylsulfonates.
"The main concern is about the analyte". But what will be the efficiency of alkylsulfonate at low pH for a basic compound which is protonated at that pH?

If your pH is low enough that the alkylsulfonate is protonated, it won't ion-pair any more.

The pKa for alkyl sulfonates are very low. Methanesulfonic acid is -2 or thereabouts. It is not so much a case of protonating the alkyl sulfonate as it is plain ion exchange competition by the high ionic strength of the acid.

Mark, what do you mean by high ionic strength here? MeSO4- is strongly hydrated thus has low selectivity?
Probably because of its relatively large benzene ring, benzene sulfonate is very high in selectivity (according to Bio-Rad on their AG 1 material), where does MeSO4- come in here? Does anybody have some related data on trifluoromethylsulfonate, which I am just trying to remove from a reaction solution (via ion exchange)?

Hans,

It depends a bit on what kind of anion exchange material you are working with but the selectivity is generally similar to bromide or nitrate for most styrenic anion exchange materials. If you start with an anion exchanger in the acetate form, this should pretty efficiently remove triflic acid.

Thanks Chris.
Methyl sulfonate has a bit higher selectivity (less hydrated?)?
The problem is a bit complex, the IX material I have at disposal presently is AG 1-X8. One portion of the task is that I am trying to take amino acids (cis & trans-fluoroproline) out of their prep reaction medium (solvent: butylmethylimmidazolium triflate, bmimOTf). If I don´t lick this soon, maybe it is best to present the details in a new chain.

Thanks Chris.
Methyl sulfonate has a bit higher selectivity (less hydrated?)?
The problem is a bit complex, the IX material I have at disposal presently is AG 1-X8. One portion of the task is that I am trying to take amino acids (cis & trans-fluoroproline) out of their prep reaction medium (solvent: butylmethylimmidazolium triflate, bmimOTf). If I don´t lick this soon, maybe it is best to present the details in a new chain.

I was using methane sulfonic acid as an example of a an alkylsulfonic acid; the pKa of octane sulfonic is not very different. In order to significantly protonate an acid with a pKA < 0 you would need [H+] > 1M. That is a very high ionic strength of protons. Ion exchange competition between protons and the base for the limited number of ion-pairing agents would take place well before protonation of the dissociated acid.

In order to actually demonstrate this you would need a polymeric RP column. A silica column would lose its bonding under such extreme conditions.