ion pair usage

[N.V.V.S.S. RAMAN]

Hello Friends,

Recently I have seen an HPLC method for Sertraline Hcl, the mobile phase contains tetra-n-butyl ammonium hydrogen sulphate and sodium lauryl sulphate in addition to potassium hydrogen sulfate. For what reason they used this ion pair reagents?

When exactly ion pair is to be used in the mobile phase, why? How to know the charge of the molecule?
Please clarify
Thanks and regards
Raman

[Uwe Neue]

I don't know your setraline, but I can tell you how such methods come about. The sodium lauryl sulfate is used to get retention of the basic compound setraline (I assume it's a base because it comes as setraline HCl). On the column that the investigators used, the peak was tailing. To suppress the tailing, they threw in the tetrabutyl ammonium ion.

Now you have a cation pairing reagent and an anion pairing reagent in the mobile phase.

[pipettemonkey]

I don't know your setraline, but I can tell you how such methods come about. The sodium lauryl sulfate is used to get retention of the basic compound setraline (I assume it's a base because it comes as setraline HCl). On the column that the investigators used, the peak was tailing. To suppress the tailing, they threw in the tetrabutyl ammonium ion.

Now you have a cation pairing reagent and an anion pairing reagent in the mobile phase.

Would you say the need for such a reagent depends more on column type or more so on the quality and condition of the column?

[Uwe Neue]

I think the reason for the addition of the tetrabutyl ammonium is due to silanols. Silanols can be very active on a column of the older less pure type of silica, no matter how old or new the column is, or they may become active on a modern high-purity and high-coverage column if the column has aged a lot.

[Ricardinio]

I don't know your setraline, but I can tell you how such methods come about. The sodium lauryl sulfate is used to get retention of the basic compound setraline (I assume it's a base because it comes as setraline HCl). On the column that the investigators used, the peak was tailing. To suppress the tailing, they threw in the tetrabutyl ammonium ion.

Now you have a cation pairing reagent and an anion pairing reagent in the mobile phase.

Can't we expect that in this case the cation pairing reagent and the anion pairing reagent form a neutral pair, thus limiting the interactions between anion pairing reagent and the basic compound?

[Chris Pohl]

Richardinio,

While the two ion pair reagents together don't truly form a neutral ion pair (in the mobile phase at least), the presence of one certainly does largely negate the effect of the other. It's possible, though, if the SDS is in substantial excess for the presence of the TBA to only minimally affect retention of a cationic solute. At the same time, the level of TBA might still be sufficient to mask adverse effects of silanols on retention of a cationic solute.