Ion pairing reagent interactions

[Ricardinio]

I used to think that the ion pair reagent would form a pair with the ionic analyte. The neutral pair would therefore result in better interaction with the apolar stationary phase. However, by reading previous posts I have the impression that the ion pair reagent rather interact direclty with the free silanol of stationary phase.
I would like to have some clarifications or comment about this.
In addition is there any difference in the action of pairing reagent in its acidic form (e.g octane sulphonic acid) compared to the salt? Does the pH have an effect on the efficiency of the pairing reagent?

[Chris Pohl]

Richardinio,

In the first place, Bidlingmeyer published an article that proved that neutral ion pairs are not present under normal conditions in the mobile phase since this requires a mobile phase dielectric constant below that observed with common HPLC solvent systems.

However, it is not correct to say that the ion pair reagent molecule is retained via silanol interaction, in general, although cationic reagents may be partially retained in this manner. There are two models for retention:

1). The ion pair reagent absorbs via hydrophobic retention to the reversed phase surface. The surface acquires a net charge and the retention process is roughly like ion exchange without the normal spatial selectivity for multivalent ions. This mode tends to dominate with lipophilic reagents and/or low solvent mobile phases. Retention can be controlled via ionic strength or solvent content (or choice of lipophilicity of the ion pair reagent).

2). An ion pair forms in the low dielectric environment of the stationary phase surface as the analyte approaches the surface. This mode tends to dominate with less lipophilic reagents and/or high solvent mobile phases. This makes the process roughly as you first described it with retention control mainly via solvent content (or choice of lipophilicity of the ion pair reagent).

In ether case, there is some “ion exchangeâ€

[Ricardinio]

Thank you Chris for ur comments.
Thus I have 2 questions:
1) How to explain the decrease of analysis time while using ion pair reagents such as tetrabutylammonium sulphate? I observed this with the analysis basic compounds.
2) TFA is also used as pairing agent. Do ur comment apply also to this pairing agent?

[Chris Pohl]

Ricardinio,

The reason that addition of TBA reduces retention of basic solutes is electrostatic repulsion induced via the acquired net positive charge of the surface. Since basic solutes will tend to have at least a partial positive charge, the result is reduced retention.

TFA (in my opinion, at least) is a borderline ion pair reagent. It mainly works in the second mode mentioned above.

[Ricardinio]

Chris,
With regard to what has been mentioned above, how come TBA is used to improve selectivity and peak shape?

[Chris Pohl]

In addition to enhancing the retention of anionic species and diminishing the retention of cationic species, TBA will also have the effect of diminishing the influence of silanols on peak shape of basic solutes. This effect is more or less analogous to what is observed when adding triethylamine to the eluent.

TBA can also improve selectivity by virtue of the opposing effects of anionic and cationic species while leaving the retention of neutral solutes relatively unchanged. For example, if you happen to have a resolution problem for a pair of analytes where one analyte is anionic and one is either neutral or cationic, you can generally achieve a better separation through the addition of TBA.