ChromFAQ:IonPair
From ChromFAQ
What is ion-pair chromatography?
In the most general sense, ion-pair chromatography refers to the addition of an ionic surfactant to a reversed-phase system in order to affect retention and/or selectivity of ionic compounds.
The name comes from one model (perhaps "mental picture" is more accurate) of the way the technique works. Retention in reversed-phase chromatography is based largely on hydrophobicity: the more hydrophobic or non-polar the analyte, the more strongly it will be retained. Ionic compounds tend to be quite hydrophilic, and so reversed-phase retention can be problematic. When a surfactant of opposite charge to the analyte is added to the mobile phase, analyte and surfactant molecules form an "ion pair" which is, overall, hydrophobic and therefore more strongly retained than the original analyte. The process is illustrated at the right: analyte and surfactant interact in solution and the resulting ion-pair interacts with (sticks to) the stationary phase
Although this is an appealingly clear picture of the process, the experimental evidence for the actual formation of ion pairs is slim.
The most common alternative explanation is the "in-situ ion exchange model".
This mental picture has the surfactant molecules adsorbed onto the surface of the stationary phase in such a fashion that the charged heads are exposed to the aqueous mobile phase. In effect, an ion exchanger is dynamically created. The process is illustrated at the right. The surfactant (ion-pair reagent) sticks to the stationary phase. Analyte ions of opposite charge then stick to the head of the surfactant.
Unlike conventional ion exchange stationary phases however, in which the ion exchange capacity (the concentration of charged groups per unit surface area or per unit column volume) is fixed, ion-pair chromatography allows the capacity to be controlled. The good news is a substantial increase in the flexibility of the technique. The bad news is a substantial increase in complexity.
In principle, any ionic surfactant can be used as an ion-pair reagent. In practice, the most commonly used reagents for base (cation) separations are the alkyl sulfonates (e.g., pentane sulfonate, hexane sulfonate, octane sulfonate), and the most commonly used reagent for acid (anion) separations tetrabutyl ammonium ion. There is nothing "magic" about these particular reagents; they were used early on and seemed to work. The specific identity of the ion pair reagent has only a weak effect on the separation in any case.
"Ion pair" chromatography is not an "all or nothing" proposition. Many ions added to the mobile phase for other purposes can act as weak ion-pair reagents if they have some affinity for the stationary phase. Trifluoroacetic acid (TFA) and heptafluorobutyric acid (HFBA), for example, has been shown to act as ion-pair reagents in the chromatography of peptides, as has triethylamine (TEA) when used with acidic analytes.
A more detailed discussion of ion-pair chromatography with inputs from Bill Tindall, Chris Pohl, Uwe Neue, and Hans Mueller (among others) can be found in this thread (http://www.sepsci.com/chromforum/viewtopic.php?t=1721) on Chromatography Forum (http://www.chromforum.com).
