Ion-Pair Chromatography MPIC

Ion-pair chromatography provides a useful alternative to ion-exchange chromatography. The selectivity of the separation in ion-pair chromatography is mainly determined by the make-up of the mobile phase, thus both anionic and cationic compounds can be separated. This universal applicability has helped ion-pair chromatography reach its present significance. 

Haney et al. [1, 2], Waters Associates [3], and Knox et al. [4, 5] - to name just a few — found that by adding lipophilic ions such as alkanesulfonic acid or quaternary ammonium compormds to the mobile phase, solute ions of opposite charge can be separated on a chemically bonded reversed phase. The term Re-versed-Phase Ion Pair Chromatography (RPIPC) has generally been adopted for this technique. The term Mobile Phase Ion Chromatography (MPIC) describes a method which combines the major elements of RPIPC with suppressed conductivity detection, previously described. Besides the above-mentioned chemically bonded reversed phases, neutral divinylbenzene resins featuring a high surface area and a weakly polar character are also used as stationary phases. 

Handbook of Ion Chromatography, Third, Completely Revised and Enlarged Edition. Joachim Weiss Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-28701-9 

The physicochemical phenomena of RPIPC, the basis for the retention mechanism, are still not fully understood. This mechanistic uncertainty is reflected by the many terms proposed for this kind of separation method in the past Here, we discuss two hypotheses, although both of them lack an unequivocal experimental basis. Horvath et al. [6,7] take the view that solute ions form neutral ion pairs with the hpophilic ions in the aqueous mobile phase. These neutral ion pairs are retained at the nonpolar stationary phase. In contrast, Huber et al, Hoffmann and Liao, and Kissinger [8-10] support the ion-exchange model, where the hpophihc reagent first adsorbs at the surface of the stationary phase. 

Handbook of Ion Chromatography, Fourth Edition. Joachim Weiss. 

---