Reversed phase ion-pair RPIP chromatography

RPIP chromatography uses a hydrocarbonaceous stationary phase and either an aqueous or aqueous-organic mobile phase which also contains the counter-ion. The stationary phase is usually an octadecyl bonded phase and the mobile phase is usually an aqueous buffer with either methanol or acetonitrile as an organic modifier. The choice of counter-ions depends on the solutes to be separated, but generally for the separation of acids a hydrophobic organic base is added to the mobile phase, while for the separation of bases a hydrophobic organic acid is added. Separations of other compounds are similarly obtained by the addition of an appropriate counter-ion. 

The advantages of this technique over other ion-pair techniques 

In this section the proposed mechanisms of the chromatographic process will be discussed together with an outUne of the influence of the parameters which may be varied on a chromatographic separation. 

The first model proposed an ion-exchange mechanism where the free Upophilic charged counter-ion is adsorbed onto the non-polar surface of the stationary phase to constitute an ion-exchange surface. The sample ions of opposite charge to the counter-ions are then partitioned between the stationary phase and the mobile phase by an ion-exchange process. If the concentration or lipophiUcity of the counter-ion is increased, the surface coverage of the stationary phase by counter-ion will be increased. 

The third mechanism, proposed by Melander and Horvath (1980b), is that of dynamic complex exchange where counter-ions are present both in the mobile phase and at the surface of the stationary phase. Sample molecules which have formed an ion-pair with the counter-ion in the mobile phase can then transfer to the bound counter-ions to form a complex at the surface of the stationary phase. A modification of this process envisages that the complex is initially formed at the stationary phase surface. 

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