Retention mechanisms and mobile phase effects in ion-exchange chromatography

2 Retention mechanisms and mobile phase effects in ion-exchange chromatography 

The competition between ions in the mobile phase and adsorbed counter ions on the stationary phase is given by equations (3.32) and (3.33) for anion exchangers and cation exchangers, respectively  

In follows from equations (3.32) and (3.33) that the ion-exchange constant (Xex) for monovalent ions, A and B is given by (Poole and Poole, 1991)  

The ion-exchange constant also determines the elution order for a series of ionic solutes and it may be related to the capacity ratio of an analyte ion, A, in the presence of a counter ion, B. Equation (3.34) may be rearranged to give equation (3.35), which relates the capacity ratio of A to the ion exchange constant for that pair of ions and the concentrations of the counter ion, B, in the mobile phase and the stationary phase 

Assuming [B]s [A]s, the concentration of counter ions in the stationary phase is equal to the concentration of ion-exchange sites in the stationary phase, Q. Q is defined as the ion-exchange capacity of the stationary phase  

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