Reversed-phase liquid chromatography of ionic compounds

4 Reversed-phase liquid chromatography of ionic compounds 

Two basic approaches have been described to enhance the affinity of ionic compounds for the stationary phase in reversed-phase systems. The first approach, sometimes inappropriately referred to as ionic suppression involves the adjustment of the pH mobile phase so that the compound of interest is in the neutral or unionized form. The second approach to enhance the retention of ionic compounds on reversed-phase supports involves the application of ion-pair extraction techniques. 

The retention of a weak acid in reversed-phase systems can be related to the fractions of the neutral, HA, and conjugate base, A, forms present in the mobile phase (Horvath et al., 1977a Foley, 1987), according to 

Substituting for the dissociation constants into equation (3.17) gives equation (3.20), which relates retention to the hydrogen ion concentration  

The appropriate expressions for the retention of weak bases and amphoteric substances are given in equations (3.21) and (3.22), respectively. 

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