HPLC of ionic compounds

1 Ion suppression. By controlling the ionisation of weakly ionic samples they are retained on reversed phase columns for example, a weak acid is retained using a eluent with a pH below the of the acid. But silica-based materials can only normally be used between pH 1.5-7.5 so increased retention for bases cannot usually be obtained in this way. 

2 Ion pairing. Correct choice of eluent pH encourages ionisation of the sample which then pairs to an oppositely charged soap-like counterion dissolved in the eluent, which leads to increased chromatographic retention of the non-ionic pair. The counter ion (e.g. octanesulphonic 

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