Ion-exchange chromatography
A type of chromatography that has charged stationary phase that is used to separate opposite-charged analytes. If the stationary phase is positively charged, it would retain negatively-charged analytes. Salt in the mobile phase acts as competing agent, thus an increasing salt gradient is used to elute out analytes. The weakly bound analytes to the stationary phase would come out first; strongly retain analytes would come out later.
If the stationary phase is positively charge and it retains negatively charged analytes, it is called anion-exchange chromatography. Negatively-charged analytes are "exchanged" by anions from the salt in the solvent. In short, both negatively-charged analytes and anions from the salt compete for the positively-charged binding sites on the stationary phase.
For cation-exchange chromatography, the opposite is true. Positively-charged analytes are retained by the negatively-charged stationary phase. Positively-charged analytes compete with cations from the salt for the negatively-charged binding sites on the stationary phase.