Ion- exchange chromatography:

There is another form of chromatography which is synonymous to ion- exchange chromatography and is known as ion-pair (or paired-ion) chromatography. It is a type of reversed phase chromatography and used for the separation of ionic species. In this, the mobile phase consists of an aqueous buffer containing an organic solvent (methanol/ acetonitrile) and an organic compound containing a counter ion of opposite in charge to the analyte. The counter ion merges along with the analyte to form a neutral ion pair. The neutral ion pair is retained on the non- polar stationary phase. It is subsequently eluted with an aqueous solution of methanol or other water soluble organic solvents. Ionic or partial ionic compounds could be chromatographed on reversed phase columns through using ion pairing reagents. Typically these reagents are long-chain alkyl anions or cations. While these reagents are used in dilute solutions, the retention of the analytical ion can increase. C₅ to C₁₀ alkyl sulphonates are commonly used cationic compounds. Tetra alkyl ammonium salts (tetramethyl-, tetrabutyl-, and ammonium salt or triethyl- (C₅ - C₈) alkyl ammonium salts are generally used in the case of anionic solutes.