Ion Pair Chromatography on Reversed-Phase Columns

Ion pair chromatography was developed in order to increase the retention of highly water-soluble ionic compounds in reversed-phase columns. This is obtained by adding a low concentration (about 5 mM) of a hydrophobic counterion to the mobile phase. The ion pair formed by the polar ion (Q q) and the counterion (X q) is more hydrophobic than the parent ion, whereby the retention is increased  

For analytes containing a carboxylic group, positively charged ions such as tetra-butylammonium bromide are used. For analytes with an amino group, counterions such as octyl sulfonate or sodium dodecyl sulfate (SDS) can be used. 

Ion pairs created by addition of large hydrophobic counterions have higher retention than the original ion pairs with smaller hydrophobic groups. 

Retention in ion pair chromatography is regulated by the type of counterion, the concentration of counterion, the concentration of organic solvent, and the concentration of salts and pH. 

Higher concentration of counterion increases retention and higher concentration of organic solvent decreases the retention. Larger size of the hydrophobic group increases retention. 

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