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amit_chem's picture

on what basis we decide to go for ion pair reagent mobile phase.what is the selection criteria.

Dr. Analytical
Dr. Analytical's picture
Ion pairing chromatography is

Ion pairing chromatography is used to increase the retention of ionized analytes.  A compound that is ionized (i.e., has a charge), is not retained much on a reversed phase system.  The ion pairing reagent is chosen because it has a charge that is opposite from the analyte.

So, if you analyte is A-, you should choose a positively charged IP reagent (IP+).  When these two combine in the column, the neutral ion pair (A-IP+) behaves like a neutral molecule and has more retention.  Also, the IP reagent also coats the surface of the stationary phase, and creates an ion-exchange surface, which also retains the analyte by ion exchange interactions.

Positive IP reagents are usually quaternary ammonium salts (trimethyl octyl ammonium sulfate).

Negative reagents are sodium octyl sulfonate.