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