please, is there anybody that can explain me where and why is convenient use this method ?
Thanks

If you come from a reversed-phase background, the short answer is to provide retention for ionic compounds that are too hydrophilic to stick to a reversed-phase column.

If you come from an ion-exchange background, the short answer it that it allows you to control the ion-exchange capacity of the column by adjusting the mobile phase.

A good longer answer can be found in Section 7.4 of the Snyder, Glajch, & Kirkland book "Practical HPLC Method Development" (1997), published by Wiley.

-- Tom Jupille

I would add, use it when all other means of retaining/separating the analyte fail, for example supression of ionization by pH control, using ion exchange or ion exclusion, running in 100% aqueous with a high retention column. The additives are expensive, particularly the high purity versions, and it is one more variable to worry about. From all the references to ion paring in this forum it is obviously used successfully by many, and many others encounter problems. I personally find it a nuissance to be avoided when possible. The availability of high retention columns that operate successfully in 100% water has lessened, but not eliminated, the need for this approach to achieving retention. It does provide some powerful means for varying selectivity as you can vary both the ionization of the analyte by pH and the ion pair equilibria by concentration and structure of ion pair reagent.