concentration effect

[Paul]

I apologize for this basic question but could you explain me the influence of the concentration of the mobile phase on the purification process (Reverse phase preparative purification), for instance AcOH 0.5% and AcOH 1%. In which cases, a change of the concentration in the mobile phase is susceptible to improve the efficiency of the reverse phase purification?
Thanks
Paul

[Uwe Neue]

The question is a little vague and unclear to me, but I will try anyway.

You are doing purifications using reversed-phase separations with two concentrations of acetic acid. Acetic acid is there to stabilize the pH. If your sample has a different pH or is an ionizable analyte in a neutral form, you can get strange peaks shapes that change with the concentration of the acetic acid. The higher the concentration, the better the peak shape.

If this is what you observe, the issue is a difference in the ionization of the analyte in the sample matrix and in the mobile phase.

If this is not the answer that you were after, you need to be more clear with your question.