What is the meaning of ion suppression mode in chromatography?

The term "ion suppression" in HPLC refers to operating under conditions where an ionizable species is rendered nonionic via adjusting the pH so that ionization is suppressed and then performing the separation via reversed phase. With most reversed phased silica columns, this is generally not easily done with basic compounds since the required pH is often too high to be practical for rugged methods although there are a few columns which vendors claim to be useful at pH 11. For that reason, ion suppression for the most part refers to operating at low pH in order to suppress the ionization of weakly acidic compounds containing, for example, carboxylic acid or phenolic functional groups.