Characterization of monoclonal antibodies (mAbs) and related products requires the identification of chromatographic peaks with mass spectrometry (MS). However, the conventional salt- and pH-gradient elution techniques used in ion-exchange chromatography (IEX) are inherently incompatible with MS. Ammonium acetate- and ammonium carbonate-based mobile phase systems have been recently applied in IEX-MS, but the influence of the eluent composition on peak shape and retention has not been discussed nor studied systematically until now. The aim of the present study was to understand the impact of ionic strength, buffer capacity, and pH-response on the retention behaviour and peak shape of mAb species.