The mobile phase in ion exchange chromatography of proteins consists of a buffer, salt and other additives such as preservatives. Initially a low salt mobile phase is used to allow the proteins to bind to the column. Then the salt concentration is increased to displace the proteins from the stationary phase and elute them from the column.
Mobile phase strength refers to the ability of the mobile phase to displace a bound protein from the stationary phase. In ion exchange chromatography, the mobile phase strength is increased by adding more salt.