A stationary phase in any given chromatographic system can be viewed as the core of the separation process. This is mainly because elution or retention of substances of an injected mixture depends on the properties of the stationary phase, in relation to those of the mobile phase. Stationary-phase properties can be studied in different ways. Characterizing stationary phases by retention profiling (Section requires the mobile phase to be kept constant. When working with different mobile-phase compositions, it is no longer correct to say that one is characterizing stationary phases, but rather the characterization of chromatographic systems is taking place. Determining the physicochemical properties of stationary phases, on the other hand, requires the use of different mobile phases (Section