HPLC is a dynamic adsorption process. Analyte molecules, while moving through the porous packing beads, tend to interact with the surface adsorption sites. Depending on the HPLC mode, the different types of the adsorption forces may be included in the retention process: Hydrophobic (non-specific) interactions are the main ones in reversed-phase (RP) separations. Dipole-dipole (polar) interactions are dominant in normal phase (NP) All these interactions are competitive. Analyte molecules are competing with the eluent molecules for the adsorption sites. So, the stronger analyte molecules interact with the surface. The weaker the eluent interaction, the longer the analyte will be retained on the surface. The resolution of peak in HPLC not only depends on chemical parameter of drug & mobile phase but also on instrumental parameter. Some extra or unexpected peaks are seen during the resolution which can be overcome by changing the condition or changing parameter