High-performance liquid chromatography(HPLC), formerly referred to as High-pressure liquid chromatography, is a powerful technique used to separate, identify, and quantify components in complex mixtures. The term "high pressure" refers to using high pressure to push the liquid mobile phase through the tightly packed columns.

In HPLC, two phases play a critical role in the separation process:

1. Mobile Phase: This is a liquid solvent that flows through the system, carrying the sample with it. The solvent is chosen based on its polarity to optimize the interaction with the analytes and the stationary phase. The "polarity index" serves as a guide to selecting the suitable solvent for separation.
2. Stationary Phase: The stationary phase is a solid material, often silica-based, packed within the column. In reverse-phase HPLC, the stationary phase is typically derivatized silica, such as octadecyl (C18) silica. The small particle size of the packed material creates a high surface area, enhancing interactions with the analytes. To reduce undesirable interactions, such as those with unreacted silanol groups, the stationary phase is often end-capped with reagents like trimethylchlorosilane

Separation occurs as the analytes interact differently with both the mobile and stationary phases, depending on their chemical properties, which leads to differential migration through the column.

Sometimes, solute molecules bind irreversibly to the stationary phase, clogging the analytical column and decreasing its performance. To prevent this, a guard column is placed before the analytical column. A precolumn called a scavenger column may also be placed between the mobile phase reservoir and the injector. This ensures that the solvent enters the analytical column, which is already saturated with silica from the scavenger column, preventing unwanted reactions and extending the lifetime of the analytical column.

After the separation, analytes are detected by various detectors, depending on the characteristics of the compounds. The results are visualized as chromatograms, with peaks corresponding to the different components of the mixture. This provides both qualitative and quantitative data about the sample.