In HPLC, what does the term reverse phase indicate about the mobile phase?

In High-Performance Liquid Chromatography (HPLC), the term reverse phase refers to the nature of the interaction between the mobile phase and stationary phase. Specifically, it indicates that the mobile phase is more polar than the stationary phase. In reverse phase chromatography, the stationary phase is typically non-polar or hydrophobic, such as octadecylsilane (C18), while the mobile phase consists of a polar solvent, like water mixed with organic solvents (e.g., methanol or acetonitrile).

This configuration allows for the separation of analytes based on their polarity, where the more polar compounds will have stronger interactions with the polar mobile phase and will elute faster, while less polar compounds interact more with the non-polar stationary phase, leading to longer retention times. This principle makes reverse phase HPLC a powerful technique for separating a wide range of substances, particularly those that are polar or semi-polar.