H
ydrophilic Interaction Liquid Chromatography or HILIC is normal phase (NP) chromatography of polar and ionic compounds under reversed phase (RP) conditions. The main separation mechanism is caused by an aqueous layer built on the stationary phase and partitioning of the analytes between the very polar stationary phase and the less polar mobile phase.
This leads to the retardation of polar and hydrophilic compounds like Uracil for example, which is used as a non retarded dead time marker in reversed phase HPLC. Unpolar compounds like Toluol for example are not retarded and can be used as a dead time marker in HILIC. This leads to HILIC having an elution order that is often inverse compared to reversed phase separations.