Understanding the possible HILIC retention mechanisms involved for a particular column-solvent-analyte system is crucial in the process of selecting the right HILIC stationary phase, leading to a successful application.
HILIC can be described as a variation of reversed phase chromatography performed using a polar stationary phase. The mobile phase employed is highly organic in nature (> 60-70% solvent, typically acetonitrile) containing a small percentage of aqueous solvent/buffer or other polar solvent. The water in the mobile phase forms an aqueous-rich layer adsorbed to the polar surface of the stationary phase into which analytes partition.
The primary advantage of HILIC as a separation technique is the strong retention of polar, hydrophilic compounds that are unretained under conventional reversed phase conditions, without the requirement for an ion-pair additive in the mobile phase.
Evidence suggests that the retention mechanism in HILIC is complex— a combination of hydrophilic partitioning, hydrogen bonding, electrostatic interactions, and van der Waals interactions.
HILIC enables retention of hydrophilic compounds that are difficult to retain by reversed phase liquid chromatography (RPLC). You can also achieve better separation efficiency than RPLC for very polar compounds under highly aqueous conditions.