What kind of pH effects do I have to be aware of with HILIC separations?

The effect of pH on analyte charge state varies based on each compound’s pKa, so pH effects must be evaluated carefully during method development. With HILIC methods, the high concentration of organic solvent in the mobile phase raises the pH, and the actual eluent pH can be 1–1.5 units higher than in the aqueous portion alone. The charge state of the column itself can also be affected. For example, in a Raptor HILIC-Si column, the bare silica has a pKa between 3.8 and 4.5, so the mobile phase pH changes the charge of the silica surface, making it neutral in very acidic conditions and ionized (negatively charged) as the pH begins to approach 3.8 and above. For this reason, if your analyte has one or more protonated amine or quaternary amine groups, it’s a good candidate for analysis on a Raptor HILIC-Si column.