Cationic EHILIC
For example, one could use a cation exchange (negatively charged) surface chemistry for ERLIC separations to reduce the influence on retention of anionic (negatively charged) groups (the phosphates of nucleotides or of phosphonyl antibiotic mixtures; or sialic acid groups of modified carbohydrates) to now allow separation based more on the basic and/or neutral functional groups of these molecules. Modifying the polarity of a weakly ionic group (e.g. carboxyl) on the surface is easily accomplished by adjusting the pH to be within two pH units of that group's pKa. For strongly ionic functional groups of the surface (i.e. sulfates or phosphates) one could instead use a lower amount of buffer so the residual charge is not completely ion paired. An example of this would be the use of a 12.5mM (rather than the recommended >20mM buffer), pH 9.2 mobile phase on a polymeric, zwitterionic, betaine-sulfonate surface to separate phosphonyl antibiotic mixtures (each containing a phosphate group). This enhances the influence of the column's sulfonic acid functional groups of its surface chemistry over its, slightly diminished (by pH), quaternary amine. Commensurate with this, these analytes will show a reduced retention on the column eluting earlier, and in higher amounts of organic solvent, than if a neutral polar HILIC surface were used. This also increases their detection sensitivity by negative ion mass spectrometry.
Read more about this topic: Hydrophilic Interaction Chromatography