Comparison of peak shape in hydrophilic interaction chromatography using acidic salt buffers and simple acid solutions

• The effect of salt buffer on performance of four different HILIC columns was studied.
• Ammonium formate gave improved results compared with formic or phosphoric acid.
• Peak shapes of ionogenic solutes were extremely poor in formic acid.
• Ammonium salts may stabilise the water layer as well as providing competing cations.
• Cation exchange occurs on all columns (including hydride) even at low pH.

The retention and peak shape of neutral, basic and acidic solutes was studied on hydrophilic interaction chromatography (HILIC) stationary phases that showed both strong and weak ionic retention characteristics, using aqueous–acetonitrile mobile phases containing either formic acid (FA), ammonium formate (AF) or phosphoric acid (PA). The effect of organic solvent concentration on the results was also studied. Peak shape was good for neutrals under most mobile phase conditions. However, peak shapes for ionised solutes, particularly for basic compounds, were considerably worse in FA than AF. Even neutral compounds showed deterioration in performance with FA when the mobile phase water concentration was reduced. The poor performance in FA cannot be entirely attributed to the negative impact of ionic retention on ionised silanols on the underlying silica base materials, as results using PA at lower pH (where their ionisation is suppressed) were inferior to those in AF. Besides the moderating influence of the salt cation on ionic retention, it is likely that salt buffers improve peak shape due to the increased ionic strength of the mobile phase and its impact on the formation of the water layer on the column surface.