In the last twenty years hydrophilic interaction liquid chromatography (HILIC) has emerged as a new type of liquid chromatography for separation of polar compounds. In HILIC, a multimodal retention mechanism is present so it is very difficult to predict the impact of chromatographic parameters on the retention and separation of the compounds. Furthermore, the relationship between the pH of the mobile phase, the degree of ionization of the compounds, the surface of the stationary phase and the absorbed water layer is very complex, which significantly complicates the HILIC method development....