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Using theoretical models, retention mechanisms of amlodipine besilate and its impurities D, E, and F were tested, as well as bisoprolol fumarate and its impurities A, C, K, and L. Appropriate theoretical retention models for tested compounds were used, which enabled the determination of retention mechanisms responsible for the separation of tested compounds on three different HILIC column: silica, amino, and diol. In order to describe the effect of ratio of aqueous and organic portion of the mobile phase on the retention behavior of tested compounds, as well as the consideration of shares of partition and adsorption processes in HILIC retention mechanism in all three columns, partitioning and adsorption retention theoretical models were calculated. For this purpose, the water content was varied in the range from 4% to 29%, corresponding to the HILIC region of mobile phase composition. During the experiment, values of other factors of mobile phase were maintained at a constant level. Regression coefficients of partition and adsorption retention models and the corresponding determination coefficients were determined (R2). Partitioning models for all tested compounds on amino column had higher R2 values, while adsorption models for all compounds tested on silica and diol column had higher R2 value. [ABSTRACT FROM AUTHOR]