Type of interaction in HPLC column

Figure 1 There are 4 types of interactions: non polar interactions, polar interactions, ionic interactions and interactions based on the molecular size.

The beads in columns are normally based on silica or a polymer. Different chains can be attached to the structure, giving different types of interactions:

• Interactions based on polarity: polar compounds will be retained by stationary phases with polar chains, such as –CN or –OH. Non polar compounds will be retained by non polar groups in the stationary phase, such as –C8 or –C18.

• Ionic interactions: depending on the pH, some groups can be protonated or deprotonated, forming ionic species. If the analyte and the stationary phase have opposite charges, the analytes will be retained in the column.

• Molecular size: Small molecules can be retained in the pores of the stationary phase whereas large molecules will not get trapped. Therefore, small molecules will stay longer in the column whereas large molecules will pass faster.