The roles of 1-alkyl-3-methylimidazolium-based ionic liquids as mobile phase additives in semi-micro high-performance liquid chromatography for separation of aromatic amines

Author(s): Yuppadee Nusai, Hitoshi Koizumi, Masaki Tachibana, Kazue Tani, Nobutoshi Kiba

The separation mechanism of semi-micro High-Performance Liquid Chromatography (Semi-micro HPLC) using ionic liquid-aqueous mobile phase was investigated for the first time with the retention behavior of eight aromatic amines. The increase of the [EMIm] [BF4] concentration resulted in the consecutive decrease of the retention factors of eight aromatic amines. This attributes the competition between the imidazoliumcations and protonated aromatic amines in forming the hydrophobic interaction with the residual silanol and octadecyl groups of octadecylsilica surface. The increase of the alkyl chain length on the imidazoliumcation of the ionic liquid caused the clear decrease of the retention factors of eight aromatic amines, because this factor leads to the increase of electrostatic repulsion force. On the contrary, the increase of the mobile phase pH caused the increase of the retention factors of most aromatic amines, this is due to most aromatic amines have a decreased dissociation ability.Moreover, the different anion types of the ionic liquids exhibited a different occurrence of cation exchange and ion-pairing interactions, these two interactions affect the major electrostatic repulsion and hydrophobic interactions. The separationmechanism involves electrostatic repulsion, hydrophobic, cation exchange and ion-pairing interactions.

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