A mechanistic study of geminal dicationic ionic liquids as mobile phase additives for improving the separation performance of high-performance liquid chromatography.

Due to the acidity and strong polarity of auxinic herbicides, separation of these compounds in food and environmental samples is a great challenge. In this study, 12 geminal dicationic ionic liquids (GDILs) were synthesized and used as mobile phase additives to separate six auxinic herbicides. The effects of the kind of dication, the length of linkage chain, the kind of anion, the concentration of GDILs, and the pH of mobile phase for the separation were investigated in detail. Compared with [C4 MIm][BF4 ], GDILs ([C8 (MIm)2 ][BF4 ]2 ) showed higher resolution, better peak shape, and lower retention factor. The separation performance of additives was in the order of [C8 (MIm)2 ][BF4 ]2  > [C8 (MPy)2 ][BF4 ]2 , [(C4 )2 MPiz][BF4 ]2 , [C8 (MMo)2 ][BF4 ]2 , [C4 MIm][BF4 ], [C8 (MPid)2 ][BF4 ]2  > [C8 (HBOc)2 ][BF4 ]2  > TBAB. GDILs showed the best separation under the following conditions: cation: imidazolium; length of linkage chain: 8; anion: BF4 - ; mobile phase pH: 3; concentration: 5 mmol L-1 . The separation mechanism of GDILs may mainly be due to their symmetric structure and the abundance of positive charge sites which could help GDILs shield the residual silanol and interact with analytes more efficiently to improve the peak shapes, resolution, and retention. In addition, the GDILs in the mobile phase and anions adsorbed on the stationary phase also greatly affected the separation. GDILs used as mobile phase additives to separate auxinic herbicides showed high efficiency separation and low damage to HPLC columns, and the developed chromatographic method had excellent linearity, accuracy, precision, and repeatability. The application and mechanism study of GDILs in HPLC would be instructive to determine and separate acid herbicides in food and environmental samples. Graphical abstract As HPLC mobile phase additives, GDILs show high separation performance due to their unique symmetric structures.