Boronate affinity-assisted MEKC separation of highly hydrophilic urinary nucleosides using imidazolium-based ionic liquid type surfactant as pseudostationary phase

In this work, we extend our investigations regarding the separation of urinary nucleosides by MEKC with the ionic liquid type surfactant 1-tetradecyl-3-methylimidazolium bromide (C(14)MImBr). We study the impact of adding alkyl- and arylboronic acids (in the presence of C(14)MImBr micelles) to the separation of these highly hydrophilic metabolites and investigate the mechanism of interaction between the negatively charged nucleosides (the negative charge is acquired either due to deprotonation of the amidic group and/or complexation with boronate) and the positively charged pseudostationary phase. This interaction is not only due to electrostatic (Coulombic) forces, but also due to hydrophobic interaction of the alkyl or aryl group of the boronate that forms a complex with the cis-diol group of the nucleoside. In this case, alkylboronates can act as a cosurfactant that increases the partitioning coefficient of the analytes into the micelles. In the presence of an alkylboronate in the BGE (employing only 20mmol/L C(14)MImBr), the retention factors of the studied analytes are increased considerably when compared to a BGE without this additive. It is shown that the concept of one-site hydrophobically assisted ion exchange can be applied to describe the observed retention behavior. The high selectivity of boronates toward cis-diol-containing compounds can be used to adjust selectively the migration behavior of members of this compound class. By adding alkylboronic acid to the BGE, the separation selectivity is fine-tuned so that interferences from matrix components can be avoided in real sample analysis.