ABSTRACT
Recent synthesis of chiral ILs paved the way for evaluation of new potential selectors for chiral separations that underscore ion-pairing in CE. Particular selectivities may be achieved by exploiting unique hydrophobic interactions, ion-dipole or ioninduced-dipole, ion-pairing effects and tailoring the molecular architecture of the IL. Even if chiral ILs did not present direct enantioselectivity with regard to model analytes (2-arylpropionic acids), in the presence of classical chiral selectors (di-or trimethyl-β-cyclodextrin), an increase in separation selectivity and resolution suggesting synergistic effects was observed in some cases [6]. Chiral counter ions were exploited in many enantiomeric separations via ion-pairing CE [7,8]
Usually bi-or multi-dimension separations must be run individually since they utilize different physico-chemical separation principles and differ greatly in selectivity. Ion-pairing and electrophoretic mobility provide two compatible separation models that may be run simultaneously in the same single separation device to achieve a bi-dimensional technique. Since they are respectively based on the complex hydrophobicity and on the analyte change, experiments demonstrated that ion-pairing
occurs in the BGE and can be optimized in terms of counter ion hydrophobicity and concentration. The major practical repercussion of ion-pairing CE is the high peak capacity that can be obtained [9-11]. This strategy represents a break from the chromatographic resemblance of CE in which the hydrophobic interaction mechanism is at an interface as in capillary electrochromatography (CEC) and capillary micellar electrokinetic chromatography (CMEKC). Ion-pairing also proved valuable in CMEKC which makes use of micellar mobile phases in reversed phase mode. Complex electrostatic hydrophobic and steric interactions exist between the solute and both stationary and mobile phases. The presence of chiral surfactants in the mobile phase afforded enantioseparation also via CMEKC [12]. Ion-pairing focusing of polyanionic heparins in a polycationic polyacrylamide gel, made by incorporating a gradient of positively charged monomers into the neutral polycrylamide backbone allows the polydisperse heparins to reach a steady-state position along the migration path and condense in an environment inducing charge neutralization. Both size and charge distribution along the oligosaccharide chains influence the separations there by confirming the multiplicity of phenomena involved in ion-pairing [13].
