Use of Tetrabutylammonium Bromide and l-Arginine-Based Deep Eutectic Mixture in Combination with Beta-Cyclodextrin for Chiral Discrimination of Amino Acids in Capillary Electrophoresis

The potential effects of deep eutectic solvents (DESs) based on eutectic mixtures of amino acids (AAs) and tetrabutylammonium bromide (TBABr) in combination with beta-cyclodextrin (β-CD) were first evaluated as chiral systems in capillary electrophoresis (CE). Three DESs: tetrabutylammonium bromide/l-arginine (TBABr/l-Arg), tetrabutylammonium bromide/l-alanine (TBABr/l-Ala) and tetrabutylammonium bromide/l-proline (TBABr/l-Pro)) were used. Markedly improved chiral discrimination for four native amino acids (dl-methionine, dl-valine, dl-serine and dl-threonine) were obtained with high resolutions ranging from 4.78 to 9.84 in the presence of TBABr/l-Arg/β-CD chiral selector. Optimal CE conditions were a BGE containing 15 mmol L−1 β-CD and 10% (v/v) TBABr/l-Arg mixed with 50 mmol L−1 borate at pH 9.5, 20 kV applied voltage and 25 °C capillary temperature. The relative standard deviations (RSDs) of intra- and inter-day variations of migration times were less than 2.2% and 3.5%, respectively. l-Arg has the highest isoelectric point (pI = 10.7 at 25 °C) among protein amino acids which is appropriate for formation of stable DES at an alkaline separation medium. Moreover, TBABr/l-Arg/β-CD possessed the lowest electroosmotic flow velocity (0.018 cm2 min−1 V−1) among the investigated chiral systems providing more opportunities for chiral recognition of analytes within 14 min. Analytical characteristics of the CE/UV developed method were evaluated and demonstrated to be adequate to apply the methodology to enantiomeric analysis of methionine in commercial dietary supplements including enantiomeric impurity of d-methionine at 0.03% level (m/m). Therefore, the proposed TBABr/l-Arg/β-CD could be used as a versatile, simple and effective selector for baseline chiral recognition in CE.