IntroductionIn the past decades, capillary electropheresis (CE) has been considered to be a powerful method, and widely applied to analyze many of different substances, ranging from small inorganic ions to large bio-macromolecules, due to many advantages, such as high separation speed and efficiency, very low running costs and low sample volume injected (e.g., 2 -10 nl). Metal-ion analysis, 1-3 however, covers a relatively small part of CE applications compared with separations in the areas of pharmaceuticals, oligonucleotides, proteins and peptides. Considering metal-ion analysis, problems occur due to (i) the similar electrophoretic mobilities and (ii) the weak UV absorption of most metal ions.The former problem is because most transition metal ions have similar mobilities due to their similar size and identical charge. 4 To achieve an electrophoretic separation, the mobilities have to be modified by introducing a complex forming equilibrium. If different degrees of complexation result for the ions during the interaction with a ligand, their apparent electrophoretic mobilities should also be different. Various reports can be found in the literature describing organic acids as complexing agents for improving the resolution in CE separation. These include oxalic acid, lactic acid, tartaric acid and hydroxyisobutyric acid. [4][5][6][7] Ethylenediaminetetraacetic acid (EDTA) has also been used for this purpose. [8][9][10][11] EDTA is known to complex with metal ions in a 1:1 ratio, and the chelates are always negalively charged whatever the charge on the cations. 4,12 As a principally electrophoretic property, metal-EDTA anionic complexes could be successfully separated due to inherent differences in the electrophoretic mobilities. However, in practice, it also suffers from poor selectivities, and is not sensitive. Until now, on-column derivatization, also called in-capillary derivatization, has been applied to determine various types of analyte by one of three different strategies: zone-passing, at-inlet and throughout capillary derivatization. 2,8,[13][14][15] These studies have proven that the sensitivity could be sharply increased by using in-capillary derivatization.For the latter problem, the weak UV-absorption of most metal ions, several preconcentration methods have been developed to overcome the sensitivity problem, such as isotachophoresis, 16 [28][29][30] In addition, the sensitivity for the metal determination could be improved by using on-line preconcentration in micellar electrokinetic chromatography 45 and CE-laser induced fluorescence. 46 Increasing the optical path length by incorporating a bubble cell 31,32 or a U-shaped cell 33,34 is one of the means to improve the sensitivity of a UV detector. U-shaped cells, usually a bent capillary, with a few millimeters of the capillary parallel to the optical path, can provide a higher sensitivity, but a loss of resolution prevents this method from further development. 35 Vancomycin is a macrocyclic antibiotic containing in its structure 18 asymmetric cent...
