New particle‐loaded monoliths for chiral capillary electrochromatographic separation

Abstract

Fritless particle-loaded monoliths for chiral capillary electrochromatographic (CEC) separation were prepared. Silica particles containing a chiral selector are suspended in a monomer solution, which is drawn into the capillary followed by in situ polymerization. Thereby the silica-based particles containing the chiral selector are embedded in a nonchiral continuous bed. This kind of chiral stationary phase is inexpensive, easy, and reproducible to prepare and circumvents the preparation of frits. As a model, teicoplanin aglycone as chiral selector bonded to 3 microm silica particles was used. The applicability of this approach is demonstrated by means of the chiral separation of aliphatic and aromatic amino acids and dipeptides. As a further application, the chiral selector ristocetin A bonded to 3 microm silica particles was used for the enantiomeric separation of chiral alpha-hydroxy acids. Since alpha-hydroxy acids migrate toward the anode, a cationic charge-providing agent was copolymerized with the matrix. This served to reverse the direction of the electroosmatic flow (EOF).

Comparison of monolithic approaches for enantioselective capillary electrochromatography involving cyclodextrins

“…Linkage to the PDMS or to the silica depends on the reaction conditions and occurs preferentially from the mono-2-position of CD rather than from the mono-6-position as previously inferred [58]. Reprinted from [21,42].…”

“…A further improvement was achieved by utilizing CDs as chiral selectors in monolithic stationary phases [14] as it was described earlier for LC by Lubda et al [15] and by Sinner and Buchmeiser [16]. The present state of the art includes three distinct types of enantioselective monolithic approaches: (i) Silicabased monoliths are prepared by the sol -gel technology [17] and the chiral selectors are immobilized by covalent bonding [18 -22], by physical adsorption [23] or by encapsulation [24,25]; (ii) Organic polymer monoliths are prepared by polymerization of organic monomers in the presence of a porogen and a crosslinker and the chiral selector is copolymerized [26 -36], grafted by polymerization [37 -39], or molecularly imprinted [40]; and (iii) monoliths are prepared by particle-fixation techniques such as embedding the silica into a sol -gel matrix [41], into an organic polymer [42], by gluing it via a sol -gel process [43], or by sintering it at high temperatures [44]. The chiral selector is linked to silica particles either prior to or after the formation of the monolith.…”

Chiral Separation of α-hydroxy Acids by Capillary Electrochromatography Using the Cathodic Detergent CTAB as Mobile Phase Additive to Speed up Analysis Time

“…To speed up the analysis time Schmid et al copolymerized the positive charge providing reagent DADMAC in the afore mentioned experiments. 16 With this method good enantioseparations in a relatively short time were achieved for α-hydroxy acids in LE-CEC both with Ristocetin A and with TAG stationary phase.…”

“…16 A monomer solution with suspended silica particles was drawn into a capillary and was polymerized. Good chiral separations of α-hydroxy acids were shown by using L-4-hydroxyproline 11 bonded to 3 µm silica material in ligand-exchange CEC and by using the macrocyclic antibiotics Ristocetin A and teicoplanin aglycone (TAG) bonded to 3 µm silica material.…”

“…Good chiral separations of α-hydroxy acids were shown by using L-4-hydroxyproline 11 bonded to 3 µm silica material in ligand-exchange CEC and by using the macrocyclic antibiotics Ristocetin A and teicoplanin aglycone (TAG) bonded to 3 µm silica material. 16 Macrocyclic antibiotics of the glycopeptide type were first introduced by Armstrong et al 17 and are nowadays wide spread chiral selectors in HPLC, CE and CEC.…”