Enantioseparation of 2-aryloxypropionic acids on chiral porous monolithic columns by capillary electrochromatography

Enantioseparations in open-tubular capillary electrochromatography: Recent advances and applications

This review highlights recent advances and applications in open-tubular capillary electrochromatography (OT-CEC) for enantioseparations during the last decade. Although extensive research has been conducted in the area of separations by use of CEC, and a big number of reviews have been published, there is not a review on exclusively the use of chiral stationary phases (CSPs) in OT-CEC for enantioseparations. Therefore, in this review, the design and synthesis of different CSPs are presented, and their potential applications in OT-CEC for enantioseparations are discussed. The different approaches to CSP development include chiral nanomaterials, porous layers, molecular imprinting, sol-gel technique, polyelectrolyte multilayer coating, polymeric coating and others.

Trimethyl-β-cyclodextrin-encapsulated monolithic capillary columns: Preparation, characterization and chiral nano-LC application

Trimethylated-β-cyclodextrin (TM-β-CD) was encapsulated within several polymer monolithic capillary columns for reversed-phase chiral nano-liquid chromatography (nano-LC). The monolithic phases were prepared using the one-pot in situ copolymerization of ethylene glycol dimethacrylate (EDMA), glycidyl methacrylate (GMA) monomers and 1-propanol, 1,4-butanediol as progenic solvents in presence of TM-β-CD solution within fused silica capillaries (150µm I.D.). The obtained chiral monolithic stationery phases were characterized by scanning electron microscopy (SEM), N₂ adsorption/desorption isotherms, wide angle x-ray diffraction (WAXRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The materials characterization demonstrated that monolithic phases with higher concentration of TM-β-CD have relatively larger surface area, smaller pore size and larger total pore volume compared to those with lower concentration TM-β-CD. The prepared columns were tested for their enantioseparation efficiency of a range of racemic pharmaceuticals. The screening results demonstrated the potential of functionalizing polymer monolithic stationary phases with TM-β-CD using the in situ encapsulation approach.

Phosphatidylcholine covalently linked to a methacrylate-based monolith as a biomimetic stationary phase for capillary liquid chromatography

In this study a strategy to immobilize phospholipids onto a polymer-based stationary phase is described. Methacrylate-based monoliths in capillary format (150×0.1mm) were modified by soybean phosphatidylcholine through 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide coupling to obtain stationary phases suitable to mimic cell surface membranes. The covalent coupling reaction involves the phosphate group in phospholipids; therefore, the described methodology is suitable for all types of phospholipids. Immobilization of soy bean phosphatidylcholine on the monolith was confirmed by attenuated total reflectance Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry of the fatty alcohol profile, generated upon reductive cleavage of the fatty acyl side chains of the phospholipid on the monolith surface with lithium aluminium hydride. The prepared stationary phases were evaluated through studies on the retention of low-molar mass model analytes including neutral, acidic, and basic compounds. Liquid chromatographic studies confirmed predominant hydrophobic interactions between the analytes and the synthesized stationary phase; however, electrostatic interactions contributed to the retention as well. The synthesized columns showed high stability even with fully aqueous mobile phases such as Dulbecco's phosphate-buffered saline solution.

A novel positively charged achiral co-monomer for β-cyclodextrin monolithic stationary phase: improved chiral separation of acidic compounds using capillary electrochromatography coupled to mass spectrometry

The work presented here demonstrates the incorporation of vinylbenzyl trimethylammonium (VBTA) as a novel positively charged achiral co-monomer to a glycidyl methacrylate-beta cyclodextrin (GMA/β-CD) based monolith, providing anion exchange sites with reversed electroosmotic flow (EOF) for capillary electrochromatography (CEC). The monolithic phases, GMA/β-CD-VBTA and GMA/β-CD (without co-monomer) were characterized by scanning electron microscopy, optical microscopy, pressure drop/flow-rate curves and nitrogen adsorption analysis. After optimizing the stationary phase and mobile phase parameters, chiral separations of 41 pairs of structurally diverse anionic chiral analytes were compared individually using the GMA/β-CD-VBTA and GMA/β-CD monolithic columns. The GMA/β-CD-VBTA monolith chiral stationary phase separated significantly more acidic compounds compared to the GMA/β-CD column. To-date there has been limited work in the development of chiral monolithic column for CEC-mass spectrometry (MS). Because of good electrodriven flow characteristics, which allow the column to maintain a stable current in the absence of outlet vial, GMA/β-CD-VBTA column was successfully coupled to single quadrupole mass spectrometer for CEC-MS of several chiral test compounds. In addition, the same monolithic CEC column when coupled to a triple quadrupole MS instrument, two orders of magnitude higher sensitivity was observed compared to a single quadrupole MS instrument.

Enantioselective capillary electrochromatography: recent developments and new trends

Since its development in the early 1970s, CEC has been studied quite extensively, but unfortunately its use is still mostly located at an academic level. Reasons for this are the limited availability of commercially available stationary phases (SPs) and columns, along with some practical limitations, such as column fragility, lack of column robustness and reproducibility. Nevertheless, CEC maintains a place among the separation techniques, probably because of its unique feature to combine two separation principles. Also in the field of chiral separations, CEC is often used as a separation technique and already showed its potential for this kind of analyses. This overview will focus on the recent applications, i.e. between 2006 and 2010, in enantioselective analysis by means of CEC. For the selected applications, the used SPs (chiral selectors) and their potential for future method development or screening purposes will be evaluated and critically discussed.

Recent advances of enantioseparations in capillary electrophoresis and capillary electrochromatography

A comprehensive survey of recent developments and applications of capillary electromigration techniques for enantioseparations from January 2006 to June 2010 is presented. The techniques include capillary electrophoresis, chip capillary electrophoresis and capillary electrochromatography. The separation principles and the chiral recognition mechanisms are discussed. Additionally, on-line preconcentrations in chiral capillary electrophoresis are also reviewed.

CEC: selected developments that caught my eye since the year 2000

During the last decade, a number of new developments have emerged in the field of CEC. This paper focuses only on monolithic columns prepared from synthetic polymers. Monolithic columns have become a well-established format of stationary phases for CEC immediately after their inception in the mid-1990s. They are readily prepared in situ from liquid precursors. Also, the control over both porous properties and surface chemistries is easy to achieve. These advantages make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials. Since the number of papers concerned with just this single topic of polymer-based monolithic CEC columns is large, this overview describes only those approaches this author found interesting.

CEC enantioseparations on chiral monolithic columns: A study of the stereoselective degradation of (R/S)-dichlorprop [2-(2,4-dichlorophenoxy)propionic acid] in soil

For the study of the stereoselective degradation of the herbicide 2-aryloxipropionic acid dichlorprop (DCPP) in soil, a porous monolithic chiral column (100 microm id) was prepared by in situ copolymerization of glycidyl methacrylate, methyl methacrylate and ethylene glycol dimethacrylate in the presence of formamide and 1-propanol as the porogen solvents. Subsequently, the epoxide groups at the surface of the monolith were reacted with (+)-1-(4-aminobutyl)-(5R,8S,10R)-terguride as the chiral selector. Optimum conditions for the herbicide resolution by CEC were found using mobile phases consisting of acetic acid/triethylamine mixtures in ACN-methanol (9:1 v/v). Under these conditions fully separation of DCPP enantiomers in the presence of clofibric acid (internal standard) was achieved in about 5 min. Experiments on the incubation of rac-DCPP in soil at room temperature showed the herbicide undergone during 23 incubation days to a degradation to levels </=20% of the initial concentration, with rates for (R)-DCPP slower than (S)-DCPP. More interesting results were observed when herbicide enantiomers were individually incubated. In both the experiments, the formation of the opposite isomer in the presence of the initial one, and reversed enantiomeric interconversion in the case of (S)-DCPP was observed. (R)-DCPP was found to be the most persistent isomer after incubation.

Recent accomplishments in the field of enantiomer separation by CEC

The present review intends to summarize recent developments in the field of enantioselective separations and analysis by CEC. It covers studies published in English language in common peer-reviewed journals within the period between 2003 and 2006. Both, methods making use of chiral mobile phase additives as well as chiral stationary phases for electrochromatographic enantiomer separations, are reviewed. Achievements that have been made on the various column technologies, such as open-tubular, particle-packed, inorganic, organic and particle-fixed (hybrid-type) monolithic as well as molecularly imprinted polymer phases, are discussed.

Recent development of monolithic materials as matrices in microcolumn separation systems

This review summarizes the development of monolithic materials, including both organic and inorganic polymers, according mainly to the papers published in the past two years. Due to their good permeability, fast mass transfer, high stability, and their ease of modification, such materials have been widely used in microcolumn separation systems, not only as stationary phases for CEC and capillary HPLC, but also as substances for sample concentration and enzyme reactor. All the research results demonstrate that monolithic materials in microseparation systems can be expected to play an increasingly important role in the analysis of complex samples.

Recent advances in the control of morphology and surface chemistry of porous polymer-based monolithic stationary phases and their application in CEC

This review focuses on developments in the field of polymer-based monolithic columns for CEC published in the literature since the beginning of the year 2005. The possibility of in-situ preparation as well as easy control over their porous properties and surface chemistries clearly make monolithic separation media an attractive alternative to capillary columns packed with particles. Different variables such as polymerization conditions, morphology, and surface chemistry are shown to directly affect performance of monolithic capillary columns in terms of efficiency, analysis time, and retention.