Capillary electrochromatography with monolithic stationary phases. II. Preparation of cationic stearyl-acrylate monoliths and their electrochromatographic characterization

A novel cationic monolithic stationary phase based on the co-polymerization of pentaerythritol diacrylate monostearate (PEDAS) with a selected quaternary amine acrylic monomer was designed for performing capillary electrochromatography at high flow velocity. While PEDAS functioned as both the ligand provider and the cross-linker, the quaternary amine acrylic monomer was introduced to control the magnitude of the electroosmotic flow (EOF). The fabrication of the cationic stearyl-acrylate monolith (designated as cationic C17 monolith) with controlled porosity was achieved by free radical polymerization using the initiator 2,2'-azobisisobutyronitrile in the presence of a ternary porogenic solvent composed of cyclohexanol, ethylene glycol and water. Four different quaternary amine acrylic monomers were investigated in order to find the optimum monomer for achieving maximum electroosmotic flow (EOF) velocity. Both photo- and thermally-initiated polymerization proved effective in producing the cationic C17 monolith, and the best monolith was achieved when [2-(acryloyloxy)ethyl]trimethyl ammonium methyl sulfate (AETA) was used as the quaternary amine acrylic monomer. Although the zeta potential of the resulting cationic C17 monolith is positive with respect to water, the magnitude and direction of the EOF was markedly affected by the nature of the electrolyte in the mobile phase. Consequently, anodal, zero or cathodal EOF was observed depending on the nature of the electrolyte, and this was attributed to the adsorption of the ionic components of the electrolyte on to the solid stationary phase, which is characterized by its amphiphilic nature consisting of C17 chains, ester functions, hydroxyl groups and quaternary amine moieties. Optimized PEDAS-AETA monoliths yielded columns with high separation efficiency and allowed rapid separations on the time scale of seconds to be achieved with short capillaries.     

Enantioseparations by capillary electrophoresis using chiral glycosidic surfactants. I. Evaluation of cyclohexyl-pentyl-beta-D-maltoside surfactant.

Chiral cyclohexyl-pentyl-beta-D-maltoside (CYMAL-5) surfactant was evaluated in the enantioseparation of charged racemic species by capillary electrophoresis. CYMAL-5 is a glycosidic surfactant (GS) with a chiral maltose polar head group and a cyclohexyl-pentyl hydrophobic tail. At concentrations above its critical micellar concentration (CMC), CYMAL-5 produces neutral micelles in aqueous media. The neutral micelles migrate at the velocity of the electroosmotic flow (EOF). As expected, the CYMAL-5 system was only useful for the enantioseparation of charged chiral solutes. The enantioresolution of the CYMAL-5 can be manipulated over a wide range of electrolyte composition, e.g., pH, ionic strength and surfactant concentration. In the presence of EOF, and in all cases, there is an optimum surfactant concentration for maximum enantioresolution, which is located at low surfactant concentration for strongly hydrophobic solutes and at high surfactant concentration for relatively hydrophilic solutes. The presence of an optimum surfactant concentration for maximum enantioresolution is attributed to the EOF. At low pH values where the EOF is negligible, enantioresolution increased with increasing surfactant concentration in the useful concentration range in a way similar to chromatography.     

Capillary electrophoresis of glucosinolates and their degradation products

Glucosinolates are important natural products occurring mainly in plants of the Cruciferae family. This review article is aimed at describing the recent progress made in capillary electrophoresis of glucosinolates and their degradation products. It describes the various electrophoretic systems and detection schemes introduced to date for the capillary electrophoresis (CE) of glucosinolates and their degradation products. Also included in this review are the applications of CE to the qualitative and quantitative determination of glucosinolates and their degradation products in plant extracts.     

High-performance liquid chromatography of sialooligosaccharides and gangliosides

Glycans were cleaved from gangliosides and separated by high-performance liquid chromatography (HPLC). The columns were packed with bonded stationary phases made of microparticulate, macroporous silica with serotonin, phenylpropanolamine or tryptamine as the biogenic amine ligate. The ganglioside oligosaccharides were eluted in the order of increasing number of sialic acid residues in the molecule and their retention decreased with the ionic strength of the mobile phase. Best selectivity was obtained in the pH range from 3.0 to 4.0. The two major sialic acids, N-acetylneuraminic and N-glycolylneuraminic acids, were separated by lectin affinity chromatography using an HPLC column packed with silica-bound wheat germ agglutinin and 10 mM phosphate buffer, pH 4.0, as the eluent. Throughout this study, isocratic elution was used and the column effluent was monitored at 195 nm.     

Capillary electrochromatography with monolithic silica columns. IV. Electrochromatographic characterization of polar bonded monolithic stationary phases having surface-bound cyano functionalities

Two polar ligands, namely 3-hydroxypropionitrile and 1H-imidazole-4,5-dicarbonitrile (IDCN) were covalently attached to epoxy-activated silica-based monolithic capillary columns via an epoxide ring-opening reaction to yield CN-OH-Monolith and 2CN-OH-Monolith, respectively. The silica monolith was prepared by a sol-gel process, and the resulting "rod-like" stationary phase was subjected to pore tailoring with an alkaline solution to convert small pore domains to mesopore domains, thus yielding a monolith with bimodal pore structure consisting of flow through pores (i.e., flow channels for mobile-phase flow) and mesopores that provide most of the adsorption capacity of the monolith toward the separated solutes. The two polar monoliths, CN-OH-Monolith and 2CN-OH-Monolith, were evaluated in normal-phase CEC with organic-rich mobile phases less polar than the stationary phase. The 2CN-OH-Monolith bearing more polar functions than the CN-OH-Monolith exhibited more retention and improved selectivity toward model polar solutes.     

Mixed ligand monolithic columns for reversed-phase capillary electrochromatography via hydrophobic and π interactions

Novel mixed ligand monoliths (MLM) for capillary electrochromatography (CEC) of a wide range of solutes differing in both polarity and size were introduced. The MLM capillary columns were based on the different compositions of octadecyl acrylate (ODA) and 2-naphthyl methacrylate (NAPM) monomers in the presence of trimethylolpropane trimethacrylate (TRIM) crosslinker and a ternary porogenic solvent made up of cyclohexanol, ethylene glycol, and water. As expected, the magnitude of the electroosmotic flow (EOF) changed with the composition of the MLM. As the percent of the monomer ODA in the polymerization mixture was increased, the EOF increased to a maximum at 50-mol% ODA and then leveled off at 75-mol% and 100-mol% ODA, an indication that the ODA ligand in general exhibited a higher binding for the mobile-phase ions than the NAPM ligand. This is due to the fact that the ODA is an acrylate-based monomer, whereas the NAPM is a methacrylate-based monomer. While ODA provided solely nonpolar interactions, NAPM exhibited both nonpolar and π interactions with certain solutes. It was found out that columns with a given composition of both ligands yielded a unique selectivity for a given set of solutes that was not matched by columns made by either ODA or NAPM alone. Several test mixtures were used in the evaluation of the MLM columns including polycyclic aromatic hydrocarbons, alkyl phenyl ketones, nitroalkanes, alkylbenzenes, toluene derivatives, peptides, and proteins. Peptide mapping of the tryptic digest of the standard lysozyme protein was also studied.     

Capillary electrochromatography with polyacrylamide monolithic stationary phases having bonded dodecyl ligands and sulfonic acid groups: evaluation of column performance with alkyl phenyl ketones and neutral moderately polar pesticides

In this report, we describe the preparation of porous polyacrylamide-based monolithic columns via vinyl polymerization. These monoliths possess in their structures bonded dodecyl ligands and sulfonic acid groups. While the sulfonic acid groups are meant to support the electroosmotic flow (EOF) necessary for moving the mobile phase through the monolithic capillary, the dodecyl ligands are introduced to provide the nonpolar sites for chromatographic retention. However, incorporating the sulfonic acid groups in the monoliths does not only support the EOF but also exhibit hydrophilic interaction with moderately polar compounds such as urea herbicides and carbamates insecticides. Consequently, mixed-mode (reversed-phase/normal phase) retention behavior is observed with neutral and moderately polar pesticides. The amount of sulfonic acid group in the monolith can be conveniently adjusted by changing the amount of vinylsulfonic acid added to the polymerization reaction. Optimum EOF velocity and adequate chromatographic retention are obtained when 15% vinylsulfonic acid is added to the reaction mixture. Under these conditions, rapid separation and high plate counts reaching greater than 400000 plates/m are readily obtained.