Capillary electrochromatography with polymeric continuous beds synthesized via free radical polymerization in aqueous media using derivatized cyclodextrins as solubilizing agents

Elucidation of the Enantiodiscrimination Properties of a Nonracemic Chiral Alignment Medium through Gel-based Capillary Electrochromatography: Separation of the Mefloquine Stereoisomers

Enantiodiscrimination and enantioseparation are two highly important processes in chemistry, often performed by using NMR spectroscopy and chromatography. For a better understanding of the mechanistic details, the same system should be studied by both methods. In addition, isotropic and anisotropic NMR parameters should be obtained, the latter using alignment media so that residual dipolar couplings and chemical-shift anisotropies can be measured. Consequently, a chiral alignment medium was used for the first time in chiral gel-based capillary electrochromatography with the four stereoisomers of the antimalaria drug mefloquine as test compounds. Chromatographic data verify that enantiodiscrimination obtained with this alignment gel is caused by differences in the equilibrium constants related to associate formation. Hence, the chromatographic separation provides physicochemical data that form a basis for the understanding and optimization of alignment processes, and vice versa.

Adamantyl-group containing mixed-mode acrylamide-based continuous beds for capillary electrochromatography. Part IV: investigation of the chromatographic efficiency dependent on the retention mode

In our previous work we have described the synthesis, characterization, and optimization of the chromatographic efficiency of a highly crosslinked macroporous mixed-mode acrylamide-based monolithic stationary phase synthesized by in situ free radical copolymerization of cyclodextrin-solubilized N-adamantyl acrylamide, piperazinediacrylamide, methacrylamide and vinylsulfonic acid in aqueous medium in pre-treated fused silica capillaries of 100μm I.D. In the present work, we study with different classes of neutral analytes (with varied hydrophobicity) the impact of the type of retention mode (influenced by the type of analyte and the mobile phase composition) and the impact of the solute functionality on the chromatographic efficiency and peak symmetry with a monolith synthesized under optimized synthesis parameters. With this monolithic capillary high separation efficiencies (up to ca. 220,000m(-1)) are obtained for the separation of different analyte classes (alkylphenones, nitrotoluenes, and phenolic compounds with k=0.2-0.55) in the reversed-phase mode, in the normal-phase mode, and in the mixed mode. For neutral alkylanilines (k<0.25) plate numbers of about 300,000m(-1) are routinely reached in the reversed-phase elution mode. For phenolic solutes separated in a mixed mode there is a solute-specific influence on peak symmetry and chromatographic efficiency. With increasing efficiency of the monolith, axial diffusion becomes an important mechanism of band broadening. For those peaks, which do not show a significant asymmetry (asymmetry factor ≤1.05), it is confirmed that plate heights gained via the tangent method are equivalent to those gained via moment analysis.

Organic monoliths for hydrophilic interaction electrochromatography/chromatography and immunoaffinity chromatography

This article is aimed at providing a review of the progress made over the past decade in the preparation of polar monoliths for hydrophilic interaction LC (HILIC)/capillary electrochromatography (HI-CEC) and in the design of immuno-monoliths for immunoaffinity chromatography that are based on some of the polar monolith precursors used in HILIC/HI-CEC. In addition, this review article discusses some of the applications of polar monoliths by HILIC and HI-CEC, and the applications of immuno-monoliths. This article is by no means an exhaustive review of the literature; it is rather a survey of the recent progress made in the field with 83 references published in the past decade on the topics of HILIC and immunoaffinity chromatography monoliths.

An insight into the mechanism of CEC separation of template analogues on a norepinephrine-imprinted monolith

A monolith molecularly imprinted polymer (MIP) column was prepared from template (-)-norepinephrine, functional monomer (itaconic acid), and a cross-linker (either ethylene glycol dimethacrylate or divinylbenzene) in porogen N,N-dimethylformamide. Understanding the molecular recognition of a template using an MIP seems feasible. However, it is hard to explain the recognition properties of their analogues on an MIP. The separation mechanism was investigated with the addition of charged surfactants, native and derivatised β-cyclodextrin (β-CD), achiral crown ether, etc. to determine the retention behaviour of the template analogues. The addition of organic modifiers and the adjustment of separation conditions were used to manipulate the selectivity. No chiral recognition was observed under most of the test conditions except the experiment with the charged β-CD on the divinylbenzene-MIP column. The different experimental conditions led to differences in the mobilities of the analytes and resulted in remarkable enantiomeric separation of the template. We confirmed the presence of mixed-mode selectivity of the stationary phase based on hydrogen bonding, hydroelectric and hydrophobic interactions, and the electrophoretic mode.

Investigation of the ion-exchange properties of methacrylate-based mixed-mode monolithic stationary phases employed as stationary phases in capillary electrochromatography

The potential of methacrylate-based mixed-mode monolithic stationary phases bearing sulfonic acid groups for the separation of positively charged analytes (alkylanilines, amino acids, and peptides) by capillary electrochromatography (CEC) is investigated. The retention mechanism of protonated alkylanilines as positively charged model solutes on these negatively charged mixed-mode stationary phases is investigated by studying the influence of mobile phase and stationary phase parameters on the corrected retention factor which was calculated by taking the electrophoretic mobility of the solutes into consideration. It is shown that both solvophobic and ion-exchange interactions contribute to the retention of these analytes. The dependence of the corrected retention factor on (1) the concentration of the counter ion ammonium and (2) the number of methylene groups in the alkyl chain of the model analytes investigated shows clearly that a one-site model (solvophobic and ion-exchange interactions take place simultaneously at a single type of site) has to be taken to describe the retention behaviour observed. Comparison of the CEC separation of these charged analytes with electrophoretic mobilities determined by open-tubular capillary electrophoresis shows that mainly chromatographic interactions (solvophobic and ion-exchange interactions) are responsible for the selectivity observed in CEC, while the electrophoretic migration of these analytes plays only a minor role.

The use of derivatized cyclodextrins as solubilizing agents in the preparation of macroporous polymers employed as amphiphilic continuous beds in capillary electrochromatography

Employing solubilization by complexation with CDs, new mixed-mode monolithic stationary phases for CEC and micro-LC were synthesized. Free radical copolymerization was performed in aqueous solution with a CD-solubilized hydrophobic monomer, a water-soluble crosslinker (piperazinediacrylamide), and a charged monomer (vinylsulfonic acid). Different hydrophobic methacrylate monomers (isobornyl, adamantyl, cyclohexyl, and phenyl methacrylate) were investigated. Chromatographic properties of the synthesized monoliths were studied with aqueous and nonaqueous mobile phases with hydrophobic and polar analytes. Due to the amphiphilic nature of the polymers synthesized, the elution orders obtained correspond to the RP mode and to the normal-phase mode dependent on the polarity of the mobile phase. However, observations made with polar solutes and polar mobile phase can only be explained by a mixed-mode retention mechanism. The influence of the total monomer concentration (%T) on the chromatographic properties and on the specific permeability was elucidated. Run-to-run, day-to-day, and capillary-to-capillary reproducibility of electroosmotic mobility and retention factors were determined. Comparison of retention data with those of a commercial octadecyl silica gel HPLC column reveals that the methylene selectivity of the monolithic capillaries prepared in this study is very similar to that of routinely used octadecyl silica gels.

Study of the complexation of different methacrylates with cyclodextrins employing a combination of electrophoretic, chromatographic, and NMR-spectroscopic methods

The present study describes the application of capillary electromigration techniques; CEC and micellar EKC (MEKC), and the application of spectroscopic methods; 1H NMR and 1H NOESY spectroscopy to investigate interactions between CDs (alpha-CD, statistically methylated beta-CD, hydroxypropyl-beta-CD, and 2-hydroxypropyl-gamma-CD) and different methacrylates (adamantyl, isobornyl, cyclohexyl, and phenyl methacrylate). It is shown that these methods complement each other. While CD-mediated MEKC is a rapid screening technique for comparing complex stabilities in aqueous media, 1H NMR chemical shift analysis provides quantitative data for very strong methacrylate-CD complexes and CD-mediated CEC provides quantitative data for complexes with lower complex forming constants. CD-mediated MEKC did not prove to be suitable for the calculation of complex forming constants. Reasons are discussed. 1H NOESY spectra were used to study spatial relationships between host and guest atoms.

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.

Evaluation of poly{-N-isopropylacrylamide-co-[3-(methacryloylamino)propyl]trimethylammonium} as a stationary phase for capillary electrochromatography

A cationic polyacrylamide-based stationary phase was synthesized and characterized for CEC. The stationary phase was prepared by radical copolymerization of N-isopropylacrylamide (NIPAAm) and (3-(methacryloylamino)propyl)trimethylammonium chloride (MAPTA), producing a copolymer attached to 5 microm porous silica particles. Fourier transform infrared spectroscopy and thermogravimetric analysis were used to characterize the copolymer. Under capillary electrochromatographic conditions, the poly-NIPAAm-co-MAPTA stationary phase showed to be stable in a wide pH range. The amino groups in the MAPTA provided an anodic EOF for CEC separation. The electroosmotic mobility changed less than 10% when the pH of the mobile phase was changed from 2 to 12. The run-to-run RSD of analyte migration time was less than 1.5% (n = 3), and the RSD of peak area was less than 3% (n = 3). The day-to-day RSD for migration time was less than 2% (n = 3). The polar groups present in the stationary phase contributed to the selectivity of the phase providing for hydrophilic interactions. In the separation of a series of neutral and acidic compounds, the stationary phase shows a mixed-mode separation mechanism with both hydrophobicity and hydrophilicity contributing to the separation.

Recent developments in the field of monolithic stationary phases for capillary electrochromatography

This review summarizes the contributions to the rapidly growing area of monolithic columns based on both silica and synthetic polymers for capillary electrochromatography and chip electrochromatography, with a focus on those published during the year 2004. A wide variety of both modified approaches to the "old" monoliths and new monoliths have been reported despite the very short period of time covered. This demonstrates that monolithic stationary phases have become a well-established format in the field of electrochromatography. The simplicity of their preparation as well as the good control over their porous properties and surface chemistries make the monolithic separation media an attractive alternative to capillary columns packed with particulate materials.