A Hydrophilic Strong Anion-Exchange Hybrid Monolith for Capillary Liquid Chromatography

A hydrophilic strong anion-exchange monolithic hybrid column was prepared by in-capillary coating 5-µm bare silica particles with the copolymers of methacryloxyethyltrimethyl ammonium chloride and pentaerythritol triacrylate in the presence of a porogen consisting of water, methanol, and cyclohexanol. The composition of the porogen and the concentration of the monomers were investigated and selected. The resulting column was characterized. The column had an uniform pore structure and could withstand a back pressure up to 3500 psi. Its permeability was comparable to that of packed columns and the swelling-shrinking behaviour negligible. Its hydrophobicity could be suppressed at acetonitrile concentrations above 40% (v/v) and the minimum theoretical plate height was about 10 µm for BrÑ. The column-to-column relative standard deviations (RSDs) were 2.2% and 3.5% (n = 9) and the batch-to-batch RSDs were 2.4% and 5.5% (n = 3) for k and H values, respectively. The column exhibited a remarkable performanceforthe separation of inorganic anions, organic weak acids, phenols, and nucleotides.

The separation characteristics and performance evaluation of the silica-based poly(pentabromostyrene) stationary phase in capillary electrochromatography

A mixed-mode capillary column packed with silica-based poly(pentabromostyrene) particles (denoted as SiO₂@pPBS) was prepared and applied to capillary electrochromatography (CEC) separation. With the presence of benzene rings and bromine atoms in polymer chains, the SiO₂@pPBS column provides a reversed-phase/hydrophilic mixed-mode retention mechanism owing to hydrophilic, hydrophobic and π-π interactions between the stationary phase and various analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons, nucleosides, phenols and anilines. In CEC mode, the separation behavior of charged solutes is not only related to the interaction with the stationary phase, but also influenced by electrophoretic effects, which may lead to different selectivities compared to high performance liquid chromatography (HPLC). A column efficiency of up to 1.22 × 10⁵ N m^-1 was achieved for p-chloroaniline. Besides, the RSDs of retention time of anilines for run to run (n = 5), day to day (n = 5) and column to column (n = 3) were all less than 4.4%. Finally, the SiO₂@pPBS capillary column was applied to the separation of coking wastewater with satisfactory results. All the results demonstrated that the SiO₂@pPBS capillary packed column with RP/HILIC mixed-mode has great application potential.

A reversed-phase/hydrophilic bifunctional interaction mixed-mode monolithic column with biphenyl and quaternary ammonium stationary phases for capillary electrochromatography

A novel RPLC/HILIC mixed-mode monolithic column with biphenyl and quaternary ammonium stationary phases is synthesized for capillary electrochromatography.

Recent advances in mixed-mode chromatographic stationary phases

Mixed-mode phases have become very popular in the last decade, and the number of new mixed/multi-mode sorbents is growing fast. Unlike single-mode stationary phases, perfectly suited for the separation of the analytes possessing similar physicochemical properties, for instance reversed-phase chromatography for hydrophobic solutes, mixed-mode sorbents providing multimodal interactions can render better separation selectivity for complex mixtures of solutes differing significantly in their physicochemical characteristics. The most frequent modern mixed-mode stationary phases are di/tri-mode sorbents embracing the following interactions, hydrophobic, electrostatic (coulombic), and hydrophilic. According to their structures, it is possible to distinguish silica-based, polymer-based, hybrid, and monolithic mixed-mode stationary phases. Herewith, newly synthesized mixed-mode sorbents developed within the last two and half years are categorized, discussed, and summarized. The main attention is devoted to the description of the synthetic routes and characterization methods applied for the new stationary phases.

Rapid Separation and Determination of Metronidazole Benzoate and Other Antiprotozoal Drugs by Pressurized Capillary Electrochromatography

A novel method for the rapid separation and determination of five polar 5-nitroimidazoles in water from different sources by pressurized capillary electrochromatography has been developed. Compared with the gradient elution mode and the packed nonpolar columns which were usually utilized for the separation of 5-nitroimidazoles, a simple isocratic elution mode and low-cost homemade polar molecularly imprinted polymer monolith were used in the experiment. Electrochromatographic conditions such as pH of buffer, organic modifier, concentration of buffer, and separation voltage were optimized. At 320 nm UV wavelengths, the five 5-nitroimidazoles could be baseline-separated rapidly in less than 11 min with the separation voltage of +20 kV in 10 mmol/L sodium dihydrogen phosphate-disodium hydrogen phosphate buffer solution (pH 4.82) containing 30% acetonitrile. Under the optimum conditions, the linear ranges of the metronidazole, secnidazole, tinidazole, ornidazole, and metronidazole benzoate were 0.50–100.00, 0.50–100.00, 0.80–500.00, 0.80–100.00, and 5.00–500.00 μg/mL, respectively, and the detection limits of these analytes were 0.11–0.73 μg/mL. Column efficiencies of 43 000, 36 000, 34 000, 14 000, and 29 000 plates/m were obtained for metronidazole, secnidazole, tinidazole, ornidazole, and metronidazole benzoate, respectively. The recoveries of different water samples were about 85.0–95.8%. Additionally, the proposed method has been successfully applied to the rapid separation of 5-nitroimidazoles in the locally available pure milk sample by simple pretreatment.