Preparation and evaluation of 2-methylimidazolium-functionalized silica as a mixed-mode stationary phase for hydrophilic interaction and anion-exchange chromatography

A C4-modified bipyridinium multi-mode stationary phase for reversed phase, hydrophilic interaction and ion exchange chromatography

A novel C4-modified bipyridinium stationary phase (Sil-DPC4) was prepared and characterized by elemental analysis (EA) and Fourier transform infrared spectrometry (FT-IR) and further investigated for multi-mode liquid chromatography. The chromatographic performances of Sil-DPC4 were evaluated by reversed-phase chromatography using polycyclic aromatic hydrocarbons (PAHs), phenylamines and phenols, hydrophilic interaction chromatography using nucleosides and nucleobases, and ion exchange chromatography using inorganic ions and organic ions. The effects of the acetonitrile content, salt concentration and pH value of the mobile phase on the retention of Sil-DPC4 were also investigated. Sil-DPC4 showed multiple retention mechanisms including π-π, hydrophobic and electrostatic interactions for PAHs, phenylamines and phenols compared with a dipyridine modified silica stationary phase (Sil-DP) and C18 in RPLC, faster separation for nucleosides and nucleobases compared with Sil-DP, and higher hydrophilicity than HILIC in HILIC, and stronger retention and better separation ability for inorganic ions and organic ions in comparison to Sil-DP in IEC. Besides, Sil-DPC4 was used successfully to detect iodide in artificial seawater and had the potential to analyze radionuclide iodine-131 in seawater. In conclusion, multiple retention mechanisms of Sil-DPC4 could make it have potential applications in complex samples.

Dendritic organic molecular gel coating with molecular shape selectivity and its application in selective separation by liquid chromatography

Dendritic organic molecular gels are a promising class of three-dimensional network compounds. Here, we have synthesized a new type of dendritic organic molecular gel stationary phase (SiO₂-G3) by using benzyl alcohol as raw material and dimethyl 5-hydroxyisophthalate as growth unit to synthesize a third-generation organic molecular gel G3, which grafted onto the silica surface by cyanogen chloride (CC). The developed stationary phase not only exhibits high molecular shape selectivity but also has a RPLC/HILIC/IEC mixed-mode characteristic for HPLC due to the ordered structure, the multiple strong π-π stacking interactions and the introduction of a hydrophilic triazine fraction during the grafting process. Compared with a commercial C₁₈ column, the developed column exhibited flexible selectivity, enhanced separation performance and excellent separation of monosubstituted benzene, polycyclic aromatic hydrocarbons (PAHs), positional isomers, nucleosides and nucleobases, benzoic acid and aniline compounds. In addition, the new column provided baseline separation of polycyclic aromatic hydrocarbon contaminants in Yellow River water, verifying its potential for application in the analysis of real samples.

Supported Ionic Liquids Used as Chromatographic Matrices in Bioseparation-An Overview

Liquid chromatography plays a central role in biomanufacturing, and, apart from its use as a preparative purification strategy, either in biopharmaceuticals or in fine chemicals industries, it is also very useful as an analytical tool for monitoring, assessing, and characterizing diverse samples. The present review gives an overview of the progress of the chromatographic supports that have been used in the purification of high-value products (e.g., small molecules, organic compounds, proteins, and nucleic acids). Despite the diversity of currently available chromatographic matrices, the interest in innovative biomolecules emphasizes the need for novel, robust, and more efficient supports and ligands with improved selectivity. Accordingly, ionic liquids (ILs) have been investigated as novel ligands in chromatographic matrices. Given herein is an extensive review regarding the different immobilization strategies of ILs in several types of supports, namely in silica, Sepharose, and polymers. In addition to depicting their synthesis, the main application examples of these supports are also presented. The multiple interactions promoted by ILs are critically discussed concerning the improved selectivity towards target molecules. Overall, the versatility of supported ILs is here considered a critical point to their exploitation as alternatives to the more conventional liquid chromatographic matrices used in bioseparation processes.

In situ photoinitiated fabrication of phosphorylcholine-functionalized polyhedral oligomeric silsesquioxane hybrid monolithic column for mixed-mode capillary electrochromatography

A phosphorylcholine-functionalized POSS hybrid monolithic column was synthesized via UV curing. It exhibits hydrophilic interaction and weak cation exchange chromatography retention mechanism for the separation of typical polar and charged compounds.

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.

Octadecylimidazolium ionic liquids-functionalized carbon dots and their precursor co-immobilized silica as hydrophobic chromatographic stationary phase with enhanced shape selectivity

In this work, 1-vinyl-3-octadecylimidazolium bromide ionic liquids ([C₁₈VIm]Br) and their derived carbon dots (ImC₁₈CDs) were prepared, [C₁₈VIm]Br and ImC₁₈CDs were grafted on the silica to obtain Sil-ImC₁₈ and Sil-ImC₁₈CDs, respectively, and they were also co-grafted on silica which named Sil-ImC₁₈/CDs. Compared with Sil-ImC₁₈ and Sil-ImC₁₈CDs columns, Sil-ImC₁₈/CDs column exhibited enhanced selectivity for separation of tetracyclic/tricyclic polycyclic aromatic hydrocarbon (PAH) isomers, and butylbenzene isomers in reversed-phase liquid chromatography, which may be due to the synergistic effect between ImC₁₈CDs and [C₁₈VIm]Br, the π-π interaction between imidazolium and analytes, etc. Meanwhile, the retention behavior of Sil-ImC₁₈/CDs was further evaluated and compared with the commercial C₁₈ column using different classes of analytes, including standard test mixtures of Tanaka, Engelhardt, SRM869b, SRM870. The results demonstrated that co-grafted column exhibited superior separation performance. And this column was applied to determine the contents of calycosin-7-glucoside, ononin, calycosin and formononetin in the extract of Radix Astragali, which were found that the concentration was 0.25 mg mL^-1, 0.15 mg mL^-1, 0.13 mg mL^-1 and 0.30 mg mL^-1, respectively.

Static vs. Dynamic Electrostatic Repulsion Reversed Phase Liquid Chromatography: Solutions for Pharmaceutical and Biopharmaceutical Basic Compounds

Many efforts have been made to separate basic compounds, which are challenging to resolve in reversed phase liquid chromatography. In this process, they are strongly retained and the peak shape undergoes significant distortion. The principal origin of this has been identified with the non-negligible interaction with residual deprotonated silanols. Consequently, all solutions that efficiently shield silanols are being sought. This review is an upgrade on the use of the electrostatic repulsion reversed phase (ERRP) approach: retention of bases, in protonated form, can be achieved by modulating the charge repulsion caused by the presence of positive charges in the chromatographic system. This study successfully (i) introduced fixed positive charges in the structure of stationary phases, (ii) used cationic and hydrophobic additives in the mobile phase, and (iii) used the ERRP-like approach employed at the preparative level for peptide purification.

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.

Surface-bonded amide-functionalized imidazolium ionic liquid as stationary phase for hydrophilic interaction liquid chromatography

The amide group modified silica materials are popular stationary phases for hydrophilic interaction liquid chromatography (HILIC). Meanwhile, surface-confined imidazolium ionic liquids (ILs) have been proved to be useful HILIC stationary phases and possess many unique properties. In this study, the synthesis of an amide-functionalized imidazolium IL was conducted which was then bonded onto silica surface to obtain a novel imidazolium-embedded amide stationary phase for HILIC. The combination of the amide group and imidazolium IL moiety might bring some advantages in selectivity or retention and therefore extended its applications. After characterizing the prepared IL and the resulting modified silica materials, the chromatographic performance and separation selectivity of the packed column were evaluated and compared with a commercial amide column. Then, the retention behavior was investigated through observing the retention factors at different chromatographic conditions using a wide range of compounds. Exceptionally, the prepared amide IL column exhibited superior separation performance towards complex samples such as flavonoids mixture, soybean flavonoids and human urine. All the results indicated that the novel amide IL column possessed an anion-exchange/HILIC mixed-mode retention mechanism and could be useful in the sample analysis as a promising candidate for HILIC stationary phase.

Hydrophobic structure of hairpin ten-ring pyrrole-imidazole polyamides enhances tumor tissue accumulation/retention in vivo

To examine the hydrophobic structure of PI polyamides on tumor accumulation in vivo, PI polyamide-fluorescein conjugates 1-5 with the distinct number of N-methylimidazole (Im) units were synthesized. There existed an inverse relationship between the Im unit number of the compounds and their hydrophobicity. Compound 1 with one Im unit and 3 with three Im units accumulated and retained preferentially in tumor tissues compared to 5 with five Im units. These results suggest the importance of a PI polyamide's primary structure, which partly contributes to its hydrophobic property, on its accumulation and/or retention in tumor tissues in vivo.

Ionic liquid-functionalized graphene quantum dot-bonded silica as multi-mode HPLC stationary phase with enhanced selectivity for acid compounds

A novel IL/GQD/SiO₂ stationary phase for reversed-phase/normal-phase/ionic exchange and hydrophilic interaction liquid chromatography.

Controllable preparation of a hydrophilic/ion-exchange mixed-mode stationary phase by surface-initiated atom transfer radical polymerization using a mixture of two functional monomers

Mixed-mode chromatographic stationary phases require functionalization with at least two functional groups to yield multiple interactions with analytes. Departing from reported methods, a mixture of two different monomers, glycidyl methacrylate and 2-dimethylaminoethylmethacrylate, was grafted onto the surface of silica by a one-step surface-initiated atom transfer radical polymerization to prepare a novel hydrophilic interaction/anion-exchange mixed-mode chromatographic stationary phase. The grafted amounts of functional groups were controlled via varying the ratio of monomers in the polymerization system. The influences of water content, salt concentration and pH in the mobile phase were investigated to illustrate the mixed interaction between the stationary phase and analytes. The retention of various solutes on three columns, especially acidic and basic solutes, showed an obvious dependence on the ratio of the two monomers in the polymerization system. The results indicated that the strategy proposed in this work was beneficial to develop various types of mixed-mode chromatographic stationary phases with adjustable selectivity to meet the needs of complex samples. Finally, the column was successfully employed in the isolation of melamine in liquid milk.