A new strategy for the preparation of core-shell MOF/Polymer composite material as the mixed-mode stationary phase for hydrophilic interaction/ reversed-phase chromatography

Novel PEI/Zein core-shell composite as mixed-mode stationary phase for high performance liquid chromatography

Based on the adhesion of polyethyleneimine (PEI), a novel PEI/zein co-modified core-shell stationary phase (PEI/Zein@SiO2) was prepared by doping zein to form a composite modification layer. The stationary phase achieved effective separation of nucleosides, bases and antibiotics in hydrophilic interaction mode on account of the hydrophilic groups of composite coating. With the hydrophobicity of zein, the flavones could be separated in reversed-phase mode. In short, the separation and analysis of hydrophilic/hydrophobic compounds were accomplished excellently by the PEI/Zein@SiO2 column with mixed double mode. The prepared chromatographic stationary phase not only avoided the dissolution of zein, but also covered the strong adsorption of some analytes caused by silica hydroxyl groups on the surface of silica spheres. The morphological structure and specific surface area of the material were reflected by various characterization techniques. Hydrophilic/hydrophobic compounds were used as tested analytes to research separation performance and retention mechanisms of PEI/Zein@SiO2 column. The stability and reproducibility of the PEI/Zein@SiO2 stationary phase were satisfied. Therefore, the modification of zein could improve the separation selectivity of stationary phase effectively for complex samples, which had the potential to be one of the significant potential application materials in stationary phase packing.

Recent advances in chiral liquid chromatography stationary phases for pharmaceutical analysis

Chirality is a common phenomenon in nature. Different enantiomers of chiral drug compounds have obvious differences in their effects on the human body. Therefore, the separation of chiral drugs plays an extremely important role in the safe utilization of drugs. High-performance liquid chromatography (HPLC) is an effective tool for the separation and analysis of compounds, in which the chromatographic packing plays a key role in the separation. Chiral pharmaceutical separation and analysis in HPLC rely on chiral stationary phases (CSPs). Thus, various CSPs are being developed to meet the needs of chiral drug separation and analysis. In this review, recent developments in CSPs, including saccharides (cyclodextrin, cellulose, amylose and chitosan), macrocycles (macrocyclic glycopeptides, pillar[n]arene and polyamide) and porous organic materials (metal-organic frameworks, covalent organic frameworks, and porous organic cages), for pharmaceutical analysis in HPLC were summarized, the advantages and disadvantages of various stationary phases were introduced, and their development prospects were discussed.

Preparation of three regioisomeric ionic liquid stationary phases and investigation of their retention behavior

The structural properties of ionic liquid stationary phases have a considerable effect on their separation selectivity. However, the difference of the chromatographic retention behavior of different regioisomeric ionic liquid stationary phases has rarely been investigated. In this study, three regioisomeric ionic liquid silane reagents were prepared by photoinitiated ene-click chemistry and bonded to silica by one-pot method to fabricate three new stationary phases (Sil-C2Im-C8, Sil-C6Im-C4, and Sil-C9Im-C1). All three stationary phases showed promising retention repeatability and efficiency. The retention behavior of the three stationary phases was investigated under various chromatographic conditions. The retention mechanism was further investigated by the linear energy solvation relationship and Van't Hoff plots. The stationary phases exhibited mixed-mode retention mechanisms. The π-π, hydrogen bonding, ion-exchange, and hydrophilic interactions with analytes were the weakest when the imidazole ions were embedded in the innermost part of the alkyl chains, while the interactions were the strongest when the imidazole ions were embedded in the middle of the alkyl chains. The three stationary phases provided great but different separation performances towards nucleosides, nucleobases, aromatic acids, alkyl benzenes, and polycyclic aromatic hydrocarbons due to the influence of imidazole ion position.

Effect of phenyl numbers in polyphenyl ligand on retention properties of aromatic stationary phases

Aromatic phase, as one type of reversed-phase stationary phases, shows complementary selectivity to the n-alkyl counterparts especially for certain challenging separation tasks. However, effect of phenyl numbers in aromatic ligands on retention behaviors has rarely been addressed compared with the alkyl stationary phases. To illustrate the issue, a series of polyphenyl stationary phases were facially prepared via the coupling chemistry of isocyanate with amine, including aniline (π¹), 4-aminobiphenyl (π²), 4-amino-p-terphenyl (π³) and [1,1':4',1'':4'',1'''-quaterphenyl]-4-amine (π⁴), respectively. The chromatographic behaviors of the new stationary phases as well as the traditional C18 were systematically compared in terms of retention mode, hydrophobic and aromatic selectivity, shape selectivity and π-π interaction by various analytes, including alkylbenzenes, polycyclic aromatic hydrocarbons congeners and substituted benzenes with electron-withdrawing groups. Due to the homologous structure of four polyphenyl ligands, the hydrophobic selectivity, aromatic selectivity and shape selectivity of stationary phases increase with phenyl numbers in the bonded polyphenyl ligands, whereas the increment becomes insignificant from U-π³ to U-π⁴. This phenomenon is explained by the insertion degree of analytes in the polyphenyl ligand brushes. Compared with the homemade C18, the polyphenyl phases indicate insignificant changes of shape selectivity with temperature. Notably, the new polyphenyl phases demonstrate the great selective separation towards the electron-deficient compounds through the π-π interaction. These findings make up for the understanding of the retention behavior of aromatic stationary phases.

Understanding the Working Mechanism of the NovelHKUST-1@BPS Composite Materials as Stationary Phases for Liquid Chromatography

Composite materials have been used based on coordination polymers or microporous metal-organic frameworks (MOFs) combined with mesoporous matrices for adsorption-related techniques, which enable outflanking some adverse phenomena manifested during pristine components operation and enhance the performance and selectivity of the resulting materials. In this work, for the first time, the novel HKUST-1@BPS composites synthesized by the microwave-assisted (MW) technique starting from microporous HKUST-1 (Cu₃(btc)₂) MOF and biporous silica matrix (BPS) with bimodal mesopore size distribution were comparatively studied as materials for liquid-phase adsorption techniques utilizing the high-performance liquid chromatography (HPLC) method and benzene as a model adsorbate. It was established that the studied HKUST-1@BPS composites can function as stationary phases for HPLC, unlike the pristine HKUST-1 and bare BPS materials, due to the synergetic effect of both components based on the preliminary enhanced adsorbate mass transfer throughout the silica mesopores and, subsequently, its penetrating into HKUST-1 micropores. The suggested mechanism involves the initial deactivation of open metal Cu²⁺ sites in the HKUST-1 framework structure by isopropanol molecules upon adding this polar component into the mobile phase in the region of the isopropanol concentration of 0.0 to 0.2 vol.%. Thereafter, at the medium range of varying the isopropanol concentration in the eluent of 0.2 to 0.3 vol.%, there is an expansion of the previously inaccessible adsorption centers in the HKUST-1@BPS composites. Subsequently, while further increasing the isopropanol volume fraction in the eluent in the region of 0.3 to 5.0 vol.%, the observed behavior of the studied chromatographic systems is similar to the quasi-normal-phase HPLC pattern. According to the obtained thermodynamic data, benzene adsorption into HKUST-1 micropores from solutions with a vol.% of isopropanol in the range of 0.4 to 5.0 follows the unique entropy-driven mechanism previously described for the MIL-53(Al) framework. It was found that HKUST-1 loading in the composites and their preparation conditions have pronounced effects on their physicochemical properties and adsorption performance, including the adsorption mechanism.

The Current Status of Metal-Organic Frameworks (MOFs) for Use in Liquid Chromatography

Metal-organic frameworks (MOFs) are self-assembled combinations of metals and inorganic ligands that result in a relatively young class of highly ordered, porous materials. Because of the number of structural and chemical possibilities, high surface area, controlled pore volume, and favorable thermal properties, MOFs are being investigated in several fields, including chromatography. Because of the enormous interest in a 2018 “Column Watch” article on the subject and the high level of research in the field, this article explores recent (2019–present) activity specifically toward the application of MOFs for liquid chromatography (LC).

Hydrogen isotope separation on nickel-containing metal-organic framework@gama-alumina-based multicomponent composite packed column: Identification of individual role of each component

A new optimized multicomponent composite, Na₂Cr₂O₇/Na₂CrO₄/NaCl/MOF-74(Ni)@γ-Al₂O₃ (S₁/S_2/S₃/MOF-74(Ni)@γ-Al₂O₃. Na₂Cr₂O₇ = S₁, Na₂CrO₄ = S₂, NaCl = S₃), was prepared and used as a gas chromatography stationary phase for the separation of H₂ and D₂ isotopes. Under the optimal chromatographic separation conditions, the resolution of the packed column for the separation of H₂/D₂ was 2.87, and the separation time was 7.15 min at 77 K. The control experiments showed that in the multicomponent composite, MOF-74(Ni), which has a chemical affinity quantum sieving effect, played a major role in the separation of H₂/D₂. As the support of MOF-74, γ-Al₂O₃ enhanced the mechanical strength of MOF-74 and reduced the gas resistance. The presence of Na₂CrO₄ in the column increased the H₂/D₂ separation resolution, while the presence of NaCl reduced the separation time, produced more symmetrical and narrow chromatographic peaks of Gaussian distribution. Furthermore, by optimizing the ratio of NaCl and Na₂CrO₄, (S₂/S₃/MOF-74(Ni)@γ-Al₂O₃) composite with a NaCl/Na₂CrO₄ mass ratio of 0.7:1 was synthesized and used to realize the high-resolution separation of H₂/D₂ (R = 2.56) with a short separation time (t = 5.91 min). Both composites also showed excellent repeatability/reproducibility for separation.

An alternative strategy to construct uniform MOFs-Grafted silica core-shell composites as mixed-mode stationary phase for chromatography separation

The preparation of the metal-organic frameworks (MOFs)@silica core-shell microspheres as the stationary phases mainly relied on the method of electrostatic interaction between the metal ions of MOFs and the silanol groups. Herein, the ligands of MOFs were preferentially modified to the surface of silica as connection points and seed crystals to connect or form the MOFs. In this way, the evenness of the MOFs particles on the silica surface was effectively improved, and the prepared composites possessed excellent reproducibility and stability, including acid-base stability. The relative standard deviation of the retention time for repeatability ranged from 0.1% to 0.26% and for stability retention time from 0.3% to 0.6%. Compared with commercial columns, the prepared stationary phase showed enhanced separation selectivity for separation of both hydrophilic and hydrophobic compounds containing alkaloids, nucleosides, antibiotics and alkylbenzenes, etc. The obtained column was used as a matrix for fast separation and analysis of antibiotics in actual samples. In short, the composites showed superior reproducibility, stability and satisfactory separation performance towards a variety of compounds in the studied conditions. It also provided another way to improve the evenness of MOFs particles on the surface of silica and enhance the stability of them under polar conditions.

High-Efficiency and Versatile Approach To Fabricate Diverse Metal-Organic Framework Coatings on a Support Surface as Stationary Phases for Electrochromatographic Separation

A large number of metal-organic frameworks (MOFs) have exhibited increasingly wide utilization in the field of chromatographic separation owing to their intrinsic fascinating properties. However, the previous studies on supported MOF coating-based chromatographic separation focused only on the synthesis and chromatographic performance of a certain kind of supported MOF coatings as stationary phases using the multiple-step, complicated, and time-consuming modification methods, which severely impeded the widespread application of MOFs in separation science. Herein, a high-efficiency and versatile methodology toward diverse supported MOF coating-based stationary phases to achieve high-efficiency chromatographic separation was first reported based on the immobilized cysteine (Cys)-triggered in situ growth (ICISG) strategy. As a proof-of-concept demonstration, four types of MOF crystals consisting of different ligands and metal ions (Zn²⁺, Cu²⁺, Fe³⁺, and Zr⁴⁺) were conveniently and firmly grown on a Cys-modified capillary using the ICISG strategy and employed as the functional stationary phase for electrochromatographic separation. A broad variety of neutral, acidic, and basic compounds were all separated in a highly efficient manner on the developed four MOF-coated columns. The maximum theoretical plate number for Cys-MIL-100(Fe)@capillary was close to 1.0 × 10⁵ plates/m, and the intraday, interday, and column-to-column repeatabilities of retention times for the four MOF-modified columns were all less than 5.25%. More interestingly, the diversified separation performance of the developed MOF-coated columns indicated that the preparation strategy and the skeletal structure of the MOF coating-based stationary phases have a significant influence on the electrochromatographic separation performance and column capacity. Benefiting from the strong universality and high applicability of the developed ICISG strategy, the present study provides an effective route to facilitate the design and fabrication of novel functional MOF-based chromatographic stationary phases.

Design and evaluation of novel MOF-polymer core-shell composite as mixed-mode stationary phase for high performance liquid chromatography

A general method was developed for preparing a metal-organic framework-polymer composite coated silica core-shell stationary phase. Silica microspheres were comodified with metal-organic framework and polyvinylpyrrolidone rather than the in situ method of silica modification by original metal-organic framework particles. Metal-organic framework particles and polyvinylpyrrolidone on silica surface were beneficial to suppress silanol activity and enhance composite material tolerance, as well as increasing the water compatibility of the original metal-organic framework-based stationary phases. The stationary phase exhibited superior hydrophilic and hydrophobic performance in terms of separation for various analytes including seven alkaloids, six sulfonamides, five antibiotics, and five polycyclic aromatic hydrocarbons. Moreover, the composite material also showed excellent stability with the relative standard deviation of the retention time of 0.4 to 0.7%. The separation performance with real samples proved that the column has good practical application potential. In summary, the poposed method provides a general way for preparing metal-organic framework-polymer composite material and changed the current status of original metal-organic framework particles modified silica as a single mode chromatographic stationary phase.