Fabrication and evaluation of tetraazahexaphenylmacrocycle-bonded stationary phase with multiple retention mechanisms

Construction of MOFs@COFs composite material as stationary phase for efficient separation of diverse organic compounds

BACKGROUND

The development of efficent chromatographic stationary phases (SP) with mixed-mode or multiple interactions in high-performance liquid chromatography (HPLC) for the separation of complex samples is a challenging task. Metal organic frameworks (MOFs)-based SP can provide desired multiple interactions and enable the separation of a diverse range of solutes, but have limitations of low column efficiency and poor stability.

RESULTS

Herein, the hybrid MOFs@Covalent organic frameworks (COFs) materials were used as SP in HPLC due to their synergistic structural features. The SiO2@NH2-UiO-66@CTF SP was synthesized by integration of NH2-UiO-66 and covalent triazine framework (CTF) onto silica surface. Due to the unique structure of SiO2@NH2-UiO-66@CTF with hierarchical-pores, this column showed higher column efficiency (up to 49,369 plates m-1 for alkylbenzenes) than the reported columns packed with MOFs-based SP. Owing to the Zr4+-N coordination bonding between CTF and NH2-UiO-66, the structural stability of SiO2@NH2-UiO-66@CTF can be improved. Furthermore, this new column exhibited remarkable column stability with relative standard deviation of retention time of <0.40% after 400 injections. With the combined advantages of multifunctional properties, high column efficiency, and good stability, SiO2@NH2-UiO-66@CTF SP showed excellent selectivity for the separation of a variety of hydrophobic, aromatic, heteroatomic, and hydrophilic analytes.

SIGNIFICANCE AND NOVELTY

This work not only offers a promising SP with multiple retention mechanisms for HPLC, but also provides an efficient strategy for development of high column efficiency MOFs-based SP with good stability. Moreover, the MOFs@COFs hybrid materials were expanded in application area through this study, and the research results can also afford the foundation for further explore its structural characteristics.

Determination of the phase ratio of a dehydroabietic-acid-bonded silica-gel chromatographic stationary phase and its effect on separation thermodynamics

Rosin-based chromatographic columns are widely used for separation purposes, but, to date, their phase ratios (Φ) have been imprecisely measured. This affects the understanding of their separation mechanism and the calculation of related thermodynamic parameters. In this study, a stationary phase was synthesized by bonding dehydroabietic acid (DA) to silica gel (Si-DO) and applied for reversed-phase liquid chromatography. The distribution coefficient (Kdm) of methyl dehydroabietate (MD), which has the same structure as the bonded phase of Si-DO, was used as a surrogate for the determination of the equilibrium coefficient (K) of Si-DO, and the Kdm values of MD in different mobile phases were measured and compared with the K values of Si-DO. It was found that the phase ratio of Si-DO varied with mobile phase composition and temperature, as shown by the Φ values: 0.039-0.122 for the methanol/water system and 0.051-0.116 for the acetonitrile/water system; in addition, the a indices were 0.552-0.757 and 0.564-0.674, respectively. The Kdm of MD was closer to the K of Si-DO than those of other surrogate models, including the octanol-water and octane-mobile phase partition coefficients. In addition, the thermodynamic parameters (ΔG°, ΔH°, and ΔS°) of n-alkylbenzenes on Si-DO were negative, indicating a spontaneous and enthalpy-driven separation process. Overall, the phase ratio of rosin-based columns is crucial for accurate thermodynamic analysis and interpretation of the separation mechanism. Finally, the MD surrogate model allows the estimation of phase ratio of Si-DO and other similar columns, providing a novel method for measuring the phase ratio of rosin-based columns and providing a validated concept and methodology for determining the phase ratios of HPLC columns.

Preparation and application of a novel imine-linked covalent organic framework@silica composite for reversed-phase and hydrophilic interaction chromatographic separations

The composites of covalent organic frameworks (COFs) and silica gel have been considered to be promising chromatographic separation materials due to the distinct advantages such as large specific surface area, good mechanical strength and high porosity. In the present study, a novel imine-linked COF@silica composite was prepared by in-situ growth of 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 2,5-dihydroxyterephthalaldehyde (DHTA) monomers on the surface of aminated silica gel (SiO2-NH2). The successful surface-modification of TAPT-DHTA-COF distinctly enhanced the separation selectivity and efficiency of SiO2-NH2. Multiple types of analyte-stationary phase interactions contributed to the selective retention of structurally similar analytes. The designed TAPT-DHTA-COF@SiO2 was observed to effectively separate hydrophobic phenyl ketones, phthalate esters and steroid hormones. Moreover, the polar amino and hydroxyl groups of TAPT-DHTA-COF facilitated the selective determination of hydrophilic nucleosides/bases. The kinetic performance and thermodynamic behavior of TAPT-DHTA-COF@SiO2 column were particularly explored. It was found that column efficiency was mainly affected by the mass transfer resistance, and the retention of nucleosides/bases on the TAPT-DHTA-COF@SiO2 column was temperature dependent. The developed versatile TAPT-DHTA-COF@SiO2 column was finally applied for detecting environmental hormones as well as water-soluble nicotinamide in real samples. In summary, the potential application of TAPT-DHTA-COF@SiO2 composite material for liquid chromatographic separations was first explored and verified. The TAPT-DHTA-COF@SiO2 was proved to be a promising chromatographic separation material.

Chromatographic separation performance of silica microspheres surface-modified with triazine-containing imine-linked covalent organic frameworks

In this work, 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and 1,3,5-tris(4-formylphenyl)benzene (TFPB) were used as monomers to construct a triazine-containing imine-linked covalent organic framework (COF), which was then bonded onto the surface of aldehydized silica (SiO₂-CHO), and finally a COF@silica composite material (TAPT-TFPB COF@SiO₂) was successfully prepared. The chromatographic separation performance of SiO₂-CHO, TAPT-TFPB COF@SiO₂ and TAPT-TFPB COF@SiO₂/SiO₂-CHO (80/20, mass ratio) was evaluated and compared. It was found that separation efficiency was obviously enhanced by adding an appropriate amount of SiO₂-CHO into TAPT-TFPB COF@SiO₂. The obtained TAPT-TFPB COF@SiO₂/SiO₂-CHO showed more favorable separation ability than SiO₂-CHO and TAPT-TFPB COF@SiO₂. Various aromatic compounds including alkylbenzenes, polycyclic aromatic hydrocarbons, environmental endocrine disruptors, foodborne stimulants and phenyl ketones were effectively separated on the TAPT-TFPB COF@SiO₂/SiO₂-CHO column in reversed phase chromatography mode. The silica microspheres surface-modified with triazine-containing imine-linked COFs proved to be a new type of promising chromatographic packing materials.

Separation of Panax notoginseng saponins on modified rosin ester-bonded silica stationary phase and its mechanism

A column prepared using a unique three-membered phenanthrene skeleton of rosin has complementary selectivity to a C18 column for some separation tasks. In this study, propylene pimaric acid (16-hydroxyethyl acrylate-34-n-butyl) ester (BRB) and propylene pimaric acid (16-hydroxyethyl acrylate-34-dodecyl) ester (BRLA) were used as functional ligands to prepare two novel stationary phases, namely BRB@SiO₂ and BRLA@SiO₂, through a "thiol-ene" click chemistry reaction. The characterization results of Fourier transform infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption-desorption measurements, and contact angle tests showed that the BRB@SiO₂ and BRLA@SiO₂ stationary phases were successfully prepared. In addition, the performance of the columns was evaluated using the Tanaka test and hydrophobic subtraction model, which showed that the stationary phases exhibited typical reversed-phase chromatography performance and good hydrophobicity, hydrophobic selectivity, and steric selectivity. The changes in the retention of Panax notoginseng saponins on a column under different chromatographic conditions (acetonitrile content, flow rate, and column temperature) were investigated. The separation effect of BRB@SiO₂ and BRLA@SiO₂ columns on P. notoginseng saponins was better than that of the C18 column and the BRLA@SiO₂ column could replace the C18 column for the detection of P. notoginseng saponins.