pH-dependent selective separation of acidic and basic proteins using quaternary ammoniation functionalized cysteine-zwitterionic stationary phase with RPLC/IEC mixed-mode chromatography

Highly sensitive and accurate measurement of underivatized phosphoenolpyruvate in plasma and serum via EDTA-facilitated hydrophilic interaction liquid chromatography-tandem mass spectrometry

Phosphoenolpyruvate (PEP) is an essential intermediate metabolite that is involved in various vital biochemical reactions. However, achieving the direct and accurate quantification of PEP in plasma or serum poses a significant challenge owing to its strong polarity and metal affinity. In this study, a sensitive method for the direct determination of PEP in plasma and serum based on ethylenediaminetetraacetic acid (EDTA)-facilitated hydrophilic interaction liquid chromatography-tandem mass spectrometry was developed. Superior chromatographic retention and peak shapes were achieved using a zwitterionic stationary-phase HILIC column with a metal-inert inner surface. Efficient dechelation of PEP-metal complexes in serum/plasma samples was achieved through the introduction of EDTA, resulting in a significant enhancement of the PEP signal. A PEP isotopically labelled standard was employed as a surrogate analyte for the determination of endogenous PEP, and validation assessments proved the sensitivity, selectivity, and reproducibility of this method. The method was applied to the comparative quantification of PEP in plasma and serum samples from mice and rats, as well as in HepG2 cells, HEK293T cells, and erythrocytes; the results confirmed its applicability in PEP-related biomedical research. The developed method can quantify PEP in diverse biological matrices, providing a feasible opportunity to investigate the role of PEP in relevant biomedical research.

Preparation of two zwitterionic polymer functionalized stationary phases and comparative evaluation under mixed-mode of reversed phase/ hydrophilic interaction/ion exchange chromatography

Zwitterions are a promising choice to prepare separation materials because of their hydrophilicity and biocompatibility. We described the preparation of two zwitterionic polymer functionalized stationary phases and evaluation under mixed-mode chromatography. A zwitterionic monomer, S-(4-vinylbenzyl) cysteine (SVC), was synthesized and bonded to silica via reversible addition fragmentation chain transfer (RAFT) polymerization to afford a zwitterionic stationary phase, Sil-SVC. A hydrophobic monomer, N-(4-phenylbutan-2-yl) acrylamide (NPA), was copolymerized with SVC onto the stationary phase (Sil-SVCNPA) for comparison. The stationary phases were characterized with FT-IR, TGA, EA, and zeta-potential measurements. Mobile phase composition (ACN content, pH and salt concentration) was varied to study the retention property. Linear solvation energy relationship and Van't Hoff plot were used to investigate the retention mechanism and how chromatographic conditions influenced it. Both stationary phases showed a mixed-mode of RPLC/HILIC/IEC and satisfactory performance in separating hydrophobic analytes (alkylbenzenes and polycyclic aromatic hydrocarbons), hydrophilic nucleotide and bases, and anions, high column efficiency of 60,000 plates·m-1 was achieved. In summary, zwitterionic polymers are attractive options to prepare stationary phases and the retention property can be easily regulated by copolymer.

Facile synthesis of a novel polymer/covalent organic framework@silica composite material in deep eutectic solvent for mixed-mode liquid chromatographic separation

The solvothermal synthesis of covalent organic framework (COF) modified silica gel usually requires the use of harmful organic solvents, tedious steps, and harsh reaction conditions. In pursuit of green chemistry, a new strategy for the facile preparation of COF@SiO2 composite material was realized in this work by using a low-toxicity and low-cost deep eutectic solvent as the reaction medium. Additionally, a flexible polyacrylic acid (PAA) was introduced for the purpose of improving the hydrophilic selectivity and separation efficiency of COF@SiO2. Based on the above ideas, a novel PAA/COF@SiO2 composite was successfully developed as a liquid chromatographic packing material. Performance evaluation of the slurry-packed PAA/COF@SiO2 column showed that diverse types of analytes were effectively separated, and the retention behavior of polar nucleosides showed a U-shaped trend, indicating mixed-mode of hydrophobic/hydrophilic retention mechanisms. Thermodynamic studies revealed that the separation mechanism was largely independent of temperature. This work verifies the feasibility of synthesizing polymer/COF@SiO2 composite material in the deep eutectic solvent. This strategy provides a theoretical reference for the green and facile preparation of COF@SiO2 as an efficient liquid chromatographic stationary phase.

Hydrophobic/hydrophilic separation performance evaluation of a mixed-mode ionic liquid embedded stearyl thioglycolate functionalized silica stationary phase

In this work, a novel imidazolium ionic liquid embedded multifunctional chromatographic stationary phase (Sil-AVI-ST) was synthesized by the radical-mediated thiol-ene click reaction. A wide range of samples including hydrophilic sulfonamides, vitamins and nucleosides/bases as well as hydrophobic phthalates, bisphenols, alkylphenols and steroid hormones were selected to evaluate the separation ability of the newly obtained Sil-AVI-ST. As expected, an efficient separation of the above tested analytes was successfully achieved in different chromatographic modes. It was proved that multiple stationary phase-analyte interaction forces promoted the selective separation. The Sil-AVI-ST column provided multiple retention mechanisms, enabling the efficient separation of diverse analytes with different polarity. More importantly, embedding a polar ligand (1-allyl-3-vinyl-imidazolium) could improve the separation efficiency of long-chain alkyl bonded stationary phases for hydrophilic analytes, and the developed Sil-AVI-ST column could also realize the detection of hydrophobic analytes under water-rich conditions, which is impossible for the conventional hydrophobic columns. Therefore, the newly prepared Sil-AVI-ST stationary phase has a good practical application potential.

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.

Dendrimer-functionalized hydrothermal nanosized carbonaceous spheres as superior anion exchangers for ion chromatographic separation

Polyamidamine (PAMAM) dendrimer-functionalized hydrothermal nanosized carbonaceous spheres (HNCSs) were prepared and utilized as latexes for agglomerated anion exchange chromatography (AEC) stationary phase. The high-concentration and scalable production of monodisperse HNCSs (73-98 nm) was accomplished via the polyquaternium-7-assisted hydrothermal carbonization of fructose. The novel PAMAM-based quaternizations of HNCSs were designed by the amidation with PAMAM and epoxy-amine addition reaction with glycidol in aqueous solution. The mild functionalization condition leads to well-kept morphology of HNCSs, which forms one even latex layer on the sulfonated surface of polystyrene-divinylbenzene microbeads for the construction of AEC packing. Under isocratic elution, seven common inorganic anions and five organic acids were baseline separated in 9 min on prepared packing with efficiencies of 54,000-79,800 plates m^-1 and asymmetry factor (As) of 1.02-1.12. The obtained separation efficiency, peak symmetry, and analysis time were superior to reported or typical commercial counterparts. The quick separation of polarizable anions in 7 min and carbohydrates in 5 min could also be carried out with symmetrical peaks (As: 1.00-1.18) and high efficiencies (49,700-62,100 N/m). Favorable stability and reproducibility were proved by continuous flushing and injection. The constructed packings were further applied to the determination of thiosulfate and sulfate in water reducer, galacturonic acid in Angelica polysaccharide hydrolysate, and fluoride samples in 4 min.