Mixed-mode Retention Mechanism Research Articles

Low swelling and high mechanical strength organosilane hybrid polydivinylbenzene microspheres for hydrophilic interaction chromatography applications.

In this work, monodisperse organosilane hybrid polymer microspheres with a particle size of about 5µm were synthesized using seed swelling polymerization. The organosilicon reagent methacryloxypropyltrimethoxysilane was introduced into the polymer framework as a copolymerization monomer, and the crosslinking degree of the microspheres was improved by the hydrolysis-condensation reaction of siloxanes. The synthesized hybrid microspheres have good mechanical strength as well as low swelling, with swelling propensity values of 0.167 and 0.348 in methanol and acetonitrile, respectively. Hybrid microspheres modified with cysteine have a hydrophilic interaction chromatography/reversed-phase liquid chromatography mixed-mode retention mechanism. Compared to the commercial cysteine-modified silica column, the synthesized stationary phase has higher separation selectivity for partially acidic or basic samples and better basic resistance for use under high pH mobile phase conditions (at least 10).

A facile creation of hydrophilic-lipophilic-balanced covalent organic frameworks with mixed-mode retention mechanisms for multiple applications in liquid chromatography

Currently, high-performance liquid chromatographic separation materials that can provide multiple retention mechanisms are highly favored. Covalent organic frameworks (COFs) have undoubtedly demonstrated promising application prospects in chromatographic separation on account of excellent stability and porosity. Nevertheless, the utilization of COFs as liquid chromatographic fillers is still in the early stages of development. Especially, developing powerful COF@SiO2 with versatile separation capabilities still poses certain difficulties. Herein, a hydrophilic-lipophilic-balanced imine-linked COF was synthesized and in-situ growth on the amino-modified silica microsphere to obtain a multifunctional TAPT-TP-COF@SiO2 composite material. The synergistic effect of multiple functional groups including hydroxyl, amino, aldehyde, triazine and aromatic ring in the TAPT-TP-COF provided multiple retention mechanisms. The selection of 2,4,6-triformylphloroglucinol (TP) as the aldehyde counterpart improved the hydrophilic effect of the developed COF-based separation material, which enabled the effective separation of hydrophilic analytes (nucleosides, sulfonamides and vitamins). The novel TAPT-TP-COF@SiO2 column displayed typical reverse-phase and hydrophilic interaction chromatographic retention behavior. The successful applications demonstrated that TAPT-TP-COF@SiO2 had enormous potential for efficient liquid chromatographic analysis.

Ionic liquid/covalent organic framework/silica composite material: Green synthesis and chromatographic evaluation

BackgroundDue to their large surface area and distinctive adsorption affinity, covalent organic frameworks (COFs) appear to be good candidates as liquid chromatographic separation materials with good application prospect. The development of COF materials in chromatographic science is currently in an exploratory stage. Especially, the practicability of COF@silica composite materials as liquid chromatographic stationary phases needs further exploration. Reasonably integrating a functional component such as ionic liquid (IL) into the COF@silica composite materials may provide customized functionality to achieve the purpose of synthesizing multi-functional COF based stationary phases. ResultsIn this study, an IL modified COF bonded silica composite material (IL-COF@SiO2) was successfully synthesized by using an environmentally friendly deep eutectic solvent as the reaction medium instead of the frequently-used organic solvent. The synthesized IL-COF@SiO2 composite material combines the excellent separation ability of COF and the excellent mass transfer function of spherical porous silica microsphere, and meanwhile, the introduction of IL endows COF@SiO2 with preferable separation performance. The slurry-packed IL-COF@SiO2 liquid chromatographic column could be applied to effectively separate hydrophobic and hydrophilic compounds with preferable separation selectivity and high column efficiency. By investigating the retention behavior and influencing factors, a mixed-mode retention mechanism was found. Multiple interaction forces endow the IL-COF@SiO2 with a hydrophilic-hydrophobic balance performance, demonstrating a good application prospect as a versatile liquid chromatographic separation material. SignificanceIn this study, a new strategy is proposed for greenly synthesizing a novel IL-COF@SiO2 composite material under mild conditions, which expands the potential application of COF materials in chromatographic science. One particular point to note is that the reaction medium in each step of the preparation process is low toxic and degradable deep eutectic solvent, which conforms to the concept of green chemistry.

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.

Chromatographic properties of hydrogenated microdiamond synthesized by high pressure and high temperature

Adsorption and chromatographic properties of oxidized and hydrogenated ‘high pressure and high temperature’ synthesised diamond (HPHT) are studied using high-performance liquid chromatography. The retention factors of organic cation (benzyltributylammonium chloride), weak base (aniline), weak acid (benzoic acid), strong acid (benzenesulfonic acid), hydrophobic toluene, and hydrophilic uracil are obtained at varied pH, organic solvent content, and ionic strength of mobile phase. Both adsorbents exhibited moderate polarity with a mixed-mode retention mechanism with a combination of electrostatic, hydrophobic and hydrophilic interactions. Unexpectedly, hydrogenated HPHT revealed significant anion-exchange properties under acidic conditions and cation-exchange properties under alkaline conditions, while only cation-exchange selectivity was noted for oxidized HPHT across the enntire pH range. The retention factors obtained for a set of model compounds including n-alkyl-, polymethyl-, nitro- and halogenated benzenes correlated well with their hydrophobicity (logP) values. The thermal stability of the adsorbent and immutability of retention mechanisms involved was confirmed by linear van't Hoff plots for the investigated compounds.

One-pot method for the synthesis of β-cyclodextrin and covalent organic framework functionalized chiral stationary phase with mixed-mode retention mechanism

As a welcomed porous material, covalent organic frameworks (COFs) have many advantages and are widely used in various aspects. Particularly, COFs have aroused great attentions of scientists in chromatographic separation field due to their outstanding advantages, such as high stability, large specific surface area and multiple voids. However, endowing COFs with chirality to construct chiral stationary phase (CSP) function is still facing many challenges. Here, we firstly prepared a β-cyclodextrin (β-CD) and covalent organic framework functional silica CSP named as COF@CD@SiO2 by one-pot method to perform high performance liquid chromatography (HPLC) chiral separation. The morphology and structure of the synthesized stationary phase were investigated by a variety of characterization methods including Fourier transform infrared spectrometry (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), N2 adsorption experiment, powder X-ray diffraction (XRD) and elemental analysis (EA). The prepared stationary phase realized fast separation of six enantiomers in a short time. The separation mechanism was mainly ascribed to the inclusion complexation of β-cyclodextrin and the mutli-interaction sites from COFs material. In conclusion, the prepared chiral column can be used to achieve fast separation of enantiomers with good stability and reproducibility. These results can open new avenue for using chiral COFs in liquid chromatographic separation.

Polymerized phosphonium ionic liquid functionalized silica microspheres as mixed-mode stationary phase for liquid chromatographic separation of phospholipids

There is a large and growing demand for the vigorous development of new high performance liquid chromatography stationary phases in order to solve complex phospholipids separation. Herein, phosphonium-based ionic liquid trioctyl(allyl)phosphonium bromide ([P888Allyl]Br) was first synthesized with trioctylphosphine and allyl bromide. With [P888Allyl]Br as the polymerizable monomer, polymerized phosphonium ionic liquid functionalized silica microsphere (PIL@SiO2) was further synthetized via click chemistry reaction. Significantly, based on the inherent amphiphilic nature of the introduced [P888Allyl]Br, the packed PIL@SiO2 column displayed hydrophilic/hydrophobic mixed-mode retention mechanisms. The PIL@SiO2 column can achieve separation of nucleic acid bases and nucleosides, sulfonamides, amides and anilines with excellent selectivity in a shorter separation time. The column efficiency reached 109,700 N/m for 2-iodoacetamide. One of the important characteristics of the PIL@SiO2 column is that both phospholipid classes and species can be efficiently separated via the same column, outperforming that of the commercial amino column. Furthermore, the application potential of the PIL@SiO2 column was further verified via separation of phospholipids extracted from soy lecithin. The proposed PIL@SiO2 column provides a promising candidate for separation of complex phospholipid samples.

Exploiting styrene-maleic acid copolymer grafting chromatographic stationary phase materials for separation of membrane lipids

High performance liquid chromatography-mass spectrometry is one of the most commonly used strategies for lipid analysis. The development of versatile chromatographic stationary phases to meet the increasing demands for separation of complex lipids is very important. Styrene-maleic acid (SMA) copolymer is an amphiphilic polymer, which has been proven to have the ability to solubilize lipid molecules of various structures. In this study, styrene-maleic anhydride copolymer coated silica was first prepared by the thiol-ene click reaction. With l-cysteine hydrochloride or dodecanol as the post-modification reagents, Sil-SMA-amino acid and Sil-SMA-dodecanol stationary phase materials were further successfully fabricated via nucleophilic ring-opening reaction. The Fourier-transform infrared, thermogravimetric analysis, and elemental analysis results confirmed the two stationary phase materials were successfully prepared. Furthermore, both the Sil-SMA-dodecanol column and the Sil-SMA-amino acid column possessed reversed-phase/hydrophilic interaction/ion exchange mixed-mode retention mechanisms. The column efficiency of the Sil-SMA-derivatives columns reached 77,300 N/m. Based on the mixed-mode retention characteristics, the Sil-SMA-derivatives columns achieved both the lipid classes and species separation via a single column. The Sil-SMA-amino acid column was further successfully used to separate lipid extract from gastric cancer cell membrane. All these results demonstrated that the SMA-based stationary phase materials have a good potential for use in lipid separation.

Preparation and application of Au nanoparticles-decorated SO3H-cofunctionalized silica stationary phase for per aqueous liquid chromatography

Au nanoparticles (Au NPs)-decorated SO3H-cofunctionalized porous silica particles was prepared as a novel mixed-mode stationary phase and used in Per aqueous liquid chromatography (PALC) mode. The obtained stationary phase was characterized by elemental analysis, FT-IR spectra, and transmission electron microscopy. To evaluate the retention behavior, the effect of a variety of chromatographic factors on the retention of different types of solutes was investigated. The obtained column exhibited a hydrophobic interaction/cation-exchange mixed-mode retention mechanism, and a typical PALC characteristic was observed in water-rich mobile phases. When the particle size of Au NPs was 10–20 nm, the analytes had a sharp peak shape and the column pressure was also moderate. Compared with the hydrophilic interaction liquid chromatography (HILIC) and C18 column, some hydrophilic compounds such as six monosaccharides were baseline separated with the enhanced resolution of the obtained column under the PALC mode. Moreover, the efficiency of the Au-S/SO3H-SiO2-2 column was significantly higher than that of the HILIC column. PALC mode was a green annlysis method and suitable for the replacement of hydrophilic interaction liquid chromatography.

One-step preparation of cyclen-containing hydrophilic polymeric monolithic materials via epoxy-amine ring-opening reaction and their application in enrichment of N-glycopeptides

Considering the special structure of 1,4,7,10-tetraazacyclododecane (cyclen) which is easy to form complexes with ions, it is beneficial to achieve particular selectivity. Cyclen was selected as a precursor to react with triglycidyl isocyanurate (TGIC), and a novel kind of hydrophilic polymeric monolithic material was facilely prepared via epoxy-amine ring-opening reaction in the presence of a binary porogenic system of acetonitrile (ACN) and polyethylene glycol. The resulting poly (TGIC-co-cyclen) monolithic column was used to separate both nonpolar alkylbenzenes using mobile phase of ACN/H2O (35/65, v/v) and polar phenolic compounds and anilines under the mobile phase of ACN/H2O (60/40, v/v) in reversed-phase capillary liquid chromatography (cLC). It should be pointed that the monolith was further used for separation of a mixture of toluene, DMF, acrylamide and thiourea under the mobile phase of ACN/H2O (95/5, v/v) by hydrophilic interaction chromatography (HILIC). These results indicated that the poly (TGIC-co-cyclen) column exhibited mixed-mode retention mechanism. As a result, the prepared monolithic material was employed for enrichment of glycosylated peptides from the tryptic digest of human immunoglobulin G (IgG) and serum protein tryptic digests. A total of 531 N-glycopeptides and 329 N-glycosylation sites, mapped to 166 glycoproteins, were identified from 2 μL human serum digest. The results indicated the prepared monolith had ability for enriching N-glycopeptides from complex biological samples.

Preparation and performance of a poly(ethyleneimine) embedded N-acetyl-L-phenylalanine mixed-mode stationary phase for HPLC

Poly(ethyleneimine) and N-acetyl-L-phenylalanine were bonded on the surface of activated silica to prepare a new multifunctional polar embedded stationary phase material named Sil-PEI-APH. The material was characterized by Fourier transform infrared spectroscopy (FT-IR), elemental analysis (EA) and thermogravimetric analysis (TGA). Then the synthesized material was loaded into the empty column tube by high pressure driving homogenization method and used as mixed-mode chromatography involving mechanisms of reversed-phase (RPLC), hydrophilic (HILIC) and ion exchange interactions (IEC). Based on these interactions, successful separation could be achieved among several groups of aromatic compounds with different polarities. In detail, the separations of alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), biphenyl isomers and flavonoids were achieved in RPLC mode. Nucleotides/nucleosides were separated in HILIC mode. Additionally, phenols, benzoic acids were separated successfully in IEC mode. The separation of anilines depended on the various separation modes. Investigations about effects of mobile phase constitution and pH on retention further demonstrated the mixed-mode retention mechanisms of the column. Meanwhile, chiral resolution ability was proved by separation of warfarin as model analyte. In a word, the developed stationary phase exhibited multiple performances and the separations for types of analytes were thus achieved.

A monolithic capillary modified with a copoplymer prepared from the ionic liquid 1-vinyl-3-octylimidazolium bromide and styrene for electrochromatography of alkylbenzenes, polycyclic aromatic hydrocarbons, proteins and amino acids.

A monolithic capillary column was prepared by single-step copolymerization of the ionic liquid 1-vinyl-3-octylimidazolium bromide (VOI) and styrene. The VOI and styrene monomers were introduced to provide multiple interaction sites and increase hydrophobicity and aromaticity of the monolithic column, respectively. The effect of porogen ratio, monomer ratio and reaction temperature on permeability was investigated. The resulting column was characterized by scanning electron microscopy, and the results suggest that the column possesses high porosity and good homogeneity. A relatively strong anodic electroosmotic flow was generated over a wide range of pH values (pH2.0-10.0), and this facilitates the rapid separation of analytes within 12min. Alkylbenzenes, polycyclic aromatic hydrocarbons (PAHs), proteins and amino acids were used to evaluate the performance of the monolithic column under capillary electrochromatography mode by UV detection. Mixed-mode retention mechanisms including hydrophobic interaction, π-stacking, ion-exclusion interaction were observed. The monolithic column exhibits high column efficiency (8.72 × 104 plates∙m-1) and satisfying separation capability (the resolution of four alkylbenzenes: 2.54, 2.86 and 4.62, four PAHs: 2.79, 4.83 and 4.77, three proteins: 4.35 and 2.86, two amino acids: 3.34). Graphical abstractSchematic representation of capillary electrochromatography (CEC) systems with ionic liquid and styrene based organic polymer monolithic column for separation of alkylbenzenes, polycyclic aromatic hydrocarbons, proteins and amino acids.

D-2-allylglycine embedded imidazolium-bridged polyhedral oligomeric silsesquioxane hybrid monolithic column for efficient separation of both small molecules and macromolecules

The development of multifarious stationary phases is still a growing demand so as to solve the tasks of ever evolving actual applications. Herein, with D-2-allylglycine hydrochloride (AG·HCl) as the hydrophilic monomer, diene ionic liquid 1-allyl-3-vinylimidazolium bromide (AVI·Br) and polyhedral oligomeric silsesquioxane methacryl substituted (POSS-MA) as the dual crosslinkers, the highly cross-linked imidazolium-bridged POSS-AVI-AG hybrid monolithic column was fabricated via the “one-pot” free radical copolymerization. The AG·HCl embedded POSS-AVI-AG column displays typical reversed-phase liquid chromatography/hydrophilic interaction liquid chromatography mixed-mode retention mechanisms. Both hydrophobic phenols, alkylbenzenes, aromatic amines and hydrophilic nucleosides/nucleic acid bases, amides and thioureas were successfully separated with high column efficiencies (up to 571,000 plates/m for amides), outperforming our previously reported AVI·Br modified POSS-AVI column. Moreover, the column was also explored for the separation of cytochrome c tryptic digests and egg white protein extraction. All these results demonstrate that the POSS-AVI-AG column has a good potential in separation of both small molecules and complex biological samples with multiple mechanisms.

Bi-functionalized mesostructured silicas as reversed-phase/strong anion-exchange sorbents. Application to extraction of polyphenols prior to their quantitation by UHPLC with ion-trap mass spectrometry detection.

Hybrid mesostructured silicas with wormhole-like pore structure were synthesized and bi-functionalized with n-octyl (C8) and quaternary ammonium (NR4+) groups to obtain new sorbent materials for dispersive solid-phase extraction (dSPE) of polyphenols. Due to their nature of being both a reversed-phase and a strong anion-exchanger, the materials display mixed-mode retention mechanism. During the synthesis, the functionalization conditions were varied to obtain materials with different functionalization degree. The resulting materials (denoted as HMS-RPC8-SAX-1, HMS-RPC8-SAX-2 and HMS-RPC8-SAX-3) show high surface area, wormhole-like framework and controlled pore size. They were evaluated for multicomponent extraction of 22 polyphenols, including phenolic acids, flavonoids and stilbenes, from spiked juice samples. The sample extracts were analyzed by ultra-high performance liquid chromatography coupled to ion-trap tandem mass spectrometry. The adsorption capability, the amount of sorbent, the eluent and the elution volume were optimized. Best performance was achieved by using HMS-RPC8-SAX-2, which is the material with the highest fraction of NR4+ groups. This material has a large extraction capability and provides high recovery values of the target analytes (70-101%) as a result of its hydrophobic and anion-exchange interactions. The detection limits for polyphenols in juice range from 1 to 560ngmL-1. Graphical abstract Schematic presentation of dispersive solid-phase extraction of polyphenols from juice samples using a novel sorbent based on the bi-functionalization of mesostructured silica with n-octyl and quaternary ammonium groups, followed by ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS) analysis. Due to the sorbent nature of being both a reversed-phase and strong anion-exchanger, the material displays mixed mode retention mechanism that improves its extraction capability.

Preparation of a poly(ethyleneimine) embedded phenyl stationary phase for mixed-mode liquid chromatography

A poly(ethyleneimine) embedded phenyl mixed-mode stationary phase was prepared through epoxide ring opening reaction. Elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) were used to characterize whether functional groups have been modified on the surface of silica successfully. The researches of chromatographic performance for designed column were divided to four parts. The retention behavior under reversed-phase mode was studied using polycyclic aromatic hydrocarbons (PAHs), positional isomers and alkylbenzenes. The retention characteristic under hydrophilic interaction mode was studied by nucleosides, nucleobases and flavonoids. The chromatographic performance of anion exchange mode was evaluated by benzoic acids and phenols. Due to a high content of amino and phenyl groups on the stationary phase, aromatic amines were chosen for the evaluation of retention mechanisms of benzene ring attraction and amino repulsion. The investigations about effects of mobile phase constitution and pH on retention demonstrated the mixed-mode retention mechanisms of the column. Furthermore, due to amino selectivity of the column, a method for the rapid separation and determination of phenylenediamines in hair dye was established. And both linear correlation coefficients of p-diaminobenzene and m-diaminobenzene in concentration range from 0.1 to 30 μg mL−1 were over 0.999. In conclude, the prepared mixed-mode stationary phase could realize various separation modes by adjusting chromatographic conditions, and it had the potential for the rapid separation and determination of aniline compounds in various complex samples.

Preparation of Cyclodextrin Type Stationary Phase Based on Graphene Oxide and Its Application in Enantioseparation and Hydrophilic Interaction Chromatography

Abstract Graphene oxide (GO) was covalently coupled to the surface of amino silica gel by amide bond. β-cyclodextrin (β-CD) was further chemically bonded with GO to prepare a novel chiral stationary phase. The resulting material was characterized by Fourier transform-infrared (FT-IR) spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), elemental analysis and thermogravimetric analysis (TGA). The separation of seven enantiomers was improved in varying degrees. Meanwhile, the stationary phase showed typical characteristics of hydrophilic interaction chromatography (HILIC), and four small nucleoside molecules were separated with the mobile phase of methanol-acetonitrile-water (45:45:10, V/V) in the HILIC mode. In addition, the separation mechanism of the stationary phase was further explored by studying the effects of mobile phase composition, temperature and pH value on the analyte retention. The low temperature was conducive to the separation of analytes at 20–60 °C. The addition of protonated solvent methanol significantly decreased the retention time of the four analytes. The change of pH affected the degree of protonation of the analyte, the interaction between analytes and the stationary phase, and retention time of analytes. The results showed that GO and β-CD played a synergistic effect in the chiral resolution of the chromatographic stationary phase. The retention of analytes in HILIC was attributed to their mixed-mode retention mechanisms including hydrophilic interaction, electrostatic interaction, hydrogen bonding, π-π stacking and so on.

Vulnerability of anthocyanins to the components of a thin-layer chromatographic system and comprehensive screening of anthocyanes in alimentary products

The aim of this study was to revisit the TLC authentication of alimentary products concept based on analysis of anthocyanes with the foodstuffs of plant origin. To this effect, we used two anthocyanins (cyanin and keracyanin) and two anthocyanidins (pelargonidin and delphinidin) as phytochemical standards. The first step was to develop a novel method making use of the RP-18 F254s stationary phase (which ensures mixed-mode retention mechanism with the localized adsorption on the non-bonded silanols) and acetic acid as the mobile phase component. Importantly, similar TLC systems are currently used for the analysis of anthocyanes. Individual steps of our method development enabled a deeper insight in vulnerability of anthocyanins to external conditions resulting in hydrolysis thereof. In this study, it was impossible to fully separate the products of hydrolytic degradation of the test anthocyanins in a single development run and it was only triple development which ensured distinct and symmetrically shaped chromatographic spots, further scrutinized with use of mass spectrometry. The identity of the hydrolytically split fractions was additionally studied with use of the p-aminobenzoic acid (PABA) test. To obtain calibration curves, triple development was employed for cyanin, keracyanin, and pelargonidin, while delphinidin was developed in one development run. The respective LOD and LOQ values were: for spot (i) derived from the cyanin standard, 0.005 and 0.016 μg spot−1; for spot (ii) derived from the cyanin standard, 0.006 and 0.017 μg spot−1; for spot (i) derived from the keracyanin standard, 0.092 and 0.274 μg spot−1; for spot (ii) derived from the keracyanin standard, 0.035 and 0.104 μg spot−1; for the pelargonidin standard, 0.013 and 0.040 μg spot−1; and for the delphinidin standard, 0.036 and 0.108 μg spot−1. The developed method was used to identify and quantify cyanin, keracyanin, pelargonidin and delphinidin in selected alimentary products (syrups, juices and herbal infusions), keeping in mind that the obtained numerical results were of semi-quantitative nature only.

Comparison of cation-exchange capillary columns used for ion chromatographic separation of biogenic amines

The selectivity for 15 biogenic amines and amino acids shown by three capillary cation-exchange columns, IonPac CS19, CS12A and CS17 (250 × 0.4 mm ID, all from Thermo Fisher Scientific), exhibiting medium, medium low and ultra-low hydrophobicity, and either carboxylic or mixed carboxylic/phosphonic acid functional groups, was investigated. A mixed mode retention mechanism was revealed with ion-exchange, hydrophobic and hydrogen bonding interactions contributing to retention of polar organic molecules on these phases. The relative impact of these interactions was evaluated via the effect of concentration and pH of the eluent (methanesulfonic acid) on the retention of fifteen structurally similar biogenic amines and amino acids. Strong hydrogen bonding interactions were observed between the solute amino acid carboxylic groups and cation-exchange groups from the ion-exchangers. This is the first time retention data correlated with logP data has revealed clustering of the solutes in two groups, according to the presence or absence of a carboxylic acid functional group. In addition, stronger retention behaviour was found for the IonPac CS12A cation-exchanger, containing both carboxylic and phosphonic functional groups. Further assessment of the orthogonality plots of retention factors for the three stationary phases revealed that the columns exhibited different complimentary selectivity that can be utilised to achieve specific separations.

Preparation and evaluation of maltose modified polymer-silica composite based on cross-linked poly glycidyl methacrylate as high performance liquid chromatography stationary phase

A new maltose modified polymer-silica composite was fabricated and applied as high performance liquid chromatography (HPLC) stationary phase. The cross-linked poly glycidyl methacrylate (pGMA) layer was chemically bonded to the outer surface as well as pore inner surface of silica beads via in-situ polymerization, and then maltose was modified onto the polymer layer via a [3 + 2] “click” reaction. The porous spherical silica (4 μm diameter) with 300 Å pore size was selected as the matrix so that the 3.25 nm-thick polymer layer fabricated on the pore inner surface would not affect its permeability. The typical ‘U-shape’ retention curves indicated a mixed-mode retention mechanism of the as-synthesized stationary phase. Both polar and non-polar analytes could be well separated on the stationary phase with column efficiency reaching 123809 plates/m for guanosine in hydrophilic interaction liquid chromatography (HILIC) mode and 46808 plates/m for fluorene in reversed-phase liquid chromatography (RPLC) mode, respectively. Nucleotides and their bases were baseline separated with good peak shape without any buffer salt in mobile phase, suggesting the effective shielding of the silanol groups. The packing material also showed excellent chromatographic repeatability with intraday RSDs of the retention time of five nucleosides less than 0.048% (n = 3) and interday RSDs less than 0.33% (n = 7) and great pH stability (from 1.5 to 10.2). Finally, the stationary phase was applied to the separation of ginseng extract.

Ultrafast preparation of a polyhedral oligomeric silsesquioxane-based ionic liquid hybrid monolith via photoinitiated polymerization, and its application to capillary electrochromatography of aromatic compounds.

An ionic liquid hybrid monolithic capillary column was prepared within 7min via photoinitiated free-radical polymerization of an ionic liquid monomer (1-butyl-3-vinylimidazolium-bis[(trifluoromethyl)sulfonyl]imide); VBIMNTF2) and a methacryl substituted polyhedral oligomeric silsesquioxane (POSS-MA) acting as a cross-linker. The effects of composition of prepolymerization solution and initiation time on the porous structure and electroosmotic flow (EOF) of monolithic column were investigated. The hybrid monolith was characterized by scanning electron microscopy and FTIR. Owing to the introduction of a rigid nanosized POSS silica core and ionic liquids with multiple interaction sites, the monolithic column has a well-defined 3D skeleton morphology, good mechanical stability, and a stable anodic electroosmotic flow. The hybrid monolithic stationary phase was applied to the capillary electrochromatographic separation of various alkylbenzenes, phenols, anilines and polycyclic aromatic hydrocarbons (PAHs). The column efficiency is highest (98,000 plates/m) in case of alkylbenzenes. Mixed-mode retention mechanisms including hydrophobic interactions, π-π stacking, electrostatic interaction and electrophoretic mobility can be observed. This indicates the potential of this material in terms of efficient separation of analytes of different structural type. Graphical Abstract Preparation of a mixed-mode ionic liquid hybrid monolithic column via photoinitiated polymerization of methacryl substituted polyhedral oligomeric silsesquioxane (POSS-MA) and 1-butyl-3-vinylimidazolium-bis[(trifluoromethyl)sulfonyl]imide (VBIMNTF2) ionic liquid for use in capillary electrochromatography.