Preparation and evaluation of a neutral methacrylate-based monolithic column for hydrophilic interaction stationary phase by pressurized capillary electrochromatography

Itaconic Acid-Based Organic-Polymer Monolithic Column for Hydrophilic Capillary Electrochromatography and Its Application in Pharmaceutical Analysis

Itaconic acid is an excellent hydrophilic monomer owing to the dicarboxylic group possessing strong polarity. This study reports on the preparation of a new organic-polymer monolithic column poly(itaconic acid-co-3-(acryloyloxy)-2-hydroxypropyl methacrylate) (poly(IA-co-AHM)) featuring excellent hydrophilic chromatography ability and its application in pharmaceutical analysis. The monolithic column was successfully synthesized by using the monomer itaconic acid and the cross-linker AHM through an in situ copolymerization method. Optical microscopy, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) were employed for the characterization of the poly(IA-co-AHM) monolithic column, and all of these demonstrated that the prepared itaconic acid-based monolithic column exhibited satisfactory permeability and a homogeneous porous structure. Owing to the carboxylic groups of itaconic acid, a cathodic electroosmotic flow (EOF) was generated on the itaconic acid-based monolithic column among the pH ranges of the mobile phase from 4.0 to 9.0. Depending on the powerful hydrophilic interactions, different kinds of polar substances, including thioureas, nucleoside drugs, sulfonamides, and polypeptides, were separated efficiently by the itaconic acid-based monoliths poly(IA-co-AHM). The separations of polar compounds were successfully realized, even at a lower level of 50% acetonitrile content on this monolithic column. The highest column efficiencies corresponding to N,N'-dimethylthiourea and idoxuridine were 102 720 and 124 267 N/m, respectively. The poly(IA-co-AHM) monolithic column displayed excellent repeatability, whose relative standard deviations (RSDs) of the retention time and peak area were both lower than 5.0%. All experimental results demonstrated that the new itaconic acid-functionalized monolithic column was greatly appropriate to separate the polar compounds under the HILIC mode.

Preparation of poly(N-vinylpyrrolidone-co-pentaerythritol triacrylate) monolithic column for hydrophilic interaction chromatography

A method for the preparation of poly(N-vinylpyrrolidone-co-pentaerythritol triacrylate copolymerization)-based monolithic capillary column was reported for the separation of polar small molecular weight compounds with nano-liquid chromatography in hydrophilic interaction chromatography mode. The monolithic columns were prepared by in situ copolymerization of N-vinylpyrrolidone and a cross-linker pentaerythritol triacrylate in a binary porogenic agent consisting of methanol and water. The composition of the polymerization solution was systematically optimized in terms of column permeability, theoretical plate number, asymmetric factor, and retention factor. A typical hydrophilic chromatography retention mechanism was observed with a mobile phase composed of a high content of organic solvent. The preparation method is simple and robust, the precursor N-vinylpyrrolidone is chemically stable, cheap, and easily available. The N-vinylpyrrolidone-based hydrophilic interaction chromatography stationary phase displays satisfactory separation selectivity for a range of polar test analytes, including benzoic acid derivatives, nucleosides, and phenols.

A benzenesulfonic acid-modified organic polymer monolithic column with reversed-phase/hydrophilic bifunctional selectivity for capillary electrochromatography

Here, a styrene-based polymer monolithic column poly(VBS-co-TAT-co-AHM) with reversed-phase/hydrophilic interaction liquid chromatography (RPLC/HILIC) bifunctional separation mode was successfully prepared for capillary electrochromatography by the in situ polymerization of sodium p-styrene sulfonate (VBS) with cross-linkers 3-(acryloyloxy)-2-hydroxypropyl methacrylate (AHM) and 1,3,5-triacryloylhexahydro-1,3,5-triazine (TAT). The preparation conditions of the monolith were optimized. The morphology and formation of the poly(VBS-co-TAT-co-AHM) monolith were confirmed by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FT-IR). The separation performances of the monolith were evaluated systematically. It should be noted that the incorporation of VBS functional monomer can provide π-π interactions, hydrophilic interactions, and ion-exchange interactions. Hence, the prepared poly(VBS-co-TAT-co-AHM) monolith can achieve efficient separation of thiourea compounds, benzene series, phenol compounds, aniline compounds and sulfonamides in RPLC or HILIC separation mode. The largest theoretical plate number for N,N'-dimethylthiourea reached 1.7 × 10⁵ plates/m. In addition, the poly(VBS-co-TAT-co-AHM) monolithic column showed excellent reproducibility and stability. This novel monolithic column has great application value and potential in capillary electrochromatography (CEC).

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.

Strong hydrophilic monolithic column functionalized with amphiphilic benzyl quinine for capillary electrochromatography and application in pharmaceutical analysis

An innovative strong hydrophilic organic polymer monolithic column of poly(N-benzylquininium chloride-co-1, 3, 5-triacryloylhexahydro-1, 3, 5-triazine) (poly(NBQ-co-TAT)) has been successfully synthesized through in situ copolymerization for capillary electrochromatography. The amphiphilic monomer NBQ and the strong polar cross-linker TAT are firstly used in hydrophilic electrochromatography by taking advantage of the exhibition of hydrophilicity at lower levels of organic solvent and ease formation of porous structure. The monolithic column poly(NBQ-co-TAT) shows powerful hydrophilic selectivity with mobile phase containing more than 60% organic solvent. The introduction of NBQ and TAT enlarges the sources of functional monomers and cross-linkers for HILIC. Due to the presence of the positively charged group in NBQ, an anodic electroosmotic flow is generated with the change of pH values from 2.0 to 12.0. The monolithic column was used for the separations of thioureas, phenols, xanthines, nucleobases, acidic substances and pharmaceuticals. The highest column efficiency for N, N'-dimethylthiourea is 1.15 × 10⁵ N m^-1. The application of the monolithic column for a real sample, cytochrome C digestion indicates its great potential in practical application.

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.

Preparation of phosphorylcholine-based hydrophilic monolithic column and application for analysis of drug-related impurities with capillary electrochromatography

A hydrophilic monolithic CEC column was prepared by thermal copolymerization of zwitterionic monomer 2-methacryloyloxyethyl phosphorylcholine (MPC), pentaerythritol triacrylate (PETA), either methacrylatoethyl trimethyl ammonium chloride (META) or sodium 2-methylpropene-1-sulfonate (MPS) in a polar binary porogen consisting of methanol and THF. A typical hydrophilic interaction LC retention mechanism was observed for low-molecular weight polar compounds including amides, nucleotides, and nucleosides in the separation mode of hydrophilic interaction CEC, when high content of ACN (>60%) was used as the mobile phase. The effect of the electrostatic interaction between the analytes and the stationary phase was found to be negligible. The poly(MPC-co-PETA-co-META or MPS) monolithic columns have an average column efficiency of 40 000 plates/m and displayed with a satisfactory repeatability in terms of migration time and peak areas. Finally, the column was successfully applied to determine the impurities of a positively charged drug pramipexole which are often separated by ion pair RP chromatography due to their high hydrophilicity. All four components can be baseline separated within 5 min with BGE consisting of ACN/20 mM ammonium formate buffer (pH 3.0; 80/20).

Thiol–ene click chemistry for the design of diol porous monoliths with hydrophilic surface interaction ability: a capillary electrochromatography study

Thiol–ene click chemistry provides an efficient surface grafting strategy for designing diol monoliths meant for hydrophilic interaction capillary electrochromatography.

Preparation and evaluation of a hydrophilic poly(2-hydroxyethyl methacrylate-co-N,N'-methylene bisacrylamide) monolithic column for pressurized capillary electrochromatography

A polar polymethacrylate-based monolithic column was introduced and evaluated as a hydrophilic interaction CEC stationary phase. The monolithic stationary phase was prepared by in situ copolymerization of a neutral monomer 2-hydroxyethyl methacrylate and a polar cross-linker N,N'-methylene bisacrylamide in a binary porogenic solvent consisting of dodecyl alcohol and toluene. The hydroxyl and amino groups at the surface of the monolithic stationary phase provided polar sites which were responsible for hydrophilic interactions. The composition and proportion of the polymerization mixture was investigated in detail. The mechanical stability and reproducibility of the obtained monolithic column preformed was satisfied. The effects of pH and organic solvent content on the EOF and the separation of amines, nucleosides, and narcotics on the optimized monolithic column were investigated. A typical hydrophilic interaction CEC was observed on the neutral polar stationary phase. The optimized monolithic column can obtain high-column efficiencies with 62,000-126,000 theoretical plates/m and the RSDs of column-to-column (n = 9), run-to-run (n = 5), and day-to-day (n = 3) reproducibility were less than 6.3%. The calibration curves of these five narcotics exhibited good linearity with R in the range of 0.9959-0.9970 and linear ranges of 1.0-200.0 μg/mL. The detection limits at S/N = 3 were between 0.2 and 1.2 μg/mL. The recoveries of the separation of narcotics on the column were in the range of 84.0-108.6%. The good mechanical stability, reproducibility, and quantitation capacity was suitable for pressure-assisted CEC applications.

Effect of ion adsorption on CEC separation of small molecules using hypercrosslinked porous polymer monolithic capillary columns

Both poly(styrene-co-vinylbenzyl chloride-co-divinylbenzene) and poly(4-methylstyrene-co-vinylbenzyl chloride-co-divinylbenzene) monolithic columns have been hypercrosslinked and for the first time used to achieve capillary electrochromatographic separations. Although these columns do not contain ionizable functionalities, electroosmotic flow was observed due to adsorption of ions from a buffer solution contained in the mobile phase on the surface of the hydrophobic polymer. An increase of more than one order of magnitude was observed with the use of both monolithic polymers. The hypercrosslinking reaction creates a large surface area thus enabling adsorption of a much larger number of ions. Alkylbenzenes were successfully separated using the hypercrosslinked monolithic columns.

A novel, neutral hydroxylated octadecyl acrylate monolith with fast electroosmotic flow velocity and its application to the separation of various solutes including peptides and proteins in the absence of electrostatic interactions

A neutral hydroxylated octadecyl monolith (ODM-OH) for reversed-phase capillary electrochromatography has been developed. The ODM-OH was prepared by the in situ polymerization of octadecyl acrylate and pentaerythritol triacrylate (PETA) in a ternary porogenic solvent. Pentaerythritol triacrylate possesses a hydroxyl functional group, which imparts the monolith with a hydrophilic group, thus the acronym ODM-OH. The ODM-OH column exhibited cathodal EOF over a wide range of pH and ACN concentration in the mobile phase despite the fact that it was devoid of any fixed charges. This ODM-OH monolith exhibited stronger EOF than its counterpart the ODM made from the in situ polymerization of octadecyl acrylate and trimethylolpropane trimethacrylate. Similar to ODM, it is believed that the EOF was due to the adsorption of ions from the mobile phase onto the surface of the monolith thus imparting the neutral monolithic column the zeta potential necessary to support the EOF. The higher EOF exhibited by ODM-OH was due to the presence of polar OH groups on its surface, which would favor stronger adsorption of ions from the mobile phase. The wide applications of the neutral ODM-OH column were demonstrated in the separation of a wide range of small and large solutes. As a typical result, the ODM-OH was able to separate proteins quite rapidly yielding 200,000 plates/m.

Organic monoliths for hydrophilic interaction electrochromatography/chromatography and immunoaffinity chromatography

This article is aimed at providing a review of the progress made over the past decade in the preparation of polar monoliths for hydrophilic interaction LC (HILIC)/capillary electrochromatography (HI-CEC) and in the design of immuno-monoliths for immunoaffinity chromatography that are based on some of the polar monolith precursors used in HILIC/HI-CEC. In addition, this review article discusses some of the applications of polar monoliths by HILIC and HI-CEC, and the applications of immuno-monoliths. This article is by no means an exhaustive review of the literature; it is rather a survey of the recent progress made in the field with 83 references published in the past decade on the topics of HILIC and immunoaffinity chromatography monoliths.

Preparation of methacrylate-based monolith for capillary hydrophilic interaction chromatography and its application in determination of nucleosides in urine

A novel poly(N-acryloyltris(hydroxymethyl)aminomethane-co-pentaerythritol triacrylate) (NAHAM-co-PETA) monolith was prepared in the 100 μm i.d. capillary and investigated for capillary liquid chromatography (cLC). The polymer monolith was synthesized by in situ polymerization of NAHAM and PETA in the presence of polyethylene glycol (PEG) in dimethyl sulfoxide (DMSO) as the porogen. The porous structure of monolith was optimized by changing the ratio of NAHAM to PETA, the molecular weight and amount of PEG. To evaluate the separation performance of the resultant polymer monolith, several groups of model compounds (including nucleosides, benzoic acids and anilines) were selected to perform cLC separation. Our results showed that these model compounds can be baseline separated on the resultant poly(NAHAM-co-PETA) monolithic column with the optimized mobile phases. The column efficiency was estimated to be 87,000 plates/m for acrylamide. In addition, this monolithic column was coupled with on-line solid-phase microextraction (SPME) for the analysis of four nucleosides (uridine, adenosine, cytidine, guanosine) in urine. The limit of detection of the proposed method was in the range from 40 to 52 ng/mL. The method reproducibility was obtained by evaluating the intra- and inter-day precisions with relative standard deviations (RSDs) less than 8.3% and 10.2%, respectively. Recoveries of the target analytes from spiked urine samples were ranged from 86.5% to 106.8%.

Sulfoalkylbetaine-based monolithic column with mixed-mode of hydrophilic interaction and strong anion-exchange stationary phase for capillary electrochromatography

A novel monolithic stationary phase with mixed mode of hydrophilic and strong anion exchange (SAX) interactions based on in situ copolymerization of pentaerythritol triacrylate (PETA), N,N-dimethyl-N-methacryloxyethyl N-(3-sulfopropyl) ammonium betaine (DMMSA) and a selected quaternary amine acrylic monomer was designed as a multifunctional separation column for CEC. Although the zwitterionic functionalities of DMMSA and hydroxy groups of PETA on the surface of the monolithic stationary phase functioned as the hydrophilic interaction (HI) sites, the quaternary amine acrylic monomer was introduced to control the magnitude of the EOF and provide the SAX sites at the same time. Three different quaternary amine acrylic monomers were tested to achieve maximum EOF velocity and highest plate count. The fabrication of the zwitterionic monolith (designated as HI and SAX stationary phase) was carried out when [2-(acryloyloxy)ethyl]trimethylammonium methylsulfate was used as the quaternary amine acrylic monomer. The separation mechanism of the monolithic column was discussed in detail. For charged analytes, a mixed mode of HI and SAX was observed by studying the influence of mobile phase pH and salt concentration on their retentions on the poly(PETA-co-DMMSA-co-[2-(acryloyloxy)ethyl]trimethylammonium methylsulfate) monolithic column. The optimized monolith showed good separation performance for a range of polar analytes including nucleotides, nucleic acid bases and nucleosides, phenols, estrogens and small peptides. The column efficiencies greater than 192 000 theoretical plates/m for estriol and 135 000 theoretical plates/m for charged cytidine were obtained.

Zwitterionic polymeric monoliths for HILIC/RP mixed mode for CEC separation applications

Polymer-based monoliths with zwitterionic surface character were synthesized in capillary columns following a two-step approach to provide versatile electrochromatographic stationary phases exhibiting potentiality of both hydrophilic interaction and RP separation modes. UV-initiated free radical copolymerization of N-acryloxysuccinimide and ethylene dimethacrylate was performed using azobisisobutyronitrile as initiator and toluene as porogen. One of the originalities of this approach relies on the dual role of the N-acryloxysuccinimide monomer that is successively used during the preparation protocol to first covalently graft chromatographic selectors on the monolith surface via simple nucleophilic substitution reaction and then to generate negative charges through hydrolysis of remaining N-hydroxysuccinimide units. In this respect, the grafting of hexyldiamine affords potential cationic surface charges. It is shown that it is possible to tune, controlling the pH of the mobile phase, the intensity and direction of the generated EOF. Moreover, the nature of the interfacial interaction process responsible for the observed separations is well governed by the composition of the mobile phase. Polymer backbone hydrophilization is proposed as an efficient way to improve the HILIC behavior of poly(N-acryloxysuccinimide-co-ethylene dimethacrylate) based monolithic CEC columns together with the grafting of an alkyldiamine incorporating a shorter aliphatic segment.

Capillary chromatography based on tube radial distribution of aqueous-organic mixture carrier solvents: introduction of double tubes having different inner diameters to the system

A tube radial distribution chromatography (TRDC) system was previously developed using an open capillary tube and an aqueous-organic solvent (water-acetonitrile-ethyl acetate) mixture as a carrier solution. In this study, we introduced double capillary tubes having different inner diameters to the system. The tubes were fused-silica capillary tubes with 100 and 250 microm i.d.; the smaller tube was inserted into the larger one through a T-type joint. Water-acetonitrile mixture (volume ratio 3:1) and acetonitrile-ethyl acetate mixture (volume ratio 4:1) solutions were delivered into the large tube from the inside through the small tube and from the outside through the joint, respectively, and then mixed through the large tube to provide a water-acetonitrile-ethyl acetate carrier solution. The carrier solution was further fed into the large tube and then to an absorption detector. By changing the flow rates of the mixture solutions in both tubes, we could control the component ratio of carrier solvents in the carrier solution, yielding either organic solvent-rich, water-acetonitrile-ethyl acetate (volume ratio 3:33:8) or water-rich, water-acetonitrile-ethyl acetate (volume ratio 24:12:1). A model analyte-mixture solution of 1-naphthol and 2,6-naphthalenedisulfonic acid was eluted in this order with the organic solvent-rich carrier solution and eluted in the reverse order with the water-rich carrier solution. We discussed the chromatographic data together with the analytical conditions from the viewpoint of the tube radial distribution of the carrier solvents.