Porous Polymer Monolithic Columns Research Articles

Porous polymer monolithic columns to investigate the interaction of humic substances with herbicides and emerging pollutants by affinity chromatography

BackgroundUnderstanding the interaction mechanisms and the relevant binding constants between humic acids and emerging or regulated pollutants is of utmost importance in predicting their geochemical mobility, bioavailability, and degradation. Fluorescence spectroscopy, UV–vis spectroscopy, equilibrium dialysis, and solid-phase extraction combined with liquid chromatography-mass spectrometry have been employed to elucidate interactions of humic acids with organic micropollutants, especially pharmaceutical drugs. These methods demand large sample volumes, long equilibration times, and laborious extraction steps which may imply analytical errors. Monolithic high-performance affinity chromatography is an alternative and simpler method to investigate these interactions and determine the binding constants. ResultsPolymer monoliths based on aminated glycidyl methacrylate and ethylene glycol dimethacrylate served to immobilize Cu(II) and then humic acid to produce monolithic affinity chromatography columns with humic acid as the active interaction phase. About 86.5 mg of humic acid was immobilized per gram of polymer. The columns enabled a comparison of the binding strength of humic acid with herbicides and emerging pollutants at 25 °C and pH 6.0 ± 0.1. Paracetamol, acetylsalicylic acid, and salicylic acid did not retain. Among the compounds that interacted with humic acid, the order of increasing affinity, estimated by the global affinity constant (nKa) or partition coefficient (KD) was: caffeine < simazine < atrazine ∼ propazine < benzophenone. The nKa (L mol−1) values ranged from (4.9 ± 0.3) × 102 for caffeine to (1.9 ± 0.3) × 103 for benzophenone, whereas KD (L kg−1) varied from 14 ± 1 to 56 ± 8 for the same compounds. Significance and noveltyTo our knowledge, this is the first paper demonstrating the use of a monolithic platform to immobilize supramolecular structures of humic acids exploiting immobilized metal affinity to comparatively evaluate their affinity towards emerging pollutants exploiting the concepts of high-performance affinity chromatography. The proposed approach needs only small amounts of humic acid, which is a relevant feature in preparing columns with humic substances isolated and purified from remote areas.

Front Cover: A thermodynamic study of capillary electrochromatographic retention of aromatic hydrocarbons on a lauryl acrylate porous polymer monolithic column with measured phase ratio

Front Cover: A thermodynamic study of capillary electrochromatographic retention of aromatic hydrocarbons on a lauryl acrylate porous polymer monolithic column with measured phase ratio

A thermodynamic study of capillary electrochromatographic retention of aromatic hydrocarbons on a lauryl acrylate porous polymer monolithic column with measured phase ratio.

The phase ratio of a chromatographic system is an important measurement that has long been estimated or calculated, but rarely directly measured. This study utilized a nanoflow liquid chromatography instrument to more accurately measure the phase ratio for a lauryl acrylate porous polymer monolith. Direct measurement of the phase ratio, and its dependence on temperature, allows for a better understanding of the thermodynamics of retention of small analytes. This study investigates the retention of an alkyl benzene series, toluene to octylbenzene, via capillary electrochromatography. The phase ratio was determined to be 0.202 at 303 K and 0.213 at 333K. Using the directly measured phase ratio, entropic contributions to retention can also be obtained. Therefore, the Gibbs free energy calculations from these measurements and methods can give insight to modes of retention. The free energy of retention for toluene is -3.97kJ/mol at 303 K and -3.78kJ/mol at 333K. The trends for enthalpy, entropy, and Gibbs free energy of transfer show that retention is enthalpically driven in this capillary electrochromatography (CEC) porous polymer monolith system.

Fast Determination of Residual Sulfonamides in Milk by In-Tube Solid-Phase Microextraction Coupled with Capillary Electrophoresis-Laser Induced Fluorescence

Abstract A fast, feasible, eco-friendly method for determination of sulfonamides using in-tube solid-phase microextraction (IT-SPME) coupled with capillary electrophoresis-laser induced fluorescence was developed. The graphene-embedded porous polymer monolithic column prepared via in-situ UV polymerization was proposed as the IT-SPME column. The method was applied to determine residual sulfonamides in a real milk sample, and an amount of 3.7 ng mL−1 sulfamethazine was detected, which was much lower than the maximum residue limit.

Polymer-based monolithic column with incorporated chiral metal-organic framework for enantioseparation of methyl phenyl sulfoxide using nano-liquid chromatography.

A new approach to the preparation of enantioselective porous polymer monolithic columns with incorporated chiral metal-organic framework for nano-liquid chromatography has been developed. While no enantioseparation was achieved with monolithic poly(4-vinylpyridine-co-ethylene dimethacrylate) column, excellent separations of both enantiomers of (±)-methyl phenyl sulfoxide were achieved with its counterpart prepared after admixing metal-organic framework [Zn2 (benzene dicarboxylate)(l-lactic acid)(dmf)], which is synthesized from zinc nitrate, l-lactic acid, and benzene dicarboxylic acid in the polymerization mixture. These novel monolithic columns combined selectivity of the chiral framework with the excellent hydrodynamic properties of polymer monoliths, may provide a great impact on future studies in the field of chiral analysis by liquid chromatography.

Integrated on-chip mass spectrometry reaction monitoring in microfluidic devices containing porous polymer monolithic columns.

Chip-based microfluidics enable the seamless integration of different functions into single devices. Here, we present microfluidic chips containing porous polymer monolithic columns as a means to facilitate chemical transformations as well as both downstream chromatographic separation and mass spectrometric analysis. Rapid liquid phase lithography prototyping creates the multifunctional device economically.

Preparation of a porous polymer monolithic column with an ionic liquid as a porogen and its applications for the separation of small molecules in high performance liquid chromatography

A monolith based on an ionic liquid as a porogen was prepared to enhance the column efficiency of small molecule separation in HPLC.

Nanostructured porous polymer monolithic columns for capillary liquid chromatography of peptides

The macroporous structure of poly(styrene-co-divinylbenzene) monolithic capillary columns has been optimized for the gradient separation of peptides. To exploit monolithic supports with porosity exceeding 70%, the thermodynamic properties of the polymerization mixture were carefully tailored to yield homogeneous monolithic materials featuring macropore and polymer microglobule sizes in the range of 50–200nm. The effects of (i) initiator content, (ii) composition of porogenic mixture, comprising tetrahydrofuran and 1-decanol, (iii) percentage of divinylbenzene crosslinker, and (iv) monomers to porogen ratio on the morphology was investigated. The resulting column structures were investigated using scanning electron microscopy and the prepared monolithic columns were tested for the separation of a tryptic digest of cytochrome c while applying a fixed flow rate and gradient time. To obtain a better understanding of the effects of macropore and microglobule size, and structure homogeneity on the separation performance in gradient elution, both in terms of peak capacity and gradient plate height, separations were also carried out at different flow rates while maintaining a constant gradient steepness. Furthermore, performance limits were determined applying ultra-high pressure conditions up to the maximum system pressure of 80MPa. The potential of monolithic nanostructured columns is demonstrated for the separation of tryptic digests of cytochrome c and bovine serum albumin.

Porous polymer monolithic columns with gold nanoparticles as an intermediate ligand for the separation of proteins in reverse phase-ion exchange mixed mode

A new approach has been developed for the preparation of mixed-mode stationary phases to separate proteins. The pore surface of monolithic poly(glycidyl methacrylate-co-ethylene dimethacrylate) capillary columns was functionalized with thiols and coated with gold nanoparticles. The final mixed mode surface chemistry was formed by attaching, in a single step, alkanethiols, mercaptoalkanoic acids, and their mixtures on the free surface of attached gold nanoparticles. Use of these mixtures allowed fine tuning of the hydrophobic/hydrophilic balance. The amount of attached gold nanoparticles according to thermal gravimetric analysis was 44.8wt.%. This value together with results of frontal elution enabled calculation of surface coverage with the alkanethiol and mercaptoalkanoic acid ligands. Interestingly, alkanethiols coverage in a range of 4.46–4.51molecules/nm2 significantly exceeded that of mercaptoalkanoic acids with 2.39–2.45molecules/nm2. The mixed mode character of these monolithic stationary phases was for the first time demonstrated in the separations of proteins that could be achieved in the same column using gradient elution conditions typical of reverse phase (using gradient of acetonitrile in water) and ion exchange chromatographic modes (applying gradient of salt in water), respectively.

A study of the methylene/perfluormethylene selectivity of porous polymer monolithic stationary phases exhibiting different fluorous/hydrophobic content

Porous polymer monolithic columns are prepared from a variety of monomers and cross-linkers and can be customized to exhibit different selectivities for separate analyte classes. The composition of the monolith can be precisely controlled by selecting different monomers and or cross-linker ratios. In this work monoliths exhibiting both fluorous and hydrophobic character were prepared using butyl methacrylate and its fluorous analogue (monomer) and 1,3-butanediol diacrylate and its fluorous analogue (cross-linker) in different ratios. The selectivity of the monoliths was probed using capillary electrochromatography with several fluorous and alkyl benzene analytes. Hydrophobic stationary phases exhibited greater methylene selectivity (α−CH2−) while those with increasing fluorous character show enhanced pefluoromethylene selectivity (α−CF2−). The Gibbs free energy change associated with the sorption of the analytes on each stationary phase composition can be calculated from migration times (i.e. capacity factor) for the addition of an individual −CF2− or −CH2− moiety. Furthermore, the Gibbs free energy change associated with a single −CF2− or −CH2− moiety (analyte) interacting with an individual −CF2− or −CH2− (stationary phase) can also be estimated by plotting fluorous column composition against ΔG−CH2−⁡or CF2−0. Furthermore ΔG−CH2−0 and ΔG−CF2−0 can be plotted versus H2O percentage in mobile phase, and a new concept, hypothetical water percentage (HWP) is proposed to evaluate the hydrophobicity/fluorophilicity of a stationary phase.

Application of cyclodextrin-modified gold nanoparticles in enantioselective monolith capillary electrochromatography

β-cyclodextrin modified gold nanoparticles (CD-GNP) were employed as the stationary phase in monolith capillary electrochromatography (CEC) to facilitate enantioseparation. CD-GNP were covalently bound to the surface of the thiolated porous polymer monolithic column. The fabricated enantioselective monolithic column was characterized by a variety of spectroscopic methods. The column exhibited steady EOF mobility over pH values ranging from 4.6 to 9.7. Additionally, the column was stable under CEC separation conditions over 180min. Moreover, the column exhibited good column-to-column reproducibility. The CD-GNP-modified monolithic column was employed in the efficient CEC separation of three pairs of drug enantiomers (chlorpheniramine, zopiclone and tropicamide). The results exhibit reproducible run-to-run enantioseparations and the monolith column can maintain its enantioselectivity for more than 1 month if the column is stored in a CD-GNP solution at 4°C.

Preparation of porous polymer monolithic column using functionalized graphene oxide as a functional crosslinker for high performance liquid chromatography separation of small molecules

A newly developed porous polymer monolith was prepared through copolymerization of 3-(trimethoxysilyl)propylmethacrylate modified graphene oxide with glycidyl methacrylate and ethylene dimethacrylate as a functional crosslinker, which was synthesized through silanization reaction of graphene oxide prepared by Hummers method with 3-(trimethoxysilyl)propylmethacrylate. The monolith was characterized by Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy, mercury intrusion porosimetry and nitrogen adsorption measurement. The monolith column was applied as the stationary phase of high performance liquid chromatography and its chromatographic performance was evaluated by separation of small molecules in the isocratic reversed-phase mode. The chromatograms of hydrophobic steroids and polar aromatic amines on the prepared monolith displayed the enhanced separation performance over those on the parent monolith. The reproducibility of the column was less than 3.5% in terms of relative standard deviation of retention time. The results demonstrate that copolymerization of functionalized graphene oxide into porous polymer monolith was an effective tool for chromatography separation enhancement of small molecules in an isocratic mode.

Preparation of porous polymer monolithic column incorporated with graphene nanosheets for solid phase microextraction and enrichment of glucocorticoids

Novel monolithic capillary columns with embedded graphene were developed and used for polymer monolith microextraction (PMME) coupled to LC–MS analysis. The column was prepared inside fused silica capillaries (320μm, i.d.) using thermally initiated free-radical polymerization with butyl methacrylate (BMA) as monomer, ethylene dimethacrylate (EDMA) as cross-linker, and 1,4-butanediol and 1-propanol as porogens. Graphene (GN) was incorporated into the poly(BMA–EDMA) monolith to enhance the loading capacity. The obtained GN and the poly(BMA–EDMA–GN) monolith were characterized by transmission electron microscope (TEM), atomic force microscopic (AFM) and scanning electron microscopy (SEM). The extraction performance of the monolithic column was evaluated by glucocorticoids (GCs) as the analytes. The operation parameters of PMME including desorption solvent, sample flow rate, sample volume, sample pH, and eluent flow rate were studied and optimized. When compared to the parent poly(BMA–EDMA) column and direct sample analysis, high enrichment capacity was observed in the case of GN-entrapped monolith. The GN incorporated monolithic capillary showed satisfactory reusability and stability during extraction. The limits of detection (S/N=3) for nine GCs were in the range of 0.13–1.93ng/mL. Relative standard deviations for the determination of the target GCs were less than 14.5%. Finally, the PMME method, based on the developed monolithic capillary as the extraction media, was successfully applied to the determination of nine GCs in cosmetics. The recoveries of GCs spiked in different matrices ranged from 83.7% to 103.8%.

On-chip sample pretreatment using a porous polymer monolithic column for solid-phase microextraction and chemiluminescence determination of catechins in green tea

A porous polymer monolithic column for solid-phase microextraction and chemiluminescence detection was integrated into a simple microfluidic chip for the extraction and determination of catechins in green tea. The porous polymer was prepared by poly(glycidyl methacrylate-co-ethylene dimethacrylate) and modified with ethylenediamine. Catechins can be concentrated in the porous polymer monolithic column and react with potassium permanganate to give chemiluminescence. The microfluidic chip is reusable with high sensitivity and very low reagent consumption. The on-line preconcentration and detection can be realized without an elution step. The enrichment factor was calculated to be about 20 for catechins. The relative chemiluminescence intensity increased linearly with concentration of catechin from 5.0 × 10(-9) to 1.0 × 10(-6) M and the limit of detection was 1.0 × 10(-9) M. The proposed method was applied to determine catechin in green tea. The recoveries are from 90% to 110% which benefits the actual application for green tea samples.

Porous polymer monolithic column with surface-bound gold nanoparticles for the capture and separation of cysteine-containing peptides.

A new porous polymer monolithic capillary column modified with gold nanoparticles that enables the selective capture of cysteine-containing peptides has been developed to reduce the complexity of peptide mixtures generated in bottom-up proteomic analysis. The column is prepared from a poly(glycidyl methacrylate-co-ethylene dimethacrylate) monolith through reaction of some of its epoxide moieties with cysteamine to afford a monolith rich in surface thiol groups. In situ reduction of chloroauric acid within the column is then used to form gold nanoparticles attached to the surface of the pores of the monolith. This process preserves the excellent hydrodynamic properties of the monolithic column while providing a means to selectively retain cysteine-containing peptides from an analyte due to their high affinity for gold. Release of the retained peptides is subsequently achieved with an excess of 2-mercaptoethanol. The loading capacity determined for l-cysteine using frontal elution is 2.58 mumol/m. Since the gold-thiol link is less stable at elevated temperatures, the adsorption capacity is recovered by washing the column at 80 degrees C for 2 h. While regeneration is easy, the multiplicity of bonds between the monolithic support and the gold nanoparticles prevents their elution even under harsh conditions such as treatment with pure 2-mercaptoethanol or treatment with boiling water for 5 h. Application of the gold modified monolith in tandem with a packed C18 capillary column is demonstrated with baseline separation of a peptide mixture achieved in a two step process. The first involves retention of cysteine-containing peptides in monolith with reversed phase separation of all other peptides, while the retained peptides are released from monolith and separated in the second step.

Characteristics and recognition mechanism of monolithic poly(methacrylic acid-ethylene glycol dimethacrylate) column

Porous polymer monolithic columns have been prepared by the direct free radical copolymerization of methacrylic acid and ethylene glycol dimethacrylate within the confines of a chromatographic column in the presence of toluene-dodecanol as a porogenic solvent. Recognition mechanism and effects of the chromatographic condition such as mobile phase composition, flow rate and temperature on the retention and separation were discussed. The results showed hydrogen-bonding interaction and hydrophobic interaction play an important role in retention and separation in this kind of monolithic column. Compared with traditional particle columns, the monolithic columns showed attractive significant interest because of their ease of preparation, high separation efficiency, low backpressure and fast analysis.

Preparation and Characterization of Monolithic Poly(methacrylic acid - ethylene glycol dimethacrylate) Columns for High Performance Liquid Chromatography

Porous polymer monolithic columns were prepared by the direct free radical copolymerization of methacrylic acid and ethylene glycol dimethacrylate within the confines of a chromatographic column in the presence of toluene-dodecanol as a porogenic solvent. The separation characteristics of the monolithic columns were tested by a homologous series of xanthine derivatives, theophylline and caffeine. The effects of the polymerization mixture composition and polymerization condition, mobile phase composition, flow rate and temperature on the retention times and separation efficiencies were investigated. The results showed that the selection of correct porogenic solvents and appropriate polymerization conditions are crucial for the preparation of the monolithic stationary phases. The separation efficiency was only extremely weakly dependent on flow rate and temperatures. Hydrogen-bonding interaction played an important role in the retention and separation. Compared with conventional particle columns, the monolithic column exhibited good stability, ease of regeneration, high separation efficiency and fast analysis.

Effect of porosity and surface chemistry on the characterization of synthetic polymers by HPLC using porous polymer monolithic columns

Monolithic 50 × 4.6 mm ID poly(styrene-co-divinylbenzene), poly(glycidyl methacrylate-co-ethylene dimethacrylate), and poly(2,3-dihydroxypropyl methacrylate-co-ethylene dimethacrylate) columns have been tested in the high-performance liquid chromatography (HPLC) separation of polystyrenes and poly(methyl methacrylate) standards. The separation process involves precipitation of the macromolecules on the macroporous monolithic column followed by progressive redissolution and elution utilizing a gradient of the mobile phase. While very good separations of individual components of mixtures of both polystyrenes and poly(methyl methacrylates) can be readily achieved using poly(styrene-co-divinylbenzene) monolith, methacrylate-based columns do not afford good separations under identical conditions. This unexpected finding appears to be the result of significant differences between the porous properties and chemistries of the two types of monolithic materials.