Comparison of thermal‐ and photo‐polymerization of lauryl methacrylate monolithic columns for CEC

Lauryl methacrylate‐based (LMA) monolithic columns for CEC, prepared using either thermal initiation or by UV‐irradiation in the presence of AIBN have been compared. Thermal polymerization was carried out at 70°C for 20 h. For UV initiation, the effects of the time exposure to UV light and irradiation energy were investigated. For each initiation process, the influence of composition of porogenic solvent (1,4‐butanediol/1‐propanol ratio) on the physical and electrochromatographic properties of the resulting monoliths was also evaluated. Photochemically lauryl methacrylate stationary phases initiated showed higher permeabilities and better efficiencies than those prepared by thermal initiation. After optimization of polymerization mixture, photopolymerized columns provided a permeability of 4.25×10^?13 m² and a minimum plate height of 13.4 μm for a mixture of polycyclic aromatic hydrocarbons. Similar column‐to‐column and batch‐to‐batch reproducibilities, with RSD values below 11.6 and 11.0 % for the thermal‐ and UV‐initiated columns, respectively, were obtained.     

Comprehensive two‐dimensional monolithic liquid chromatography of polar compounds

Two‐dimensional liquid chromatography largely increases the number of separated compounds in a single run, theoretically up to the product of the peaks separated in each dimension on the columns with different selectivities. On‐line coupling of a reversed‐phase column with an aqueous normal‐phase (hydrophilic interaction liquid chromatography) column yields orthogonal systems with high peak capacities. Fast on‐line two‐dimensional liquid chromatography needs a capillary or micro‐bore column providing low‐volume effluent fractions transferred to a short efficient second‐dimension column for separation at a high mobile phase flow rate. We prepared polymethacrylate zwitterionic monolithic micro‐columns in fused silica capillaries with structurally different dimethacrylate cross‐linkers. The columns provide dual retention mechanism (hydrophilic interaction and reversed‐phase). Setting the mobile phase composition allows adjusting the separation selectivity for various polar substance classes. Coupling on‐line an organic polymer monolithic capillary column in the first dimension with a short silica‐based monolithic column in the second dimension provides two‐dimensional liquid chromatography systems with high peak capacities. The silica monolithic C₁₈ columns provide higher separation efficiency than the particle‐packed columns at the flow rates as high as 5 mL/min used in the second dimension. Decreasing the diameter of the silica monolithic columns allows using a higher flow rate at the maximum operation pressure and lower fraction volumes transferred from the first, hydrophilic interaction dimension, into the second, reversed‐phase mode, avoiding the mobile phase compatibility issues, improving the resolution, increasing the peak capacity, and the peak production rate.     

Rapid determination of sterols in vegetable oils by CEC using methacrylate ester‐based monolithic columns

A method for the determination of sterols in vegetable oils by CEC with UV–Vis detection, using methacrylate ester‐based monolithic columns, has been developed. To prepare the columns, polymerization mixtures containing monomers of different hydrophobicities were tried. The influence of composition of polymerization mixture was optimized in terms of porogenic solvent, monomers/porogens and monomer/crosslinker ratios. The composition of the mobile phase was also studied. The optimum monolith was obtained with lauryl methacrylate monomer at 60:40% (wt:wt) lauryl methacrylate/ethylene dimethacrylate ratio and 60 wt% porogens with 20 wt% of 1,4‐butanediol (12 wt% 1,4‐butanediol in the polymerization mixture). Excellent resolution between sterols was achieved in less than 7 min with an 85:10:5 v/v/v ACN–2‐propanol–water buffer containing 5 mM Tris at pH 8.0. The limits of detection were lower than 0.04 mM, and inter‐day and column‐to‐column reproducibilities at 0.75 mM were better than 6.2%. The method was applied to the determination of sterols in vegetable oils with different botanical origins and to detect olive oil adulteration with sunflower and soybean oils.     

Photografted methacrylate‐based monolithic columns coated with cellulose tris(3,5‐dimethylphenylcarbamate) for chiral separation in CEC

A chiral capillary monolithic column for enantiomer separation in capillary electrochromatography was prepared by coating cellulose tris(3,5‐dimethylphenylcarbamate) on porous glycidyl methacrylate‐co‐ethylene dimethacrylate monolith in capillary format grafted with chains of [2(methacryloyloxy)ethyl] trimethylammonium chloride. The surface modification of the monolith by the photografting of [2(methacryloyloxy)ethyl] trimethylammonium chloride monomer as well as the coating conditions of cellulose tris(3,5‐dimethylphenylcarbamate) onto the grafted monolithic scaffold were optimized to obtain a stable and reproducible chiral stationary phase for capillary electrochromatography. The effect of organic modifier (acetonitrile) in aqueous mobile phase for the enantiomer separation by capillary electrochromatography was also investigated. Several pairs of enantiomers including acidic, neutral, and basic analytes were tested and most of them were partially or completely resolved under aqueous mobile phases. The prepared monolithic chiral stationary phases exhibited a good stability, repeatability, and column‐to‐column reproducibility, with relative standard deviations below 11% in the studied electrochromatographic parameters.     

Longevity and other practical benefits of monolithic silica columns in the analysis of samples with complex matrices

At the turn of the millennium, the monolithic columns invoked new chances in HPLC. Even more than their organic polymer-based siblings, the inorganic silica-based monoliths targeted the territory of classical fully porous particle-packed columns, promising many benefits. Based on the number of published articles, the monoliths attracted academics just in the first few years after their introduction to the market. Lately, as superficially porous particles and sub-2-micron fully porous particles dominated the market, they stayed in the focus of routine laboratories and those who really appreciated the high porosity of the monolithic bed. The monoliths' practical benefits cannot be easily traced in the literature when they gradually lose academics' interest. Nevertheless, after more than 20 years of our experience, we still favor silica monoliths for their low back pressure and longevity when analyzing samples of clinical, pharmaceutical, and environmental origin. At the same time, the high permeability of monoliths enabled the birth of sequential injection chromatography, the medium-pressure separation technique based on the flexible flow manifold. This minireview aims to check, discuss, and summarize the practical aspects of monolithic silica columns in HPLC and medium-pressure sequential injection chromatography (SIC) that may not be visible at first sight but are evident retrospectively.     

Photo‐polymerized lauryl methacrylate monolithic columns for CEC using lauroyl peroxide as initiator

Lauryl methacrylate (LMA)‐ester based monolithic columns photo‐polymerized using lauroyl peroxide (LPO) as initiator were prepared, and their morphological and CEC properties were studied. The composition of the polymerization mixture (i.e. ratios of monomers/porogenic solvents, 1,4‐butanediol/1‐propanol and LMA/crosslinker) was optimized. The morphological and chromatographic properties of LMA columns were evaluated by means of SEM pictures and van Deemter plots of PAHs, respectively. The polymerization mixture selected as optimal provided a fast separation of a mixture of PAHs with excellent efficiencies (minimum plate heights of 8.9–11.1 μm). Satisfactory column‐to‐column (RSD<4.5%) and batch‐to‐batch reproducibilities (RSD<6.3%) were achieved. The LMA columns photo‐polymerized with LPO were compared with those prepared with AIBN. Using PAHs, alkylbenzenes and basic compounds for testing, the columns obtained with LPO gave the best compromise between efficiency, resolution and analysis time.     

Electrical and optical simulations of a polymer‐based phosphorescent organic light‐emitting diode with high efficiency

A comprehensive numerical device simulation of the electrical and optical characteristics accompanied with experimental measurements of a new highly efficient system for polymer‐based light‐emitting diodes doped with phosphorescent dyes is presented. The system under investigation comprises an electron transporter attached to a polymer backbone blended with an electronically inert small molecule and an iridium‐based green phosphorescent dye which serves as both emitter and hole transporter. The device simulation combines an electrical and an optical model. Based on the known highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of all components as well as the measured electrical and optical characteristics of the devices, we model the emissive layer as an effective medium using the dye's HOMO as hole transport level and the polymer LUMO as electron transport level. By fine‐tuning the injection barriers at the electron and hole‐injecting contact, respectively, in simulated devices, unipolar device characteristics were fitted to the experimental data. Simulations using the so‐obtained set of parameters yielded very good agreement to the measured current–voltage, luminance–voltage characteristics, and the emission profile of entire bipolar light‐emitting diodes, without additional fitting parameters. The simulation was used to gain insight into the physical processes and the mechanisms governing the efficiency of the organic light‐emitting diode, including the position and extent of the recombination zone, carrier concentration profiles, and field distribution inside the device. The simulations show that the device is severely limited by hole injection, and that a reduction of the hole‐injection barrier would improve the device efficiency by almost 50%. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Simultaneous determination of NG‐monomethyl‐l‐arginine,NG,NG‐dimethyl‐l‐arginine,NG,NG′‐dimethyl‐l‐arginine,and l‐arginine using monolithic silica disk‐packed spin columns and a monolithic silica column

We have developed and validated a high‐performance liquid chromatography method that uses monolithic silica disk‐packed spin columns and a monolithic silica column for the simultaneous determination of N^G‐monomethyl‐l ‐arginine, N^G,N^G‐dimethyl‐l ‐arginine, and N^G,N^G′‐dimethyl‐l ‐arginine in human plasma. For solid‐phase extraction, our method employs a centrifugal spin column packed with monolithic silica bonded to propyl benzenesulfonic acid as a cation exchanger. After pretreatment, the methylated arginines are converted to fluorescent derivatives with 4‐fluoro‐7‐nitro‐2,1,3‐benzoxadiazole, and then the derivatives are separated on a monolithic silica column. l ‐Arginine concentration was also determined in diluted samples. Standard calibration curves revealed that the assay was linear in the concentration range 0.2–1.0 μM for methylated arginines and 40–200 μM for l ‐arginine. Linear regression of the calibration curve yielded equations with correlation coefficients of 0.999 (r²). The sensitivity was satisfactory, with a limit of detection ranging from 3.75 to 9.0 fmol for all four compounds. The RSDs were 4.3–4.8% (intraday) and 3.0–6.8% (interday). When this method was applied to samples from six healthy donors, the detected concentrations of N^G‐monomethyl‐l ‐arginine, N^G,N^G‐dimethyl‐l ‐arginine, N^G,N^G′‐dimethyl‐l ‐arginine and l ‐arginine were 0.05 ± 0.01, 0.41 ± 0.07, 0.59 ± 0.11, and 83.8 ± 30.43 μM (n = 6), respectively.     

有机聚合物整体柱的制备与应用的研究进展

整体柱具有通透性能良好和传质速度快等特点,可实现快速、高效、高通量的分离,近年来已引起人们的热切关注。聚合物整体柱是其中应用最为广泛的一种,它是由单体、交联剂、致孔剂和引发剂等通过原位聚合得到的连续均一的棒状聚合物,具有取材广泛,使用pH范围比较宽,生物兼容性好等特点,通过化学修饰,可以用作多种色谱模式的固定相。该文主要综述了2003年至2006年期间有关聚合物整体柱制备和应用的研究进展。     

Novel affinity monolithic column modified with cuprous sulfide nanoparticles for the selective enrichment of low‐molecular‐weight electron‐rich analytes

A novel monolithic column modified with cuprous sulfide nanoparticles was developed and its affinity characteristics towards low‐molecular‐weight electron‐rich analytes were investigated. In the synthesis process, home‐made cuprous oxide nanocubes were immobilized on the surface of monolithic skeleton with the moderate thickness based on the strong interaction between imidazole groups and cuprous oxide, then the cuprous oxide layer was transformed into the more stable cuprous sulfide layer through the treatment by sodium sulfide. The resulting cuprous sulfide modified monolithic column presented good permeability and stability in a wide pH range from 2 to 10. Two kinds of typical electron‐rich analytes, kanamycin A and purine, were chosen to assess its affinity characteristics. Compared with the commercial Cu²⁺‐ and Ni²⁺‐based affinity sorbents, a larger binding capacity of cuprous sulfide modified column toward kanamycin A was obtained under basic condition and the recovery of kanamycin A in a milk sample was over 70%. Moreover, the binding capacity of cuprous sulfide modified column for purine was up to 5.57 mg/mL in frontal elution mode. These results suggested that the Cu₂S column has a promising application for the enrichment of electron‐rich analytes.     

Study of elution behaviour with gradient voltage in CEC using methacrylate monolithic columns

A theoretical study on the retention behaviour and chromatographic performance of neutral solutes using a lauryl methacrylate‐based monolithic column under voltage gradient mode in CEC was carried out. Through a flexible mathematical function based on a modified Gaussian model, the peak shape of compounds was firstly fitted under constant and gradient voltage. Using the peak shape parameters and retention time, the estimation of global chromatographic performance, efficiency and peak capacity under several voltage conditions was performed. The influence of voltage gradient on the separation efficiency is discussed and simple equations are presented to calculate retention and peak widths under voltage gradient conditions. A comparison in terms of chromatographic performance of a test mixture of neutral solutes under constant and gradient voltage modes was also carried out. The experiments carried out under gradient voltage showed better efficiencies (172 000 plates/m) and lower peak widths than those obtained under constant voltage (52 000 plates/m).     

Organic polymer‐based monolithic capillary columns and their applications in food analysis

In recent years, the use of organic polymer monolithic capillary columns in separation science has gained popularity due to the fact that they are easy to fabricate and do not require retaining frits. These materials have been applied in different fields including foods, proteomics, and pharmaceuticals. The interest in food analysis still needs to develop in order to increase the sensitivity towards micro/nano‐scale food applications for food samples of < 5 μg (e.g., foodomics). In this regard, polymer monolithic capillary columns offer great separation capability in the food analytical separation science. We review the most important applications in food analysis using polymer monolithic capillary columns. In addition, several examples of the use of capillary separation methods combined with mass spectrometry detection in food analysis are summarized.     

整体柱制备技术的新进展及其在蛋白质组学中的应用

整体柱是通过在柱管内原位聚合或固化的方法制备得到的具有多孔结构的整体棒状固定相,与传统的填充柱相比,具有通透性好、传质速率快、容易制备等优点,因此在分离分析领域特别是生物分离分析中发挥的作用日益增大。整体柱的制备及应用近年来也得到了快速发展,层出不穷的新型整体柱已被广泛用于色谱高效分离分析、固相萃取及酶反应器等方面,大大推动了分离分析科学的发展。本文主要总结了近五年来整体柱的制备技术及其在蛋白质组学应用中的一些最新研究进展。     

Study of peak shape and efficiency in butyl acrylate-based monolithic columns for capillary electrochromatography

The study and modelling peak shape of in capillary electrochromatography (CEC), obtained using butyl acrylate (BA)-based monolithic columns, is described in this article. A modified-Gaussian model, which is a function of conventional experimental parameters: retention time (t_R), peak height (H₀) and standard deviation (σ₀) at the peak maximum, and left (A) and right (B) halfwidths, was used to describe the peaks of thiourea and several polyaromatic hydrocarbons compounds, which were eluted under several applied voltages. A mean relative error below 2% in the prediction of peak shape is obtained. Based on peak parameters, an easy and reliable estimation of global chromatographic performance, efficiency and peak capacity of BA-based monoliths was also considered. A comparison in terms of chromatographic performance of BA-based monoliths in CEC with CE mode and RPLC microparticulate columns was also performed.     

Fabrication of an ionic‐liquid‐based polymer monolithic column and its application in the fractionation of proteins from complex biosamples

An ionic‐liquid‐based polymer monolithic column was synthesized by free radical polymerization within the confines of a stainless‐steel column (50 mm × 4.6 mm id). In the processes, ionic liquid and stearyl methacrylate were used as dual monomers, ethylene glycol dimethacrylate as the cross‐linking agent, and polyethylene glycol 200 and isopropanol as co‐porogens. Effects of the prepolymerization solution components on the properties of the resulting monoliths were studied in detail. Scanning electron microscopy, nitrogen adsorption–desorption measurements, and mercury intrusion porosimetry were used to investigate the morphology and pore size distribution of the prepared monoliths, which showed that the homemade ionic‐liquid‐based monolith column possessed a relatively uniform macropore structure with a total macropore specific surface area of 44.72 m²/g. Compared to a non‐ionic‐liquid‐based monolith prepared under the same conditions, the ionic‐liquid‐based monolith exhibited excellent selectivity and high performance for separating proteins from complex biosamples, such as egg white, snailase, bovine serum albumin digest solution, human plasma, etc., indicating promising applications in the fractionation and analysis of proteins from the complex biosamples in proteomics research.     

A molecularly imprinted organic–inorganic hybrid monolithic column for the selective extraction and HPLC determination of isoprocarb residues in rice

An IPC‐imprinted (IPC is isoprocarb) poly(methacrylic acid)/SiO₂ hybrid monolithic column was prepared and applied for the recognition of the template. The hybrid monolithic column was synthesized in a micropipette tip using methyltrimethoxysilane as the inorganic precursor, 3‐(methacryloxy)propyltrimethoxysilane as the coupling agent, and ethylene glycol dimethacrylate as the cross‐linker. The synthesis conditions, including the porogenic solvent, coupling agent, volume ratio of the inorganic alcoholysate and organic part, were optimized. The prepared monolithic column was characterized by SEM and FTIR spectroscopy. A simple, rapid, and sensitive method for the determination of IPC in rice using the imprinted monolithic column microextraction combined with HPLC was developed. Several parameters affecting the sample pretreatment were investigated, including the eluent, washing solution, and loading sample volume. The linearity of the calibration curve was observed in the range of 9.0–1000 μg/kg for IPC in rice with the correlation coefficient (r²) of 0.9983. The LOD was 3.0 μg/kg (S/N = 3). The assay gave recovery values ranging from 91 to 107%. The proposed method has been successfully applied for the selective extraction and sensitive determination of IPC in rice and a satisfactory result was obtained.     

Preparation of a long‐alkyl‐chain‐based hybrid monolithic column with mixed‐mode interactions using a “one‐pot” process for pressurized capillary electrochromatography

A simple “one‐pot” approach for the preparation of a new vinyl‐functionalized organic–inorganic hybrid monolithic column is described. In this improved method, the hydrolyzed alkoxysilanes of tetramethoxysilane and triethoxyvinylsilane were used as precursors for the synthesis of a silica‐based monolith, while 1‐hexadecene and sodium ethylenesulfonate were used as vinyl functional monomers along with azobisisobutyronitrile as an initiator. The effects of reaction temperature, urea content, and composition of organic monomers on the column properties (e.g. morphology, mechanical stability, and chromatographic performance) were investigated. The monolithic column was used for the separation of neutral solutes by reversed‐phase pressurized capillary. Furthermore, the monolith can separate various aromatic amines, which indicated its excellent cation‐exchange capability and hydrophobic interactions. The baseline separation of the aromatic amines was obtained with a column efficiency of up to 78 000 plates/m.     

Radial distribution of the contributions to band broadening of a silica‐based semi‐preparative monolithic column

Using an on‐column local electrochemical microdetector operated in the amperometric mode, band elution profiles were recorded at different radial locations at the exit of a 10 mm id, 100 mm long silica‐based monolithic column. HETP plots were then acquired at each of these locations, and all these results were fitted to the Knox equation. This provided a spatial distribution of the values of the eddy diffusion (A), the molecular diffusion (B), and the resistance to the kinetics of mass transfer (C) terms. Results obtained indicate that the wall region yields higher A values and smaller C values than the central core region. Significant radial fluctuations of these contributions to band broadening occur throughout the exit column cross‐section. This phenomenon is due to the structural radial heterogeneity of the column.     

Monolithic columns based on a poly(styrene-divinylbenzene-methacrylic acid) copolymer for capillary liquid chromatography of small organic molecules

A very simple and readily performed method is described for the preparation of poly(styrene-divinylbenzene-methacrylic acid) monolithic columns for capillary liquid chromatography. The effect of the methacrylic acid content on the morphological and chromatographic properties has been investigated. Methacrylic acid is shown to be essential for isocratic separations of small organic analytes by capillary liquid chromatography. Column efficiencies of about 28,000 theoretical plates/m have been obtained for all the test compounds. The batch-to-batch and run-to-run repeatability of the retention times is better than 1.5%.     

Silica‐supported polymeric monolithic column with a mixed mode of hydrophilic and strong cation‐exchange interactions for microcolumn liquid chromatography

A novel sulfonic acid group containing hydrophilic strong cation‐exchange monolith was prepared by in situ coating 5 μm bare silica particles with the copolymers of glycidyl methacrylate and pentaerythritol triacrylate and further sulfonating the prepared polymer matrix with Na₂SO₃ inside a 150 μm id capillary. The preparation conditions were investigated, and the method was described in detail. The prepared column was characterized by comparing with its counterparts reported previously in terms of matrix morphology, preparation reproducibility, permeability, swelling–shrinking behavior, mechanical stability, hydrophilicity, binding capacity, and column efficiency. The swelling–shrinking behavior of the present column in solvents of different polarities was negligible, the hydrophobicity could be suppressed at the acetonitrile concentrations higher than 40% v/v, and the binding capacities were 256 μequiv/mL and 20.1 mg/mL for Cu²⁺ and lysozyme, respectively. The minimum theoretical plate heights were 8, 10, and 13 μm, and the values of the C term in van Deemter equation were 9, 12, and 35 ms for the test analytes of Na⁺, thiourea, and cytidine 5ʹ‐monophosphate, respectively. This column exhibited an excellent performance in the separations of monovalent inorganic cations, uncharged polar, and charged polar compounds.