Macroporous polymeric monoliths as stationary phases in gas adsorption chromatography

The influence of the conditions of preparation of divinylbenzene-based polymer monoliths on the properties of monolithic capillary columns for use in gas adsorption chromatography was examined. It was found that the polymerization time and the temperature and composition of the polymerization mixture have an effect on the dynamic and chromatographic properties of the columns. The monolithic capillary columns prepared under the optimal synthesis conditions had the height equivalent of a theoretical plate at the level of 20–30 μm, which is an order of magnitude below that of conventional open tubular columns.     

Poly(alkylmethylsiloxanes) thermally immobilized on silica as stationary phases for high-performance liquid chromatography

Modifications have been made to the method of ion-exclusion pre-separation followed by ion exchange with conductivity detection for the determination of trace levels of chloride, sulfate and nitrate in concentrated phosphoric acid. Ion-exclusion separation and pre-concentration of impurity anions is performed using Dionex AS6-ICE and AS11-HC (4 mm) columns, respectively, with water eluent. Final separation is performed using Dionex AG11-HC and AS11-HC (2 mm) columns, KOH gradient elution, and suppressed conductivity detection. Improvements to the method include addition of an autosampler and eluent generator, and use of external standard calibration. These instrumental and procedural changes significantly improve the method's throughput, while the method's capability relative to phosphoric acid specifications is maintained, as verified through statistical evaluation of control sample analyses. Detection limits of 60, 680, and 4,0 ppb (w/w) are obtained vs. semiconductor-grade phosphoric acid specifications of 1000, 12,000, and 5000 ppb for chloride, sulfate, and nitrate, respectively.     

Capillary liquid chromatography and capillary electrochromatography using silica hydride stationary phases

A hydride-based octadecyl stationary phase on both 4.0 and 1.8 microm silica particles is tested in both the capillary LC and the pressurized capillary electrochromatography (pCEC) modes. These two materials are compared to standard C18 stationary phase made by organosilanization and to the hydride material packed into a convention 4.6mm I.D. column. The performance of the capillary columns is evaluated in terms of analysis times for various mixtures as well as efficiency. Of particular interest are the differences between the LC mode where only laminar flow is present and pCEC operation where a flat electrodriven flow profile is superimposed on the parabolic pressurized flow. Differences in performance between columns packed with 4.0 and 1.8 microm particle silica are also evaluated.     

核壳型二氧化硅色谱填料的研究进展

复杂样品的高效快速分离分析是分离科学家所面临的挑战。近年来,核壳型二氧化硅色谱填料以其高效、快速和低背压的特点被广泛用于小分子、大分子和复杂样品的快速分离分析。该文系统综述了二氧化硅核壳色谱固定相快速分离的机理,制备方法及其在小分子、多肽和生物大分子快速分离分析方面的应用,同时对核壳型色谱固定相的发展进行了展望。     

Organic polymer monoliths as stationary phases for capillary HPLC

Modern rigid porous polymer monoliths were conceived as a new class of stationary phases in classical columns in the early 1990s and later extended to the capillary format. These monolithic materials are typically prepared using a simple molding process carried out within the confines of the capillary. Polymerization of a mixture comprising monomers, initiator, and porogenic solvent affords macroporous materials with large through-pores that enable applications in a rapid flow-through mode. Since all the mobile phase must flow through the monolith, convection considerably accelerates mass transport within the monolithic separation medium and improves the separations. As a result, monolithic columns perform well even at very high flow rates. Various mechanisms including thermally and UV initiated free radical polymerization as well as ring opening metathesis copolymerizations were demonstrated for the preparation of monolithic capillary columns. The versatility of these preparation techniques was demonstrated by their use with hydrophobic (styrene, divinylbenzene, butyl methacrylate, ethylene dimethacrylate), hydrophilic (2-hydroxyethyl methacrylate, methacrylamide, methylenebisacrylamide), ionizable (vinylsulfonic acid, 2-acrylamido-2-methyl-propanesulfonic acid), and tailor-made (norborn-2-ene, 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethanonaphthalene) monomers. Variation of polymerization conditions enables control of the porous properties of the monolith over a broad range and mediates the hydrodynamic properties of the monolithic columns. The applications of polymer-based monolithic capillary columns are demonstrated for numerous separations in the microHPLC mode.     

Acrylamide-based monoliths as robust stationary phases for capillary electrochromatography

A method is described for the synthesis of rigid, macroporous polymers (monoliths) to be used as stationary phases in capillary electrochromatography (CEC). The procedure reproducibly results in columns with good mechanical and chemical stability. Once the procedure was optimized, it yielded the desired CEC columns in nearly 100% of the cases. The batch-to-batch standard deviation of the migration of the electroosmotic flow (EOF) marker for nine randomly chosen columns was 5%. The polymerization is carried out inside the capillary, an aqueous phase is used as solvent. Monomers based on acrylamides with varying hydrophilicity were used to introduce the interactive moieties together with piperazine diacrylamide as cross-linker and vinylsulfonic acid as provider of the charged, EOF-producing moieties. The pore size of the monoliths was adjusted by adding varying amounts of ammonium sulfate to the reaction mixture. In this manner, the average pore size of a given monolith could be reproducibly adjusted to values ranging from 50 nm to 1.3 microm. The procedure was optimized for four particular types of monoliths, which differed in hydrophobicity. The latter was adjusted by introducing suitable co-monomers, such as alkyl chain-bearing molecules, into the monolithic structure. Attempts to systematically investigate the chromatographic behavior of the monolithic stationary phases were made, using a model mixture of aromatic compounds as sample. The standard deviations for the run-to-run reproducibility of the retention times for unretained and retained analytes were <1.5%. Flat Van Deemter curves were measured even at elevated flow-rates (2 mm/s). Plate heights between 10 and 15 microm were measured in this range. The retention order was taken as the principal indication for the chromatographic mode. The separation was found to be governed neither by pure reversed-phase nor by pure normal-phase chromatography, even on monoliths, where large amounts of C6 ligands had been introduced.     

Fluorocarbon stationary phases for liquid chromatography applications

This article presents an overview on fluorocarbon stationary phases for liquid chromatography (LC) applications. Fluorocarbons developed as alternative reverse phases have revealed previously unknown separation mechanisms and special utilities. Solvophobicity and fluorophilicity of the fluorinated phases provide enhanced selectivity for organofluorine compounds. The dual normal- and reverse-phase characteristics make fluorinated phases suitable for analysis of polar pharmaceutical and biological samples such as proteins, peptides, nucleotides, steroids, and alkaloids. Fluorinated phases for other applications including supercritical fluid chromatography (SFC), micellar electrokinetic liquid chromatography (MEKC), ion chromatography (IC), open tubular electrochromatography (OTEC), and liquid chromatography-mass spectrometry (LC-MS) are also highlighted.     

Polypyrrole-coated silica as a new stationary phase for liquid chromatography

Summary Polypyrrole chloride and polypyrrole dodecylsulfate coated-silica packing materials have been synthesized. Reverse-phase chromtographuy was employed to characterise both packings. A series of test compounds with known properties was used as molecular probes. They included benzene and derivatives, basic drugs, and polyaromatic hydrocarbons. A commericial C₁₈ column was also used for the purpose of comparison in some cases.     

Ionic liquid stationary phases for gas chromatography

This article provides a summary of the development of ionic liquids as stationary phases for gas chromatography beginning with early work on packed columns that established details of the retention mechanism and established working methods to characterize selectivity differences compared with molecular stationary phases through the modern development of multi-centered cation and cross-linked ionic liquids for high-temperature applications in capillary gas chromatography. Since there are many reviews on ionic liquids dealing with all aspects of their chemical and physical properties, the emphasis in this article is placed on the role of gas chromatography played in the design of ionic liquids of low melting point, high thermal stability, high viscosity, and variable selectivity for separations. Ionic liquids provide unprecedented opportunities for extending the selectivity range and temperature-operating range of columns for gas chromatography, an area of separation science that has otherwise been almost stagnant for over a decade.     

Polymer coatings as stationary phases in high-performance liquid chromatography

Modern biochromatography demands highly sophisticated packing materials in terms of biocompatibility, (substrate-) selectivity and recovery. Polymers can be designed in a wide variety and therefore deliver solutions to specific chromatographic problems. Thus, tailor-made polymer coatings are an alternative to the classical chemically bonded stationary phases.     

Phenylcarbamate derivatives of cellulose and amylose immobilized onto silica gel as chiral stationary phases for high-performance liquid chromatography

Three polysaccharide phenylcarbamate derivatives [cellulose 2,3-bis(3,5-dimethylphenylcarbamate)-6-(3,5-dimethylphenylcarbamate)/(2-methacryloyloxyethylcarbamate), cellulose 2,3-bis(3,5-dichlorophenylcarbamate)-6-(3,5-dichlorophenylcarbamate)/(2-methacryloyloxyethylcarbamate), and amylose 2,3-bis(3, 5-dimethylphenylcarbamate)-6-(3,5-dimethylphenylcarbamate)/(2-methacryloyloxyethylcarbamate)] with vinyl groups were prepared and coated onto silica gel to immobilize them via radical copolymerization with 2,3-dimethylbutadiene. The copolymerization efficiently proceeded, and the coated polysaccharide derivatives were mostly immobilized on the surface of the silica gel. The immobilized polysaccharide derivatives showed high chiral recognition abilities similar to those of the corresponding coated polysaccharide derivatives. They could be used with an eluent containing chloroform, which dissolved the polysaccharide derivatives. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4704–4710, 2004     

Using Prussian blue analogue nanoparticles confined into ordered mesoporous silica monoliths as precursors of oxides

Powdered Prussian blue analogues (PBAs) and PBAs confined in ordered mesoporous silica monoliths were used as oxide precursors through thermal treatment under an oxidizing atmosphere. The study focuses on the transformation of the alkali cation-free CoCo PBA of chemical formula K_0.1Co^II₄[Co^III(CN)₆]_2.7·20 H₂O. The compounds were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), IR spectroscopy and small-angle X-ray scattering (SAXS), and the magnetic properties of the calcined samples were investigated. In both cases, powdered and confined PBAs, the coordination polymers are transformed into well-crystallized Co₃O₄ spinel oxide. In the case of the confined PBA, isolated Co₃O₄ single crystals confined within the ordered mesoporosity of the monoliths were evidenced by HRTEM. A preliminary study shows an effect of particle size and confinement on the magnetic properties of the confined oxide particles.     

A family of ordered mesoporous carbons derived from mesophase pitch using ordered mesoporous silicas as templates

A variety of ordered mesoporous carbons (OMCs) were synthesized using ordered mesoporous silicas (OMSs) as hard templates and the mesophase pitch (MP) as a carbon precursor. The synthesis included the mixing of OMS with MP, the infiltration of OMS with MP at 450–550?°C and the carbonization of MP in OMS/MP composite followed by the dissolution of the OMS template. OMCs with structures of two-dimensional hexagonal arrays of nanorods and three-dimensional arrays of nanospheres were obtained through the replication of silica templates, including large-pore SBA-15, KIT-6, large-pore FDU-12 and SBA-16. In particular, 2-D hexagonal array of carbon nanorods (CMK-3 carbon) with (100) interplanar spacing of ～13 nm as well as an array of carbon nanospheres arranged in the face-centered cubic structure with the unit-cell parameter of 33 nm were successfully prepared. The specific surface areas of the resulting carbons were up to 400 m²/g, and the total pore volumes were up to 0.43 cm³/g, with the highest values achieved when the MP infiltration temperature was 500?°C. The OMCs exhibited narrow mesopore size distributions. As inferred from XRD, the frameworks of OMCs featured semi-graphitic structures even though moderate carbonization temperature (850?°C) was employed.     

Adsorption hysteresis in ordered mesoporous silicas

We review some recent progress in experimental studies of the adsorption hysteresis of simple molecules in ordered mesoporous silicas. We show that the nature of the adsorption hysteresis due to capillary condensation can be examined with less ambiguity by measuring the hysteresis loop for the ordered mesoporous silicas with three types of pore geometries (cylindrical, interconnected cylindrical, and interconnected spherical) over a wide temperature range. The adsorption hysteresis arises from the metastability of a confined phase and the temperature at which the hysteresis disappears is lower than the critical temperature of vapor-liquid equilibrium in pores. The hysteresis occurs mainly on the desorption rather than adsorption branch, irrespective of the pore geometries.     

Monolithic stationary phases for liquid chromatography and capillary electrochromatography

A monolithic stationary phase is the continuous unitary porous structure prepared by in situ polymerization or consolidation inside the column tubing and, if necessary, the surface is functionalized to convert it into a sorbent with the desired chromatographic binding properties [J. Chromatogr. A 855 (1999) 273]. Monolithic stationary phases have attracted considerable attention in liquid chromatography and capillary electrochromatography in recent years due to their simple preparation procedure, unique properties and excellent performance, especially for separation of biopolymers. This review summarizes the preparation, characterization and applications of the monolithic stationary phases. In addition, the disadvantages and limitations of the monolithic stationary phases are also briefly discussed.     

p-tert-Butylcalix[8]arene-bonded silica monoliths for liquid chromatography

Monolithic silica columns have inspired considerable research interests in the separation science because of their unique properties in permeability, mass transfer, efficiency and throughput. In this paper, a chemically p-tert-butylcalix[8]arene-bonded silica monolith was prepared as the promising candidate for versatile LC separations. Micrometer-sized macropores and nanometer-sized mesopores in this derivatized silica monolith reduce the diffusion path length and provide both low backpressure and high column efficiencies, leading to high-speed and high-throughput separations. Since p-tert-butylcalix[8]arene possesses a pi-donors cavity composed of benzene rings while polycyclic aromatic hydrocarbons, anthraquinones, phenol regio isomers and fullerenes are pi-systems with appreciable electron affinity, they may have a chance to get involved in forming host-guest inclusion complexes through non-covalent interactions, e.g. hydrophobic and pi-pi interactions. Compared with RP-18e, the prepared calixarene-bonded monolith exhibited better selectivity to molecules which contains more pi-electrons and more condensed cyclic moieties. The column efficiency was about 22,000 plates/m. The calixarene-bonded monolith also showed its good performances in separation of fullerenes and dihydropyridines, indicating a promising approach for purification of fullerenes with high purity from the carbon soot.     

Fullerene-impregnated ionic liquid stationary phases for gas chromatography

A new method has been developed to facilitate the use of fullerenes as stationary phases (SPs) in gas chromatography (GC). In this method, ionic liquids (ILs) are used as solvents to coat fullerenes (C(60), amino-C(60) and hydroxy-C(60)) onto GC columns. However, the ILs serve not just as coating solvents but also act synergistically with fullerenes to provide unique properties as stationary phases, namely dual modal characteristics. They act as non-polar SPs when separating non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures), and as polar SPs for polar analytes (e.g., alcohol mixtures). The polarity of the stationary phase can be adjusted by changing either the type of the IL and/or by adding either C(60) (or its amino or hydroxy derivatives) to the IL. It was found that C(60) and its derivatives produce not only a change in the polarity of the SP but also substantial enhancement in separation efficiencies for both non-polar and polar analytes. More importantly, when added to the IL SP, C(60) improves separation efficiencies not just for non-polar analytes (aromatic hydrocarbon mixtures and alkane mixtures) but also for polar analytes (mixtures of ortho-, meta- and para-xylene and alcohol mixtures) as well. Moreover, C(60) SP provides higher efficiencies than amino-C(60) and hydroxy-C(60) for separation of polar analytes. This is rather surprising considering that not only are amino-C(60) and hydroxy-C(60) more polar than C(60), but that the IL used to coat the amino- and hydroxy-C(60) (i.e., N-ethylpyridinium trifluoroacetate, [EtPy(+) CF(3)COO(-)]) is more polar than the IL used to coat the C(60) (i.e., octylmethylimidazolium bis(trifluoromethyl)sulfonyl)amide], [OMIm(+) (CF(3)SO(2))(2)N(-)]). Moreover, compared to its amino and hydroxy derivatives, the concentration of C(60) in the column was 10 times lower.     

Capillary-channeled polymer fibers as stationary phases in liquid chromatography separations

A method utilizing capillary-channeled polymer (C-CP) fibers as stationary phases in high-performance liquid chromatographic separations has been investigated. Polymeric fibers of differing backbones (polypropylene and polyester) having nominal diameters of approximately 50 and approximately 35 microm and a channeled structure on their periphery were packed into stainless steel tubing (305 x 4.6 mm I.D.) for use in reversed-phase separations of various mixtures. The fibers have eight channels running continuously along the axis which exhibit very high surface activity. As such, solvent transport is affected through the channels through wicking action. Bundles of 1000-3000 fibers are loaded co-linearly into the tubing, providing flow channels extending the entire length of the columns. As a result, backing pressures are significantly lowered (approximately 50% reduction) in comparison to packed-sphere columns. In addition, the capital costs of the fiber material (< US$0.25 per column) are very attractive. Flow-rates of up to 5 ml/min can be used to achieve near baseline separation of related compounds in reasonable run times, indicating very fast mobile phase mass transfer (C-terms). The polymer stationary phases demonstrate high selectivity for a wide variety of analytes with gradient elution employed successfully in many instances. Specifically, separations of three polyaromatic hydrocarbons (benzo[a]pyrene, chrysene, pyrene), mixtures of both organic and inorganic lead compounds [chlorotriethyllead, chlorotriphenyllead, lead nitrate, lead(II) phthalocyanine], and a lipid standard of triglycerides were accomplished on the polymeric stationary phases. Other species of biological interest, including groups of aliphatic and aromatic amino acids have also been effectively separated. The reversed-phase nature of the fiber surfaces is supported through atomic force microscopy measurements using hydrophilic and hydrophobic functionalized polystyrene beads as the probe tips. Separations of the various analytes demonstrate the feasibility of utilizing C-CP fibers as stationary phases in reversed-phase LC. It is envisioned that columns of this nature would be particularly useful in prep-scale separations as well as for immobilization matrices for organic constituents in aqueous environments.     

手性有序无机介孔硅用作气相色谱固定相

手性介孔材料在手性分离、不对称催化、手性传感等领域具有广泛的应用价值。手性有序无机介孔硅是一类介孔结构高度有序、不含有机成分的手性材料。该文采用D-苯丙氨酸为手性源合成手性有序无机介孔硅(COIMS),将其用聚硅氧烷(OV-1701)稀释后用作固定相制备毛细管气相色谱手性柱,并对该手性柱的分离性能进行了考察,8种手性化合物在该手性柱上得到了拆分。COIMS柱对直链烷烃、醇的分离也表现出良好的选择性。该柱还具有分析时间短、在较高温度下测定稳定等优点,其具有开发成高温手性固定相的潜力。