Effect of Pluronic F127 on the pore structure of macrocellular biodegradable polylactide foams

Thermally induced phase separation technique was utilized to fabricate biodegradable poly(l ‐lactic acid) (PLLA) macrocellular foams which were capable of being applied in tissue engineering. The block copolymer Pluronic F127 composed of (polyethyleneoxide)‐(polypropyleneoxide)‐(polyethyleneoxide) [(PEO)‐(PPO)‐(PEO)] was used as a porogen. Water/dioxane mixtures with different volume ratios were used as solvents. The addition of Pluronic F127 could induce an appearance of large pores (50–200 μm) besides small pores (10–20 μm) or a change from a solid–liquid phase separation to a liquid–liquid phase separation. The role of Pluronic F127 depends on the water/dioxane ratios in the PLLA/dioxane/water system. The X‐ray diffraction patterns and porosity measurement results showed that Pluronic F127 was crystallized and existed on the pore wall. The effect of Pluronic F127 on changing pore structure is attributed to the occurrence of the interaction of the lipophilic PPO blocks in Pluronic F127 with PLLA clews, consequently, this results in PLLA aggregation and early phase separation on cooling. Copyright © 2004 John Wiley & Sons, Ltd.     

Hierarchical porous silica materials with a trimodal pore system using surfactant templates

Porous silica exhibiting a hierarchically ordered trimodal pore system with a well-defined reverse opal microstructure and bimodal mesoporosity in the walls has been prepared by using polystyrene latex spheres, a novel block copolymer and an ionic liquid surfactant as templates. The resulting materials exhibit hierarchical order at three length scales (small mesopores: 2-3 nm; large mesopores: 11-12 nm; macropores: 360 nm).     

Fast HPLC with a silica-based monolithic ODS column

Fast high-performance liquid chromatography is becoming routine in laboratories that require high throughput or for combinatorial libraries. Reduced analysis time is commonly achieved by using shorter columns and higher flow rates. Shorter columns require smaller particles in order to maintain efficiency. However, smaller particles increase backpressure, which limits both column length and higher flow rates for typical LC pumps. This disadvantage has been addressed by the emergence of monolithic liquid chromatographic columns (1). Unlike particle-base columns, monolithic columns consist of a continuous rod-shaped porous network with a bimodal pore distribution. In this study, a commercially available 50- x 4.6-mm silica-based octadecyl silane monolithic column (Chromolith SpeedROD RP18e, EM Science, Gibbstown, NJ) was used to separate a seven-component test mixture with a wide range of polarity. The primary goals of this investigation were: (a) to study the chemistry (selectivity) of the new silica-based monolithic columns and (b) to study their run-to-run and column-to-column performance (retention times and peak areas). The selectivity (alpha factor) is a ratio of partition coefficients and, if comparable for a variety of solutes, would mean that methods could be readily transferred between particulate and monolithic columns.     

Applications of silica-based monolithic HPLC columns

The recent invention and successive commercial introduction of monolithic silica columns has motivated many scientists from both academia and industry to study their use in HPLC. The first paper on monolithic silica columns appeared in 1996. Currently about 200 papers have been published relating to applications and characterization of monolithic silica columns, including monolithic capillaries. This review attempts to give an overview covering various aspects of this new column type in the field of high throughput analysis of drugs and metabolites, chiral separations, analysis of pollutants and food-relevant compounds, as well as in bioanalytical separations such as in proteomics. Some of the applications are described in greater detail. The numerous publications dealing with the physicochemical and chromatographic characterization of monolithic silica columns are briefly summarized.     

Robust monolithic silica-based on-chip electro-osmotic micro-pump

A robust, compact, on-chip, electro-osmotic micro-pump (EOP) for micro-flow analysis, based on parallel, encased, 10 x 0.1 mm I.D. monolithic silica capillary columns has been developed. A 15 x 40 x 2 mm poly(methyl methacrylate) (PMMA) chip, containing a total of nine parallel EOP systems was fabricated, allowing the use of single, double or triple monolithic columns to produce increased flow as required. The monolithic silica was compatible with both aqueous and organic solvents without swelling or shrinking problems, with the triple column EOP capable of generating flow of up to 0.6 microL min(-1) under zero pressure load and over 0.1 microL min(-1) with an applied pressure of ca. 2.4 bar using an applied voltage of just 2 kV. Current generated at the 2 kV applied voltage for a 2 mM acetate buffer solution (pH 4.5) was under 4 microA, allowing stable, bubble-free flow. The developed triple column EOP was incorporated within a micro-fluidic chip (5.0 x 2.0 x 0.4 cm) integrated with a second single 10 x 0.1 mm column EOP, for combined sample injection and simple on-chip micro-flow analysis.     

Ionic liquids-assisted fabrication of silica-based monolithic columns

The mesopores of a monolithic silica column are very important and useful for chromatographic separation since they can offer sufficiently large surface area. In this paper, a novel method with the assistance of an ionic liquid (1-butyl-3-methylimidazolium tetrafluoroborate ([bmin]BF₄)) was developed for the preparation of a C18-modified monolithic silica column for the first time, in which, the through pores and mesopores were formed simultaneously during the sol-gel reaction. The method is effective to simplify the preparation process of the silica-based monolithic columns. The factors influencing the sol-gel process, including the content of methanol and pH, were studied. The chromatographic performance of the prepared monolithic column was evaluated by the separation of alkylbenzenes.     

Preparation and performance of porous titania with a trimodal pore system as anode of lithium ion battery

A porous titania has been prepared by using polystyrene spheres and tri-block copolymer ((EO)₂₀–(PO)₇₀–(EO)₂₀, P123) as templates, and its structure, composition, and performance as anode of lithium ion battery are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, and galvanostatic charge/discharge test. The results from SEM and TEM indicate that the prepared porous titania has a trimodal pore system, in which the pores are in ordered arrangement and interconnected with the same pore diameter and uniform wall thickness. The charge/discharge tests show that the battery using the prepared porous titania as anode exhibits good rate capacity and cycle stability.     

Structure and performance of silica-based monolithic HPLC columns

Silica-based monolithic columns were prepared for HPLC with systematic variations of the tetramethoxysilane (TMOS) and polyethylene oxide (PEO) content as reactants in a sol-gel process accompanied by phase separation. The resulting monoliths showed differences in the macropore and silica skeleton diameter as well as in the corresponding domain sizes (the sum of macropore and skeleton diameter). All monoliths were synthesized with a diameter of 4.6 mm and cladded with a suitable polyaryletheretherketone (PEEK) polymer in a standardized and optimized manner for the subsequent chromatographic evaluation of the resulting monolithic HPLC columns. The columns were tested under normal phase conditions using n-heptane/dioxane (95:5 v/v) as a mobile phase and 2-nitroanisole as a test compound for the determination of separation efficiency and permeability. Two different sets of columns were prepared: the first one in which the amount of PEO was stepwise decreased to yield monoliths with identical macropore volumes and variations in the domain sizes. The second group of materials was synthesized adjusting both TMOS and PEO quantities to yield monolithic columns with identical macropore diameters of about 1.80 microm but different skeleton diameters and macropore volumes. The chromatographic results suggest that an increase in the column performance cannot be achieved by just arbitrarily decreasing the domain size of a given column. From a certain point of "downsizing" the monolithic structure a loss of structural homogeneity can be observed, which is apparently responsible for a lower chromatographic performance.     

Structural radial heterogeneity of a silica-based wide-bore monolithic column

The radial distribution of the main characteristics (elution time and standard deviation) of the elution profiles of a flat injected band recorded at the exit of a monolithic column were determined. These distributions provide the radial distributions of the average mobile phase velocity, the elution time and the maximum height of the peak of an analyte, the column efficiency and the analyte concentration. The band profiles were measured at the exit of a 10-mm i.d., 100-mm long silica-based monolithic column. An on-column local electrochemical amperometric detector allowed the recording of the elution profiles at different spatial positions throughout the column cross-section. The local spatial distribution of the mobile phase velocity does not follow a piston-flow behavior but exhibits radial heterogeneity. The local efficiency near the wall is lower than that near the column center. The radial distribution of the maximum concentration of the peaks varies throughout the column exit section, partially due to the radial variations of the column efficiency. These results might explain the rather large value of the A term of the Van Deemter or the Knox equations reported previously for monolithic columns.     

Adsorption equilibria of butyl- and amylbenzene on monolithic silica-based columns

The adsorption isotherms of butyl- and amylbenzene on silica monolithic columns were measured by frontal analysis. The external, internal and total porosities of these columns were determined by inverse size-exclusion chromatography. The adsorption isotherms are concave upward in the entire concentration range investigated. They were fitted to the anti-Langmuir model, an unusual model in liquid-solid and liquid-liquid phase equilibria. Band profiles under overloaded conditions were recorded. They were in good agreement with the profiles calculated using th,e lumped pore diffusion model of chromatography and these adsorption isotherms.     

Reversed-phase and weak anion-exchange mixed-mode silica-based monolithic column for capillary electrochromatography

A silica-based CEC monolithic column with mixed modes of RP and weak anion-exchange (WAX) was successfully prepared by using the sol-gel technique at mild temperature. The synthesizing procedure was optimized by changing the ratios of tetraethoxysilane (TEOS), aminopropyltriethoxysilane (APTES), and octyltriethoxysilane (C(8)-TEOS) in the mixture. While serving as WAX group, the amino group dominated the charge on the surface of the capillary column and generated an EOF from cathode to anode at low pH. At pH above 7.5, a cathodic EOF was observed due to the full ionization of silanol group and the suppression in the ionization of amino group. The morphology of monolithic columns was examined by SEM, and the performance of column was evaluated in detail by separating different kinds of compounds. As expected, the monolithic column exhibited RP chromatographic behavior for neutral solutes. Fast and efficient separation of six aromatic acids was obtained using acidic mobile phase with column efficiency up to 160,000 plates/m. Symmetrical peaks can be obtained for aromatic amines because positively charged amino groups on the surface can effectively minimize the adsorption of positively charged analytes to the stationary phase.     

Tailoring the macroporous structure of monolithic silica-based capillary columns with potential for liquid chromatography

The present work aims at the optimisation of the synthesis of methyl-silsesquioxane monolithic capillary columns using a sol-gel based protocol. The influence of reaction conditions such as temperature, reaction mixture composition and catalyst concentration has been examined. The morphology of the products was studied by scanning electron microscopy and nitrogen adsorption. Monolithic capillary columns were obtained with a skeleton-like structure with open pores. Pore diameters vary from 0.8 to 15 microm, diameters of the xerogel network vary from 0.4 to 12 microm, respectively. Specific surface areas up to 334 m2/g have been observed, however, many materials did not possess areas above few m2/g which represents the limit of detection of the nitrogen porosimetry measurements. Excellent adhesion to the capillary wall was observed in all cases, and drying was possible at ambient conditions without the formation of cracks.     

Achieving rapid low-pressure ion chromatography separations on short silica-based monolithic columns

Short silica-based monolithic columns (0.5-1 cm) are coated with the surfactant didodecyldimethylammonium bromide (DDAB) and used for fast ion exchange separations of small inorganic anions. Sources of extra-column band broadening are assessed and minimized to obtain separations of seven analytes (iodate, chloride, nitrate, bromide, nitrite, phosphate, sulphate) in two minutes at 2 mL/min. Eluents used are 6 and 9 mM 4-cyanophenol at pH 7.3-7.4 or 5 mM 4-hydroxybenzoic acid at pH 5.6. DDAB coating stability is improved by 15-fold by the addition of a DDAB coated pre-column before the injection valve. Separations are obtained using a low-pressure glass syringe.     

硅胶整体柱离子对色谱快速分析碘离子

建立了用硅胶整体柱和直接电导检测的离子对色谱快速分析碘离子的方法。采用Chromolith Speed ROD RP-18e色谱柱,以氢氧化四丁铵(离子对试剂)-邻苯二甲酸+乙腈(有机改进剂)为淋洗液,讨论了离子对试剂浓度、有机改进剂浓度、pH、流速和色谱柱温度对碘离子保留的影响。确定最佳色谱条件为:0.25 mmol/L氢氧化四丁铵-0.18 mmol/L邻苯二甲酸+体积分数7%乙腈(pH5.5)作为淋洗液,流速6.0 mL/min,色谱柱温30℃。在此条件下,碘离子的保留时间在0.5 min之内,其它常见阴离子(Cl-、NO3-、SO42-)及SCN-、ClO4-不干扰测定。方法的检出限为0.86 mg/L,标准曲线的线性范围为1.6～85.0 mg/L,峰面积的相对标准偏差为2.3%。将方法应用于测定地下水和果汁中的碘离子,加标回收率为98.5%～104.2%。     

Synthesis of monolithic meso-macroporous silica and carbon with tunable pore size

Monolithic porous silica and carbon structures have been obtained by the synthesis of silica inside the aqueous phase of a sponge-like Swollen Liquid Crystal, and the parallel preparation of carbon replica.     

A silica-based monolithic column in capillary HPLC and CEC coupled with ESI-MS or electrospray-atmospheric-pressure laser ionization-MS

We describe the successful coupling of CEC and capillary HPLC with the recently developed atmospheric-pressure laser ionization (APLI) method. APLI is suitable for selectively and sensitively ionizing nonpolar aromatic compounds at ambient pressure for subsequent mass-selective detection. The polycyclic aromatic hydrocarbons used as analytes are first separated either by CEC on a silica-based monolithic column or by capillary HPLC. The eluent, along with a sheath flow, is volatilized by microelectrospray and then selectively ionized by excimer laser (KrF*) radiation via two-photon excitation. A QTOF-MS is used as mass-selective detector. This interface combination makes soft ionization of thermally labile nonpolar aromatic analytes possible.     

反相硅胶整体柱离子色谱法快速分析BrO-3

研究了基于反相硅胶整体柱的离子色谱快速分析BrO;的方法.以氢氧化四丁铵(TBA)为流动相动态修饰反相硅胶整体柱,采用直接电导检测,在普通高效液相色谱仪上实现了BrO3-和常见阴离子的快速分析.实验考察并讨论了流动相、色谱柱温度和流速对所测阴离子保留和分离的影响.确定分离测定BrO3-的最佳色谱条件是:以0.75 mm...     

Pressure-assisted capillary electrochromatography with electrospray ionization-mass spectrometry based on silica-based monolithic column for rapid analysis of narcotics

A pressure-assisted CEC (pCEC) with ESI-MS based on silica-based monolithic column was developed for rapid analysis of narcotics. Combining the extremely high permeability and separation efficiency of silica-based monolithic column with the high selectivity and sensitivity of pCEC-ESI-MS, the developed system exhibited its prominent advantages in separation and detection. A systematic investigation of the pCEC separation and ESI-MS detection parameters was performed. Experiment results showed that the optimized separation efficiency could be obtained at 8 bar assisted pressure with 25 kV separation voltage, using the solution containing 65% ACN v/v and 20 mmol/L ammonium acetate with pH 6.0 as running buffer. 3 microL/min of sheath liquid was considered as the optimized flow rate since it could provide the maximum signal intensity. Under the optimum conditions, the tested five narcotics could be completely separated within 10 min with the detection limit in the range of 2.0-80 nmol/L. The proposed method has been successfully used for detection of narcotics in real urine samples.     

Comparison of nonporous silica-based ion exchangers and monolithic ion exchangers in separations of inorganic anions

Low capacity anion exchangers for IC have been prepared by modification of nonporous uniformed silica MICRA microbeads and by modification of the organic polymeric monolithic matrixes prepared in situ in quarz capillary. Due to the small particle size (1.5 microm) high-performance adsorbents were prepared allowing to obtain up to 190,000 tp/m. However, the column possesses a very high back-pressure and can be used in a short length up to 50 mm only to meet the requirements of conventional chromatographic equipment. An analysis of a test mixture of seven anions was completed within 3 min with a back column pressure of about 350 bar (HETP of about 5.5 microm, where HETP is the height equivalent to the theoretical plate). Monolithic capillary columns provide lower efficiency per column unit length than MICRA columns; however, they can be used at a longer length because of their low flow resistance. Monolithic column of ca. 40 cm length has workable pressure below 10 bar and allows separation of a five anions test mixture within less than 10 min. A better efficiency of monolithic column (HETP approximately 75 microm) can be achieved at reduced flow rates when the analysis time is not a critical parameter.     

Design and evaluation of synthetic silica-based monolithic materials in shrinkable tube for efficient protein extraction

Sample pretreatment is a required step in proteomics in order to remove interferences and preconcentrate the samples. Much research in recent years has focused on porous monolithic materials since they are highly permeable to liquid flow and show high mass transport compared with more common packed beds. These features are due to the micro-structure within the monolithic silica column which contains both macropores that reduce the back pressure, and mesopores that give good interaction with analytes. The aim of this work was to fabricate a continuous porous silica monolithic rod inside a heat shrinkable tube and to compare this with the same material whose surface has been modified with a C(18) phase, in order to use them for preconcentration/extraction of proteins. The performance of the silica-based monolithic rod was evaluated using eight proteins; insulin, cytochrome C, lysozyme, myoglobin, β-lactoglobulin, ovalbumin, hemoglobin, and bovine serum albumin at a concentration of 60 μM. The results show that recovery of the proteins was achieved by both columns with variable yields; however, the C(18) modified silica monolith gave higher recoveries (92.7 to 109.7%) than the non-modified silica monolith (25.5 to 97.9%). Both silica monoliths can be used with very low back pressure indicating a promising approach for future fabrication of the silica monolith inside a microfluidic device for the extraction of proteins from biological media.