Ruonong Fu

Monolithic graphene fibers for solid-phase microextraction

Monolithic graphene fibers for solid-phase microextraction (SPME) were fabricated through a dimensionally confined hydrothermal strategy and their extraction performance was evaluated. For the fiber fabrication, a glass pipeline was innovatively used as a hydrothermal reactor instead of a Teflon-lined autoclave. Compared with conventional methods for SPME fibers, the proposed strategy can fabricate a uniform graphene fiber as long as several meters or more at a time. Coupled to capillary gas chromatography (GC), the monolithic graphene fibers in a direct-immersion (DI) mode achieved higher extraction efficiencies for aromatics than those for n-alkanes, especially for polycyclic aromatic hydrocarbons (PAHs), thanks to π-π stacking interaction and hydrophobic effect. Additionally, the fibers exhibited excellent durability and can be repetitively used more than 160 times without significant loss of extraction performance. As a result, an optimum extraction condition of 40°C for 50min with 20% NaCl (w/w) was finally used for SPME of PAHs in aqueous samples. For the determination of PAHs in water samples, the proposed DI-SPME-GC method exhibited linear range of 0.05-200μg/L, limits of detection (LOD) of 4.0-50ng/L, relative standard deviation (RSD) less than 9.4% and 12.1% for one fiber and different fibers, respectively, and recoveries of 78.9-115.9%. The proposed method can be used for analysis of PAHs in environmental water samples.

Simple HPLC Method for Simultaneous Determination of Acetaminophen, Caffeine and Chlorpheniramine Maleate in Tablet Formulations

SummaryA simple, rapid and accurate, routine-HPLC method is described for simultaneous determination of acetaminophen, caffeine and chlorpheniramine maleate in a new tablet formulation Chromatographic separation of the three pharmaceuticals was achieved on a Hypersil CN column (150×5.0 mm, 5 μm) using a mobile phase comprising a mixture of acetonitrile, an ion-pair solution and tetrahydrofuran (13:14:87, v/v,pH4.5). The flow-rate was changed from 1.0 mL min−1 (in 0≈7.5 min) to 1.8 mL min−1 (after 3.5 min). was complete in <10 min. The method was validated for system suitability, linearity, accuracy, precision, limits of detection and quantitation, and robustness. Linearity, accuracy and precision were found to be acceptable over the ranges 31.6≈315.8 μg mL−1 for acetaminophen, 9.5≈94.6 μg mL−1 for caffeine and 1.4≈13.8 μg mL−1 for chlorpheniramine maleate.

Capillary zone electrophoretic separation of proteins using a column coated with epoxy polymer

Abstract Fused-silica capillaries used in capillary zone electrophoresis were covalently coated with epoxy polymer and then cross-linked with ethylenediamine to form a stable layer. This coating was of sufficient thickness and hydrophilicity to reduce both protein adsorption and electroosmotic flow. The electroosmotic flow-rate showed about a twofold decrease but was still sufficient to carry both positive and negative species to the detector. As a consequence, high separation efficiencies were obtained in the pH range 2–10. The coatings were stable after more than 350 injections in 2 months. Further, reproducible separations were achieved from run to run and day to day.

LC method for the determination of oxcarbazepine in pharmaceutical preparations

A simple and accurate HPLC method for determination of oxcarbazepine (OXC) in a new tablet formulation is described. Chromatographic separation was achieved on a Diamonsil C18 column using a mobile phase consisting of acetonitrile, potassium phosphate monobasic buffer (pH 6.8) and water (36:8:56, v/v) at a flow rate of 1.0 ml/min. Absorbance was monitored at 255 nm where OXC has maximum absorption. The linear range of detection for OXC was from 9.96 to 99.6 microg/ml. The proposed method was validated for selectivity, precision, accuracy and limits of detection and quantitation, etc.

Performance of graphene sheets as stationary phase for capillary gas chromatographic separations.

This work presents the investigation of graphene as a new type of stationary phase for capillary GC separations. Graphene capillary column (0.25 mm, i.d.) achieved column efficiency of 3100 plates/m determined by n-dodecane at 120 °C. The obtained McReynolds constants suggested the weakly polar nature of graphene sheets as GC stationary phase. As evidenced, graphene stationary phase differs from the conventional phase (5% phenyl polysiloxane) in its resolving ability and retention behaviors, and achieved better separation for the Grob and other mixtures. The advantages of graphene stationary phase may mainly originate from its specific π-π stacking interaction as well as H-bonding interaction. Furthermore, graphene column exhibited good repeatability with relative standard deviation (RSD%) in the range of 0.01-0.07% for run-to-run and 2.5-6.7% for column-to-column, respectively.

Versatile two-phase solvent system for alkaloid separation by high-speed counter-current chromatography.

In order to find a versatile high speed counter-current chromatography solvent system that can be used as a general prefractionation system for most alkaloids, the crude extracts of five Chinese traditional medicinal herbs, Cortex phellodendri, Semen strychni, green tea, Sophora flavescens ait, and Datura mete L. were resolved. All separations were performed only with a two-phase system composed of CHCl3-CH3OH-water (4:3:2). The water had different acidities controlled by adding NaH2PO4 or HCl to each sample. The fractionated components were identified by thin-layer chromatography, which confirmed this solvent system was versatile and very useful for the separation of alkaloids.

Use of crown ethers in gas chromatography

Abstract Two new kinds of crown ethers, 4,4-dipentadecyl- or 4,3′0dipentadecyldibenzo-30-crown-10 and 3-pentadecylbenzo-15-crown-5, were coated on glass capillary columns and fused-silica capillary columns, and their chromatographic characteristics studied. The polarity, selectivity and stability of the crown ethers were characterized. The role of the crown ether ring in separating solutes is also mentioned.

Epoxy resin coatings for capillary zone electrophoretic separation of basic proteins

Abstract Epoxy resins, combining with a cross-linker, were coated on organosilanes modified fused-silica capillaries. After cross-linking, a tough three-dimensional network was attached to capillary surface via SiOSiC bonds. The most effective coating consisted of epoxy resins cross-linked with diaminodiphenylmethane, which was stable over the pH range 2–12 and suppressed protein adsorption effectively. The electroosmotic flow was also greatly reduced and maintained constant over the pH range 6–10. High resolutions and separation efficiencies for basic proteins were obtained in the pH range 3–10 and reproducible separations were achievedafrom run to run and day and day.

Cucurbit[n]urils as a new class of stationary phases for gas chromatographic separations.

Cucurbit[n]urils (CBs) possess unique structures and physicochemical properties as well as excellent thermal stability. These characteristics concur to make them good candidates for stationary phases in capillary gas chromatographic (GC) separations. Herein, CB7 and CB8 in neat (CB7, CB8) and binary (CB7-CB8) forms were investigated for this purpose. After they were statically coated onto fused silica capillary columns, the CB columns were evaluated in terms of chromatographic parameters, separation performance, thermal stability and column repeatability. The columns had efficiencies ranging from 1060 to 2200 plates per meter determined by n-dodecane at 100°C and exhibited nonpolar to weakly polar nature. These CBs columns showed good separation performance for a wide range of analytes such as n-alkanes, aromatic hydrocarbons, esters, aldehydes, ketones, alcohols and the Grob mixture, and exhibited nice peak shapes for analytes that are liable to peak-tailing in GC analysis. The results also proved the good column repeatability and thermal stability of the CB columns. No noticeable decreases in both retention times and resolution or appreciable baseline drift were observed after the columns were conditioned up to 250°C (CB8 and CB7-CB8 columns) or 280°C (CB7 column). This work demonstrates the promising future of CBs as a new class of GC stationary phase. To the best of our knowledge, this is the first report on using CB stationary phases in capillary GC separations.

Nonequilibrium thermodynamic separation model in capillary electrophoresis

Abstract This paper describes a separation model in capillary electrophoresis (CE) based on the entropy equation of nonequilibrium thermodynamics. We first relatedΔS, the mixed entropy change of the solute system, to plate height (H) andΔSs, the contributionΔS only due to the net separation process, to resolution(Rs) and resolution product (IIRs). In particular, we determined the entropy flow of the solute system, which is composed of both energetic and material exchange terms relating to capillary cooling and relative migrations among solute zones, respectively. It is just the CE separation system, as exterior surroundings, that contributes to the enhanced separation efficiency. The more the CE system (except the solute system) provides the solute system with negative entropy flow, the better the separation efficiency of the CE system. We also determined six thermodynamic forces and their thermodynamic fluxes corresponding to six irreversible processes; heat conduction, four kinds of diffusion (electrical field, axial concentration gradient, electrophoretic dispersion and wall adsorption) and viscous flow, respectively. Entropy production is thus composed of the six terms corresponding to time-dependent CE efficiency loss factors. The bigger the entropy production, the greater the loss of separation efficiency. The objective functions were built based on the entropy equation of solute systems developed between CE separation efficiency (ΔSS) and the optimizing parameters (electrical strength, coolant temperature; the composition and concentration of buffer; the radius, length and wall adsorption of the capillary; the concentration, charge, molecular weight and conformation of solutes; injection conditions, etc.). The more negativeΔSS is, the better the separation efficiency. This model was supported by the results of our experiments and data in the literature.