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Yafeng Guan

Publications and source records attributed to Yafeng Guan.

At least 19 recordsLinked to original sources
PubMed2006

Multidimensional liquid chromatography system with an innovative solvent evaporation interface.

An orthogonal two-dimensional liquid chromatographic (2D-LC) system was developed by using a vacuum-evaporation loop-type valve interface. Normal-phase liquid chromatography (NPLC) with a bonded CN phase column was used as the first dimension, and reversed-phase liquid chromatography (RPLC) with a C(18) column was used as the second dimension. All the solvents in the loop of the interface were evaporated at 90 degrees C under vacuum conditions, leaving the analytes on the inner wall of the loop. The mobile phase of the second dimension dissolved the analytes in the loop and injected them onto the secondary column, allowing an on-line solvent exchange of a selected fraction from the first dimension to the second dimension. The chromatographic resolution of analytes on the two dimensions was maintained at their optimal condition. Sample loss due to evaporation in the interface was observed that depended on the boiling point of the compound. Separation of sixteen polycyclic aromatic hydrocarbon mixtures and a traditional Chinese medicine Angelica dahurica was demonstrated.

Angelica↗
PubMed2006

On-line preconcentration of protein in capillary electrophoresis with an end-column cellulose acetate-based porous membrane.

A simple on-line preconcentration method of protein for capillary electrophoresis (CE) using a cellulose acetate (CA)-coated porous membrane was proposed. CA membrane is fabricated at one of the ends of the column that allows the passage of buffer ions but excludes larger protein molecules. Protein sample is continuously electrokinetically loaded and trapped by the membrane. When injection is completed, the direction of the electric field is switched and the trapped proteins are then separated by conventional CE procedure. The results achieved showed that the preconcentration mechanism of this method was based on size-exclusion effect. Bovine serum albumin (BSA) was used for model protein sample, and signal enhancement of 550-fold with 15 min injection time was achieved.

Cellulose↗
PubMed2006

Study of conical columns with 10 degrees opening angle for preparative liquid chromatography.

The efficiency and sample capacity of conical liquid chromatographic columns with 10 degrees opening angle were studied at different ratio of cross section areas of inlet to outlet (A(in)/A(out)) and column dimension. As the A(in)/A(out) ratio changed from 4 to 2.25, the reduced plate height (h) was reduced and the h value decreased 12% when the column dimension was scaled up proportionally because of relatively smaller dead volume on both end of the column. Compared to cylindrical columns having corresponding lengths and volumes, the conical columns with 10 degrees opening angle were superior in column efficiency, resolution and the maximum peak concentration at column outlet; the loadability of conical column was improved 30-40% on injection volume and 50-60% on sample mass, respectively, over cylindrical columns.

Chromatography, Liquid↗
PubMed2006

[Determination of organophosphorous pesticide residues in red wine by solid phase microextraction-gas chromatography].

A method for the determination of 12 organophosphorus pesticide residues (OPs) in red wine by fiber solid phase microextraction (SPME) coupled with gas chromatography (GC) was developed and validated. The SPME phase was prepared by sol-gel technology of physical incorporation. The extraction conditions were optimized with the results of stirring rate of 1,250 r/min, NaCl mass concentration of 150 g/L, and extraction time of 30 min. With the sample volume of 25 mL, the relative standard deviations (RSD) of peak areas for most of OPs were below 5%, and the detection limits of OPs were in the range of 5 ng/L-0.38 microg/L. It can be seen from the results that this method has the potential to analyze OPs in other beverages and soft drinking materials.

Beverages↗
PubMed2005

Physically incorporated extraction phase of solid-phase microextraction by sol-gel technology.

A sol-gel method for the preparation of solid-phase microextraction (SPME) fiber was described and evaluated. The extraction phase of poly(dimethysiloxane) (PDMS) containing 3% vinyl group was physically incorporated into the sol-gel network without chemical bonding. The extraction phase itself is then partly crosslinked at 320 degrees C, forming an independent polymer network and can withstand desorption temperature of 290 degrees C. The headspace extraction of BTX by the fiber SPME was evaluated and the detection limit of o-xylene was down to 0.26 ng/l. Extraction and determination of organophosphorus pesticides (OPPs) in water, orange juice and red wine by the SPME-GC thermionic specified detector (TSD) was validated. Limits of detection of the method for OPPs were below 10 ng/l except methidathion. Relative standard deviations (RSDs) were in the range of 1-20% for pesticides being tested.

Chromatography, Gas↗
PubMed2005

Determination of organophosphorus pesticides in cucumber and potato by stir bar sorptive extraction.

Organophosphorus pesticides (OPPs) in vegetables were determined by stir bar sorptive extraction (SBSE) and capillary gas chromatography with thermionic specific detection (TSD). Hydroxy-terminated polydimethylsioxane (PDMS) prepared by sol-gel method was used as extraction phase. The effects of extraction temperature, salting out, extraction time on extraction efficiency were studied. The detection limits of OPPs in water were < or = 1.2 ng/l. This method was also applied to the analysis of OPPs in vegetable samples and matrix effect was studied. Linear ranges of OPPs in vegetable samples were 0.05-50 ng/g with detection limits < or =0.15 ng/g and the repeatability of the method was less than 20% relative standard deviation.

Chemical Fractionation↗
PubMed2005

Application of a high-pressure electro-osmotic pump using nanometer silica in capillary liquid chromatography.

A novel designed electro-osmotic pump (EOP) with simple structure was assembled using three 20 cm x 530 microm i.d. fused-silica capillaries packed with 20 +/- 5 nm silica grains for capillary liquid chromatography. It was found that the pump could generate pressures over 20 MPa and several microL/min flow rate for most of the liquids being delivered with the applied voltage less than 10 kV. By increasing the pressure, decreasing the applied voltage and the electrical current, the thermodynamic efficiency was about 1-4%. A practical application of the EOP in a 20cm x 150 microm i.d. 3 microm C18 fused-silica analytical capillary column demonstrated the applicability of the pump.

Chromatography, Liquid↗
PubMed2005

Simultaneous light emitting diode-induced fluorescence and contactless conductivity detection for capillary electrophoresis.

A combined detection system of simultaneous contactless conductometric and fluorescent detection for capillary electrophoresis (CE) has been designed and evaluated. The two processes share a common detection cell. A blue light-emitting diode (LED) was used as the excitation source and an optical fiber was used to collect the emitting fluorescence for fluorescent detection (FD). Inorganic ions, fluorescein isothiocyanate (FITC)-labeled amino acids and small molecule peptides were separated and detected by the combined detector, and the detection limits (LODs) of sub-microM level were achieved.

Journal Article↗
PubMed2005

Determination of heavy metal ions by capillary electrophoresis with contactless conductivity detection after field-amplified sample injection.

A method has been developed for determining of heavy metal ions by field-amplified sample injection capillary electrophoresis with contactless conductivity detection. The effects of the 2-N-morpholinoethanesulfonic acid/histidine (MES/His) concentration in the sample matrix, the injection time and organic additives on the enrichment factor were studied. The results showed that MES/His with a low concentration in the sample matrix, an increase of the injection time and the addition of acetonitrile improved the enrichment factor. Four heavy metal ions (Zn2+, Co2+, Cu2+ and Ni2+) were dissolved in deionized water, separated in a 10 mM MES/His running buffer at pH 4.9 and detected by contactless conductivity detection. The detection sensitivity was enhanced by about three orders of magnitude with respect to the non-stacking injection mode. The limits of detection were in the range from 5 nM (Zn2+) to 30 nM (Cu2+). The method has been used to determine heavy metal ions in tap water.

Journal Article↗
PubMed2005

[Capacitively coupled contactless conductivity detection in capillary electrophoresis].

Capacitively coupled contactless conductivity detection (C4D) is a new detection technique has been developed in recent years and used mainly in capillary electrophoresis and microchip electrophoresis. The characteristics of C4D detector are simple in structure, easy in miniaturization and integration, and free of electrodes contamination, which are common problems in an electrochemical detection. The principles and progresses of C4D in capillary electrophoresis are reviewed.

Conductometry↗
PubMed2005

[Separation and identification of isoflavonoids in Pueraria lobata extracts and its preparations by reversed-phase capillary liquid chromatography coupled with electrospray ionization quadrupole time of flight mass spectrometry].

Isoflavonoids in Pueraria lobata extract and its preparations were separated and identified by reversed-phase capillary liquid chromatography (RP-CapLC) coupled with photodiode array (PDA) detector and negative electrospray ionization quadrupole time of flight (Q-TOF) mass spectrometry. The separation was performed on a 150 mm x 0. 32 mm i. d. , 5 microm C18 capillary column, using mobile phase of 0.1% aqueous trifluoroacetic acid solution and acetonitrile containing 0.1% trifluoroacetic acid under gradient elution. The product ion spectra of the deprotonated ions allowed for the identification of puerarin, daidzin and daidzein in the sample. Puerarin was found to be the most abundant component in the extract (about 13%, mass fraction) and its preparations (19.28 - 24.34 mg per tablet). The structures of trace amount of unknown isoflavonoids were deduced based on the spectra of known compounds. They were proposed to be 3'-methoxypuerarin and 3'-methoxydaidzin.

Chromatography, High Pressure Liquid↗
PubMed2004

Preparation and characterization of long methacrylate monolithic column for capillary liquid chromatography.

Long methacrylate monolithic columns (100 cm x 320 microm i.d.) were prepared from silanized fused-silica capillaries of 320 microm i.d. by in situ copolymerization of butyl methacrylate (BMA) with ethylene dimethacrylate (EDMA) in the presence of a suitable porogen. The separation performance and selectivity of the column were evaluated and compared with a 25 cm x 320 microm i.d. column prepared in the same way by capillary high-performance liquid chromatography (micro-HPLC). The results showed that the 1 m long monolithic column can generate 33 x 10(3) plate number and exhibited good permeability, higher sample loadability, and separation capability.

Chromatography, High Pressure Liquid↗
PubMed2004

On-line introduction of high-pressure gas-liquid sample for capillary gas chromatographic analysis.

An on-line sample introduction technique in capillary gas chromatograph (CGC) for the analysis of high-pressure gas-liquid mixtures has been designed and evaluated. A sample loop of 0.05 microL and a washing solvent loop of 0.5 microL are mounted on a 10-port switching valve, which serves as the injection valve. A capillary resistor was connected to the vent of sample loop in order to maintain the pressure of the sample. Both the sample and the washing solvent are transferred into the split-injection port through a narrow bore fused silica capillary inserted into the injection liner through a septum. The volume of the liner is used both as the pressure-release damper and evaporation chamber of the sample. On-line analysis of both reactants and resultants in ethylene olimer reaction mixture at 5 MPa was carried out, which demonstrated the applicability of the technique.

Chromatography, Gas↗
PubMed2004

Preparation of stir bars for sorptive extraction using sol-gel technology.

A sol-gel coating method for the preparation of extractive phase on bars used in sorptive microextraction is described. The extraction phase of poly(dimethylsiloxane) is partially crosslinked with the sol-gel network, and the most part is physically incorporated in the network. Three aging steps at different temperatures are applied to complete the crosslinking process. Thirty-micrometer-thick coating layer is obtained by one coating process. The improved coating shows good thermal stability up to 300 degrees C. Spiked aqueous samples containing n-alkanes, polycyclic aromatic hydrocarbons and organophosphorus pesticides were analyzed by using the sorptive bars and GC. The results demonstrate that it is suitable for both aploar and polar analytes. The detection limit for chrysene is 7.44 ng/L, 0.74 ng/L for C19 and 0.9 ng/L for phorate. The extraction equilibration can be reached in less than 15 min by supersonic extraction with the bars of 30 microm coating layer.

Chromatography↗
PubMed2004

Study on conical columns with different conical angles for semi-preparative liquid chromatography.

The dynamic flow profiles and separation performances in conically shaped preparative liquid chromatographic columns (inlet i.d. larger than outlet i.d.) with three different angles (7, 10 and 15) were studied and compared with cylindrical column of the same length and internal volume. The shapes of dynamic flow profiles were studied by on-column visualization method. The transparent chromatographic columns made of polymethyl methacrylate (PMMA), packed with C18 bonded silica, were immerged into a cubic pool filled with glycerol to eliminate the cylindrical and conical lens effect. The flow profiles of colored iodine solution in the columns were observed clearly using cyclohexane as mobile phase since the refractive indices of C18, column wall and the mobile phase are very close. In the conical column of 15 degrees (20-7 mm i.d.) the mobile phase in the central region migrated slower than in wall region as it moved toward the column outlet, while in the conical column of 7 degrees (17-11 mm i.d.) the mobile phase in the central region migrated faster than in wall region just like in cylindrical column. We found that a plug-like flow profile was generated in the conical column of 10 degrees (18-9 mm i.d.) during the whole migration process. A carmine and brilliant blue mixture was used as a probe to test the separation ability of the columns. The resolutions of the two compounds on the conical column of 7, 10, 15 and on the cylindrical column were 0.6, 1.57, 1.29 and 0.8, respectively.

Benzenesulfonates↗
PubMed2004

Study of an electroosmotic pump for liquid delivery and its application in capillary column liquid chromatography.

A packed-bed electroosmotic pump (EOP) was constructed and evaluated. The EOP consisted of three capillary columns packed in parallel, a gas-releasing device, Pt electrodes and a high-voltage power supply. The EOP could generate output pressure above 5.0 MPa and constant flow rate in the range of nl/min to a few microl/min for pure water, pure methanol, 2 mM potassium dihydrogenphosphate buffer, the buffer-methanol mixture and the pure water-methanol mixture at applied potentials less than 20 kV. The composition of solvent before/after pumping was quantitatively determined by using a gas chromatograph equipped with both flame ionization detector and thermal conductivity detector. It was found that there were no apparent changes in composition and relative concentrations after pumping process for a methanol-ethanol-acetonitrile mixture and a methanol-water mixture. Theoretical aspect of the EOP was discussed in detail. An capillary HPLC system consisting of the EOP, an injection valve, a 15 cm x 320 microm i.d., 5 microm Spherigel C18 stainless steel analytical column, and an on-column UV detector was connected to evaluate the performance of the EOP. A comparative study was also carried out with a mechanical capillary HPLC pump on the same system. The results demonstrated that the reproducibility of flow rate and the pulsation-free flow property of the EOP are superior to that of mechanical pump in capillary HPLC application.

Acetonitriles↗
PubMed2004

[Analysis of group components in oils and aromatics in fuel oils].

On-line coupling of capillary liquid chromatography-capillary gas chromatography (CGC) technique for detailed group component analysis of aviation kerosene, diesel fuels, lubricating oils, extraction oils, and residuum, is described. Aromatics in fuel oils were separated according to their ring number and structures. Different eluents were stored in loops on interface, and introduced sequentially onto the CGC for detailed separation and quantitative anal- pre-column and the components of interest (aromatics plus heavier compounds) eluting from an analytical column were all directed to the same flame ionization detector, which performed quantitative analysis of aromatic hydrocarbons in oils by correction factor normalization method. Real oil samples were analyzed and the results showed that the reproducibility, accuracy and reliability of the method can fulfill the requirements of practical application.

Chromatography, Gas↗