Mingyu Ding

One-step synthesis of magnetic graphene oxide nanocomposite and its application in magnetic solid phase extraction of heavy metal ions from biological samples.

A novel magnetic graphene oxide nanocomposite was synthesized by one-step coprecipitation method and characterized by transmission electron microscopy, X-ray photoelectron spectroscopy and vibrating sample magnetometer. The nanocomposite beard many intriguing properties, including chemical stability, high adsorption capacity, and superparamagnetic. These properties evoked great interest and desire of its exploration in magnetic solid-phase extraction of heavy metal ions from complex samples. Several parameters effecting the analytical performance, such as the sample pH, amounts of adsorbent, sample volumes, elution volumes, and coexisting ions, had been investigated in detail. The adsorbed metal ions were easy eluted by controlling the pH condition and the materials could be reused more than 20 times. Under the optimized conditions, the limits of detection were 0.016, 0.046, 0.395, 0.038, 0.157 μg L(-1) for Co(2+), Ni(2+), Cu(2+), Cd(2+), and Pb(2+), respectively. The intra-day relative standard deviations (n=5) were in the range of 1.8-5.5% at 10 μg L(-1). The proposed method was successfully applied to biological sample analysis and got excellent recoveries in the range of 81-113% even the matrix was complex.

Determination of parabens in cosmetic products by solid‐phase microextraction of poly(ethylene glycol) diacrylate thin film on fibers and ultra high‐speed liquid chromatography with diode array detector

The fabrication of a solid-phase microextraction (SPME) fiber through UV-induced polymerization of poly(ethylene glycol) diacrylate (PEG-DA) for determination of parabens in cosmetic products is presented in this work. The PEG-DA polymer coating was covalently attached to the fiber by introducing a surface modification with 3-(trichlorosilyl)propyl methacrylate (TPM). The PEG-DA polymer thin film coated on the fiber was homogeneous and wrinkled, which led to an increase of the surface area and high extraction efficiency. The extraction performances of the prepared SPME fibers were assessed by preconcentration of parabens including methylparaben, ethylparaben, propylparaben and benzylparaben from cosmetic products. The analysis was performed on an ultra high-speed liquid chromatography with diode array detector. The prepared SPME fibers exhibited good repeatability (for one fiber) and reproducibility (fiber-to-fiber) with RSDs of 5.4 and 6.9%, respectively. The optimized SPME method supported a wide linear range of 0.50-160 μg/mL and the detection limits for parabens were in the range of 0.12-0.15 μg/mL (S/N=3). The developed method was successfully applied for determination of parabens in cosmetic products with different natures.

Preparation of hydrophilic carbon-functionalized magnetic microspheres coated with chitosan and application in solid-phase extraction of bisphenol A in aqueous samples

Hydrophilic carbon-functionalized magnetic microspheres coated with chitosan (Fe(3)O(4)@C@CHI) were synthesized for the first time. The hydrophilic carbonaceous layer on the Fe(3)O(4) microspheres was formed by the hydrothermal reaction of Fe(3)O(4) microspheres and glucose, and then chitosan was coated on surface of the carbon-functionalized microspheres by an easy zero-length reaction with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as cross-linkers. The Fe(3)O(4)@C@CHI microspheres were spherical core-shell structure with uniform size at about 400 nm. The microspheres had a high saturation magnetization of 60.22 emu g(-1), which was sufficient for magnetic separation. The Fe(3)O(4)@C@CHI microspheres presented good hydrophilic property due to the abundant carboxyl and hydroxyl groups on the carbonaceous layer of the microspheres. The successful modification of chitosan on the surface of carbon-functionalized magnetic microspheres was validated by the adsorption of Eriochrome Black T (EBT). The prepared microspheres were applied as magnetic solid-phase extraction materials for extraction of bisphenol A (BPA) in aqueous samples. The extraction recoveries of spiked samples were in the range of 99.4-102.6% with low relative standard deviations of 2.8-3.9%. The results demonstrated that the novel Fe(3)O(4)@C@CHI microspheres were good candidate as magnetic SPE materials for extraction of analytes from aqueous samples.

Preparation and retention mechanism study of graphene and graphene oxide bonded silica microspheres as stationary phases for high performance liquid chromatography.

Graphene oxide (GO) bonded stationary phase for high performance liquid chromatography (HPLC) was fabricated by coating GO sheets onto aminosilica microspheres via covalent coupling. Graphene (G) functionalized HPLC stationary phase was then prepared through hydrazine reduction of GO bonded silica (GO@SiO2) composite, which was the first example of using graphene as stationary-phase component for HPLC. Effective separations of the tested neutral and polar compounds on both GO@SiO2 and graphene bonded silica (G@SiO2) columns were achieved under the optimal experimental conditions. Compared with commercial C18 column, the different chromatographic performances of GO and graphene bonded columns were ascribed to their unique retention mechanisms. The polyaromatic scaffold of GO and graphene gives π-π stacking property and hydrophobic effect, and other retention mechanisms, such as π-π electron-donor-acceptor (EDA) interaction for the separation of nitroaromatic compounds and hydrogen bonding for hydroxyl and amino compounds, may also be taken into consideration. Experimental results indicated that the mixed-mode retention mechanism can facilitate the separation of analytes with similar hydrophobicity, which is a unique property compared with C18 column. Additionally, G@SiO2 showed higher affinity to aromatic analytes in contrast with GO@SiO2 and its retention mechanism was not consistent with the typical reversed phase behavior. The separation of aromatic compounds on G@SiO2 column relies primarily on the π-π stacking interaction and then the hydrophobicity, while the two interactions have equal shares on GO@SiO2 column.

Simultaneous ion-exclusion chromatography and cation-exchange chromatography of anions and cations in environmental water samples on a weakly acidic cation-exchange resin by elution with pyridine-2,6-dicarboxylic acid.

A non-suppressed conductivity detection ion chromatographic method using a weakly acidic cation-exchange column (Tosoh TSKgel OApak-A) was developed for the simultaneous separation and determination of common inorganic anions (Cl-, NO3- and SO4(2-)) and cations (Na+, NH4+, K+, Mg2+ and Ca2+). A satisfactory separation of these anions and cations on the weakly acidic cation-exchange column was achieved in 25 min by elution with a mixture of 1.6 mmol L-1 pyridine-2,6-dicarboxylic acid and 8.0 mmol L-1 18-crown-6 at flow rate of 1.0 mL min-1. On this weakly acidic cation-exchange resin, anions were retained by an ion-exclusion mechanism and cations by a cation-exchange mechanism. The linear range of the peak area calibration curves for all analytes were up to two orders of magnitude. The detection limits calculated at S/N = 3 ranged from 0.25 to 1.9 mumol L-1 for anions and cations. The ion-exclusion chromatography-cation-exchange chromatography method developed in this work was successfully applied to the simultaneous determination of major inorganic anions and cations in rainwater, tap water and snow water samples.

Preparation and retention mechanism exploration of mesostructured cellular foam silica as stationary phase for high performance liquid chromatography.

Siliceous mesostructured cellular foam (MCF) with highly interconnected porous structure, ultralarge pore size and relatively uniform particle size (3-5μm) was prepared to achieve the mixed-mode and efficient separation of intact proteins. And molecular sieving effect for the first time played an important role in protein separation using mesoporous silica materials as HPLC stationary phase. The spherical silica particles were synthesized via hydrothermal method and the pore size was easily regulated by adding NH4F as well as altering the aging time. After aminopropyl derivatization, the chromatographic performance of functionalized mesoporous silica particles was investigated in comparison with those without modification and commercial NH2 column, and their mixed-mode retention mechanisms were investigated in detail. The superior separation performance for the retention of proteins was obtained on our home-made column in comparison with commercial NH2 column. The influences of aminopropyl derivatization and mobile phase composition on the column property were also investigated. Moreover, the home-made column showed similar performance for separation of polar anilines and neutral PAHs with the commercial column, owing to mixed-mode retention mechanisms including p-π stacking, electron interaction, hydrophobic effect, π-π EDA interaction and hydrogen bonding. All these results indicated that the aminopropyl modified MCF would be promising in the mixed-mode and efficient separation of biomolecules in addition with small molecules.

In-syringe solid-phase extraction for on-site sampling of pyrethroids in environmental water samples

On-site sampling is an analytical approach that can ensure the accuracy of monitoring data and enhance the effectiveness of environmental protection measures. In the present work, an in-syringe solid-phase extraction (SPE) device was designed for on-site sampling of trace contaminants in environmental water samples followed by gas chromatography-mass spectrometry (GC-MS) analysis. Template assisted freeze casting followed by hydrazine vapor reduction approach was used to synthesize a hierarchical porous graphene aerogel (HPGA), which was used as the sorbent in the in-syringe SPE device. Environmental degradable pyrethroids were selected as the model analytes. Owing to the large specific surface area and hydrophobicity of HPGA, the target molecules could be completely extracted during one aspirating/dispensing cycle. The analytes were stable on the sorbent for at least 72 h when the device was stored under airtight and light-free conditions, and were not affected by the pH value of sample solution. All results demonstrated that the device could meet the requirements of on-site sampling. For practical application, the limits of detection were found to be in the range of 0.012-0.11 ng mL-1 under the optimized conditions, and satisfactory recoveries in the range of 65.7-105.9% were obtained for the analysis of real samples. The results of this study demonstrate the immense potential of HPGA for the enrichment of trace environmental pollutants, and meanwhile promote the application of the in-syringe SPE technique as a promising candidate for on-site sampling.

SIMULTANEOUS DETERMINATION OF ANIONS AND CATIONS IN ENVIRONMENTAL SAMPLES BY USING A WEAKLY ACIDIC CATION-EXCHANGE COLUMN WITH 2,6-PYRIDINEDICARBOXYLIC ACID, SULFOSALICYLIC ACID, AND 18-CROWN-6 AS ELUENT

Weakly acidic cation exchange resin can be used for simultaneous separation of anions and cations when acidic eluents were employed. On the weakly acidic cation exchange resin, anions were retained by ion exclusion mechanism, while cations by cation exchange mechanism. The selection of eluent is very important for the separation of interesting analytes. In this study, a mixture of 0.6 mmol/L 2,6-pyridinedicarboxylic acid (PDCA), 0.75 mmol/L sulfosalicylic acid (SSA), and 6.0 mmol/L 18-crown-6 was employed as eluent for the simultaneous separation of common inorganic anions (Cl−, NO3 − and SO4 2−) and cations (Na+, NH4 +, K+, Mg2+, and Ca2+). A good separation of these anions and cations on the weakly acidic cation exchange column (Tosoh TSKgel OApak-A) was achieved in 16 min. The linear range of peak area calibration curves for all analytes were up to two orders of magnitude. The conductivity detection limits calculated at S/N=3 were from 0.2 to 1.6 μmol/L for anions and cations. The method developed in this work was successfully applied to the simultaneous determination of major inorganic anions and cations in several environmental water samples.