3‐D graphene‐supported mesoporous SiO2@Fe3O4 composites for the analysis of pesticides in aqueous samples by magnetic solid‐phase extraction with high‐performance liquid chromatography

Three‐dimensional graphene‐supported mesoporous silica@Fe₃O₄ composites (mSiO₂@Fe₃O₄‐G) were prepared by modifying mesoporous SiO₂‐coated Fe₃O₄ onto hydrophobic graphene nanosheets through a simple adsorption co‐condensation method. The obtained composites possess unique properties of large surface area (332.9 m²/g), pore volume (0.68 cm³/g), highly open pore structure with uniform pore size (31.1 nm), as well as good magnetic separation properties. The adsorbent (mSiO₂@Fe₃O₄‐G) was used for the magnetic solid‐phase extraction of seven pesticides with benzene rings in different aqueous samples before high‐performance liquid chromatography. The main parameters affecting the extraction such as adsorbent amount, volume of elution solvent, time of extraction and desorption, salt effect, oscillation rate were investigated. Under the optimal conditions, this method provided low limits of detection (S/N = 3, 0.525–3.30 μg/L) and good linearity (5.0–1000 μg/L, R² > 0.9954). Method validation proved the feasibility of the developed adsorbent, which has a high extraction efficiency and excellent enhancement performance for pesticides in this study. The proposed method was successfully applied to real aqueous samples, and satisfactory recoveries ranging from 77.5 to 113.6% with relative standard deviations within 9.7% were obtained.     

Preparation and evaluation of mesoporous cellular foams coating of solid-phase microextraction fibers by determination of tetrabromobisphenol A,tetrabromobisphenol S and related compounds

Two kinds of mesoporous cellular foams (MCFs), including mesoporous silica materials (MCF-1) and phenyl modified mesoporous materials (Ph-MCF-1), were synthesized and for the first time used as fiber-coating materials for solid-phase microextraction (SPME). By using stainless steel wire as the supporting core, four types of fibers were prepared by sol–gel method and immobilized by epoxy-resin method. To evaluate the performance of the home-made fibers for SPME, seven brominated flame retardants (BFRs), including tetrabromobisphenol A (TBBPA), tetrabromobisphenol S (TBBPS) and related compounds were selected as analytes. The main parameters that affect the extraction and desorption efficiencies, such as extraction temperature, extraction time, desorption time, stirring rate and ionic strength of samples were investigated and optimized. The optimized SPME coupled with high performance liquid chromatography (HPLC) was successfully applied to the determination of the seven BFRs in water samples. The linearity range was from 5.0 to 1000 μg L⁻¹ for each compound except TBBPS (from 1.0 to 1000 μg L⁻¹), with the correlation coefficients (r²) ranging from 0.9993 to 0.9999. The limits of detection of the method were 0.4–0.9 μg L⁻¹. The relative standard deviations varied from 1.2 to 5.1% (n = 5). The repeatability of fiber-to-fiber and batch-to-batch was 2.5–6.5% and 3.2–6.7%. The recoveries of the BFRs from aqueous samples were in the range between 86.5 and 103.6%. Compared with three commercial fibers (100 μm PDMS, 85 μm PA and 65 μm PDMS/DVB), the MCFs-coated fiber showed about 3.5-fold higher extraction efficiency.     

A practical method for computing the largest M‐eigenvalue of a fourth‐order partially symmetric tensor

In this paper, we consider a bi‐quadratic homogeneous polynomial optimization problem over two unit spheres arising in nonlinear elastic material analysis and in entanglement studies in quantum physics. The problem is equivalent to computing the largest M‐eigenvalue of a fourth‐order tensor. To solve the problem, we propose a practical method whose validity is guaranteed theoretically. To make the sequence generated by the method converge to a good solution of the problem, we also develop an initialization scheme. The given numerical experiments show the effectiveness of the proposed method. Copyright © 2009 John Wiley & Sons, Ltd.