Study of enantioselectivity on an immobilized amylose carbamate stationary phase under subcritical fluid chromatography

In this paper, we describe the enantiomeric separation of a chiral alcohol under subcritical fluid chromatography conditions using a 3 μm particle size bonded amylose carbamate stationary phase. Linear and branched alcohols were used as polar modifiers in CO₂. The studies with linear alcohols showed a decrease in selectivity factor as the number of carbons in the linear chain increased. For branched alcohols, as the bulk of substituents at the α carbon atom increases the separation factor decreases. Thermodynamic studies showed that in the presence of the alcohols studies, except methanol and ethanol, a positive ΔΔS was observed. Molecular mechanics simulation brought more insights into the mechanism of enantiomeric separation on this stationary phase under subcritical fluid chromatography.     

Chemical recycling of carbon fibre reinforced epoxy resin composites in subcritical water: Synergistic effect of phenol and KOH on the decomposition efficiency

The combination of phenol and potassium hydroxide (KOH) was used to chemically recycle carbon fibre reinforced epoxy resin cured with 4,4′-diaminodiphenylmethane in subcritical water. This combination had a synergistic effect on decomposing this kind of epoxy resin. The main decomposition products from the epoxy resin were identified by means of GC-MS, and a possible free-radical reaction mechanism for the decomposition of epoxy resin is proposed. The recovered carbon fibres were characterized using single fibre tensile tests, scanning electron microscopy and X-ray photoelectron spectroscopy. Compared to virgin carbon fibres after sizing removal, the surface compositions of the recovered carbon fibres had little change and the tensile strength of the recovered carbon fibres was well retained.     

The quasi-stationary structure of radiating shock waves. I. The one-temperature fluid

We calculate the quasi-stationary structure of a radiating shock wave propagating through a spherically symmetric shell of cold gas by solving the time-dependent equations of radiation hydrodynamics on an implicit adaptive grid. We show that this code successfully resolves the shock wave in both the subcritical and supercritical cases and, for the first time, we have reproduced all the expected features – including the optically thin temperature spike at a supercritical shock front – without invoking analytic jump conditions at the discontinuity. We solve the full moment equations for the radiation flux and energy density, but the shock wave structure can also be reproduced if the radiation flux is assumed to be proportional to the gradient of the energy density (the diffusion approximation), as long as the radiation energy density is determined by the appropriate radiative transfer moment equation. We find that Zel'dovich and Raizer's (1967) analytic solution for the shock wave structure accurately describes a subcritical shock but it underestimates the gas temperature, pressure, and the radiation flux in the gas ahead of a supercritical shock. We argue that this discrepancy is a consequence of neglecting terms which are second order in the minimum inverse shock compression ratio [, where is the adiabatic index] and the inaccurate treatment of radiative transfer near the discontinuity. In addition, we verify that the maximum temperature of the gas immediately behind the shock is given by , where is the gas temperature far behind the shock. Received 21 September 1998/ Accepted 2 February 1999     

Existence of solution for a nonvariational elliptic system with exponential growth in dimension two

We prove existence of solution for an elliptic system on a bounded domain in dimension two. We use the Galerkin scheme in the product of Hilbert spaces. The nonlinearities may have subcritical or critical exponential growth.     

Determination of 13 endocrine disrupting chemicals in sediments by gas chromatography–mass spectrometry using subcritical water extraction coupled with dispersed liquid–liquid microextraction and derivatization

In this study, a sample pretreatment method was developed for the determination of 13 endocrine disrupting chemicals (EDCs) in sediment samples based on the combination of subcritical water extraction (SWE) and dispersed liquid–liquid microextraction (DLLME). The subcritical water that provided by accelerated solvent extractor (ASE) was the sample solution (water) for the following DLLME and the soluble organic modifier that spiked in the subcritical water was also used as the disperser solvent for DLLME in succession. Thus, several important parameters that affected both SWE and DLLME were investigated, such as the extraction solvent for DLLME (chlorobenzene), extraction time for DLLME (30 s), selection of organic modifier for SWE (acetone), volume of organic modifier (10%) and extraction temperature for SWE (150 °C). In addition, good chromatographic behavior was achieved for GC–MS after derivatisation by using N,O-bis(trimethylsilyl) trifluoroacetamide (BSTFA). As a result, proposed method sensitive and reliable with the limits of detection (LODs) ranging from 0.006 ng g⁻¹ (BPA) to 0.639 ng g⁻¹ (19-norethisterone) and the relative standard deviations (RSDs) between 1.5% (E2) and 15.0% (DES). Moreover, the proposed method was compared with direct ASE extraction that reported previously, and the results showed that SWE–DLLME was more promising with recoveries ranging from 42.3% (dienestrol) to 131.3% (4,5α-dihydrotestosterone), except for diethylstilbestrol (15.0%) and nonylphenols (29.8%). The proposed method was then successfully applied to determine 13 EDCs sediment of Humen outlet of the Pearl River, 12 of target compounds could be detected, and 10 could be quantitative analysis with the total concentration being 39.6 ng g⁻¹, and which indicated that the sediment of Humen outlet was heavily contaminated by EDCs.     

Determination of neonicotinoid insecticides residues in eels using subcritical water extraction and ultra-performance liquid chromatography–tandem mass spectrometry

A rapid, sensitive, and environmental-friendly multi-residue method has been developed for the simultaneous determination of seven neonicotinoid insecticides (dinotefuran, nitenpyram, thiamethoxam, imidacloprid, clothianidin, acetamiprid, and thiacloprid) residues in eel samples. Subcritical water extraction was investigated as a novel and alternative technology for the extraction of neonicotinoids from eel matrices and the results were compared with the conventional ultrasonic and shaking extraction. The target compounds were identified and quantitatively determined by ultra-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (UPLC–MS/MS) operated in multiple reaction monitoring mode. Under the current optimized chromatographic conditions, each LC run was completed in 5 min. Average recoveries of the seven analytes from fortified samples ranged between 84.6% and 102.0%, with relative standard deviations (RSD) lower than 10.8%. The limits of detection (LODs) and quantification (LOQs) for neonicotinoids were in the ranges of 0.12–0.36 μg kg⁻¹ and 0.42–1.12 μg kg⁻¹, respectively. The proposed method is fast, sensitive, easy to perform, water-based thus more environmentally acceptable, making it applicable for high-throughput monitoring of insecticides residues in aquatic products.     

利用亚临界丁烷分离茶树鲜叶中的阿维菌素

以茶树鲜叶为原料,利用亚临界丁烷流体分离鲜叶表面残留的阿维菌素.在料溶比为1∶10(m/V)的条件下,利用响应面法(RSM)的Box-Benhnken Design(BBD)设计实验,考察不同温度、时间、分离次数等条件对多酚氧化酶活性的影响,同时分析亚临界丁烷处理对茶树鲜叶生理特性的影响.结果表明,亚临界丁烷流体在保持茶叶物理结构和多酚氧化酶活性基本不变的情况下,可有效分离茶叶中的阿维菌素;通过Design Expert软件分析,得出分离阿维菌素的最优工艺参数为温度45℃,时间30 min,分离次数1次,料液比为1∶10(m/V),分离效率大于91%,多酚氧化酶相对活性为25.73%.通过扫描电镜观察,茶树鲜叶结构在丁烷处理前后无明显变化.本研究结果表明,亚临界丁烷流体可有效分离茶叶中的农药残留,为亚临界流体在去除天然植物农残的应用中提供了实验依据和技术支持.     

Structural characteristics of detonation expansion from a small channel to a larger one

We report on numerical simulations of the evolution of two-dimensional detonation waves that are expanded from a small channel to a larger one. In accordance with experimental data, the simulations predict three different types of evolution, namely, supercritical, critical and subcritical detonations. In a supercritical detonation, the reaction zone remains always attached to the precursor shock, whereas in a critical one it temporarily detaches and then re-attaches to the front. In the subcritical type, the extinction is permanent, i.e., the detonation quenches. The effects of the fuel’s activation energy and the channel-width ratio are studied via a parametric study. It is found that sufficiently large values of these two parameters can result to flows of the critical and even the subcritical type. Finally, three-dimensional simulations have also been performed and are briefly discussed herein.     

Properties of subcritical water as an eluent for reversed-phase liquid chromatography—Disruption of the hydrogen-bond network at elevated temperature and its consequences

The use of subcritical water as an eluent for reversed-phase liquid chromatography is further explored. Shape selectivity as well as thermodynamic values for solute transfer were measured and compared to those seen with traditional ambient methanol/water and acetonitrile/water mobile phases. Linear solvation energy analysis was also used to analyze extrapolated values of the retention factor in pure water at ambient temperatures (k^′_w${k^{\prime }}_w$) for subcritical water and ambient hydroorganic mobile phases. Results indicate that it is likely that a large disruption in the hydrogen-bonding network of water at high temperatures causes unique chromatographic selectivity, as well as prohibits accurate extrapolation from high temperature to ambient conditions using pure water. Additionally, subcritical water was not found to be a suitable mobile phase for determining k^′_w${k^{\prime }}_w$ for use in estimating octanol/water partition coefficients.