Simultaneous separation and determination of six arsenic species in rice by anion‐exchange chromatography with inductively coupled plasma mass spectrometry

The simultaneous separation and determination of arsenite As(III), arsenate As(V), monomethylarsonic acid (MMA), dimethylarsinic acid (DMA), arsenobetaine (AsB), and arsenocholine (AsC) in rice samples have been carried out in one single anion‐exchange column run by high‐performance liquid chromatography with inductively coupled plasma mass spectrometry. To estimate the effect of variables on arsenic (As) speciation, the chromatographic conditions including type of competing anion, ionic strength, pH of elution buffer, and flow rate of mobile phase have been investigated by a univariate approach. Under the optimum chromatographic conditions, baseline separation of six As species has been achieved within 10 min by gradient elution program using 4 mM NH₄HCO₃ at pH 8.6 as mobile phase A and 4 mM NH₄HCO₃, 40 mM NH₄NO₃ at pH 8.6 as mobile phase B. The method detection limits for As(III), As(V), MMA, DMA, AsB, and AsC were 0.4, 0.9, 0.2, 0.4, 0.5, and 0.3 μg/kg, respectively. The proposed method has been applied to separation and quantification of As species in real rice samples collected from Hunan Province, China. The main As species detected in all samples were As(III), As(V) and DMA, with inorganic As accounting for over 80% of total As in these samples.     

梯度洗脱测定植物源调节剂中内源激素方法探讨

建立了梯度洗脱高效液相色谱法同时测定内源激素赤霉素(GA3)、吲哚乙酸(3-IAA)、脱落酸(ABA)、细胞分裂素(6-BA)和α-萘乙酸(α-NAA)等5种植物内源激素的方法,采用Hypersil ODS C18柱和紫外检测器,以甲醇、乙腈和0.6%的乙酸为流动相梯度洗脱,分别在各组分的保留时间段设置其最佳检测波长,在12 min内可将上述5种内源激素的各组分分离完全,峰形良好,该方法的回收率可达到90%～96%,线性相关系数(r)大于0.998,检出限在0.02～0.3 μg/g之间.还探讨了各组分的最佳检测条件和梯度洗脱存在的问题及解决方法.     

电场条件下高分子共混物组分浓度梯度化的研究

研究了聚乙烯醇/聚丙烯酸高分子共混物水溶液中,共混物各组分在电场诱导条件下沿电场方向的浓度梯度分布情况.通过测定不同时刻PVA/PAA高分子共混物水溶液在电场的不同区域内的pH值,研究了电场诱导下共混物各组分沿电场方向的迁移过程.结果表明,PAAn-向电场正极迁移,同时由于浓差梯度,PVA向负极迁移,并形成浓度梯度分布.随时间的延长,高分子共混物的组分梯度程度逐渐加大.     

Extrapolated Smoothing Descent Algorithm for Constrained Nonconvex and Nonsmooth Composite Problems*

In this paper, the authors propose a novel smoothing descent type algorithm with extrapolation for solving a class of constrained nonsmooth and nonconvex problems,where the nonconvex term is possibly nonsmooth. Their algorithm adopts the proximal gradient algorithm with extrapolation and a safe-guarding policy to minimize the smoothed objective function for better practical and theoretical performance. Moreover, the algorithm uses a easily checking rule to update the smoothing parameter to ensure that any accumulation point of the generated sequence is an (affine-scaled) Clarke stationary point of the original nonsmooth and nonconvex problem. Their experimental results indicate the effectiveness of the proposed algorithm.     

Subset representations and eigenvalues of the universal intertwining matrix

We solve a combinatorial question concerning eigenvalues of the universal intertwining endomorphism of a subset representation.     

梯度淋洗离子色谱法同时分析有机酸与无机酸阴离子的应用研究

利用梯度离子色谱法和抑制电导检测器对有机酸与无机酸阴离子的色谱分析条件进行了研究,建立了最佳梯度程序。通过浓缩柱的富集,可使方法的检出限达1×10-3μg·L-1,各离子的相对标准偏差大部分在5%以下。此方法可用于火电厂水汽系统中痕量低分子有机酸和无机酸阴离子的快速、准确分析,结果满意。     

Problems of the Flamant–Boussinesq and Kelvin Type in Dipolar Gradient Elasticity

This work studies the response of bodies governed by dipolar gradient elasticity to concentrated loads. Two-dimensional configurations in the form of either a half-space (Flamant–Boussinesq type problem) or a full-space (Kelvin type problem) are treated and the concentrated loads are taken as line forces. Our main concern is to determine possible deviations from the predictions of plane-strain/plane-stress classical linear elastostatics when a more refined theory is employed to attack the problems. Of special importance is the behavior of the new solutions near to the point of application of the loads where pathological singularities and discontinuities exist in the classical solutions. The use of the theory of gradient elasticity is intended here to model material microstructure and incorporate size effects into stress analysis in a manner that the classical theory cannot afford. A simple but yet rigorous version of the generalized elasticity theories of Toupin (Arch. Ration. Mech. Anal. 11:385–414, 1962) and Mindlin (Arch. Ration. Mech. Anal. 16:51–78, 1964) is employed that involves an isotropic linear response and only one material constant (the so-called gradient coefficient) additional to the standard Lamé constants (Georgiadis et al., J. Elast. 74:17–45, 2004). This theory, which can be viewed as a first-step extension of the classical elasticity theory, assumes a strain-energy density function, which besides its dependence upon the standard strain terms, depends also on strain gradients. The solution method is based on integral transforms and is exact. The present results show departure from the ones of the classical elasticity solutions (Flamant–Boussinesq and Kelvin plane-strain solutions). Indeed, continuous and bounded displacements are predicted at the points of application of the loads. Such a behavior of the displacement fields is, of course, more natural than the singular behavior present in the classical solutions.      

Instability of the melt flow in VGF growth with a traveling magnetic field

The linear instability of a thermally stratified melt flow in the VGF configuration driven by a traveling magnetic field (TMF) is considered numerically and experimentally. The dependency of the instability threshold on the governing parameters is found for several cuts through the parameter space covering a wide range of possible applications. In a first approximation the linear instability occurs when the dimensionless TMF forcing parameter reaches the magnitude of the Grashof number. This is particularly true in a medium-sized crucible where the first instability is axisymmetric and sub-critical. As the Grashof number increases the flow develops self-similar boundary layers and the instability becomes three-dimensional. The instability originates in the bottom boundary layer where the convection tends to suppress the imposed temperature gradient in the central part of the melt zone. It is shown that the TMF may serve as a tool to control the phase interface shape without causing flow instationarity when the crucible diameter exceeds a certain value. This value is estimated to be around 6 cm for GaAs. The flow stays stable if the TMF is used for a reversal of the meridional flow with the aim to remove a possible dopant concentration peak on the axis.     

New approach to linear gradient elution used for optimisation in reversed-phase liquid chromatography

A new mathematical treatment concerning the gradient elution in reversed-phase liquid chromatography when the volume fraction psi of an organic modifier in the water-organic mobile phase varies linearly with time is presented. The experimental ln k versus psi curve, where k is the retention factor under isocratic conditions in a binary mobile phase, is subdivided into a finite number of linear portions and the solute gradient retention time tR is calculated by means of an analytical expression arising from the fundamental equation of gradient elution. The validity of the proposed analytical expression and the methodology followed for the calculation of tR was tested using eight catechol-related solutes with mobile phases modified by methanol or acetonitrile. It was found that in all cases the accuracy of the predicted gradient retention times is very satisfactory because it is the same with the accuracy of the retention times predicted under isocratic conditions. Finally, the above method for estimating gradient retention times was used in an optimisation algorithm, which determines the best variation pattern of psi that leads to the optimum separation of a mixture of solutes at different values of the total elution time.     

Adiabatic shear banding and scaling laws in chip formation with application to cutting of Ti–6Al–4V

The phenomenon of adiabatic shear banding is analyzed theoretically in the context of metal cutting. The mechanisms of material weakening that are accounted for are (i) thermal softening and (ii) material failure related to a critical value of the accumulated plastic strain. Orthogonal cutting is viewed as a unique configuration where adiabatic shear bands can be experimentally produced under well controlled loading conditions by individually tuning the cutting speed, the feed (uncut chip thickness) and the tool geometry. The role of cutting conditions on adiabatic shear banding and chip serration is investigated by combining finite element calculations and analytical modeling. This leads to the characterization and classification of different regimes of shear banding and the determination of scaling laws which involve dimensionless parameters representative of thermal and inertia effects. The analysis gives new insights into the physical aspects of plastic flow instability in chip formation. The originality with respect to classical works on adiabatic shear banding stems from the various facets of cutting conditions that influence shear banding and from the specific role exercised by convective flow on the evolution of shear bands. Shear bands are generated at the tool tip and propagate towards the chip free surface. They grow within the chip formation region while being convected away by chip flow. It is shown that important changes in the mechanism of shear banding take place when the characteristic time of shear band propagation becomes equal to a characteristic convection time. Application to Ti–6Al–4V titanium are considered and theoretical predictions are compared to available experimental data in a wide range of cutting speeds and feeds. The fundamental knowledge developed in this work is thought to be useful not only for the understanding of metal cutting processes but also, by analogy, to similar problems where convective flow is also interfering with adiabatic shear banding as in impact mechanics and perforation processes. In that perspective, cutting speeds higher than those usually encountered in machining operations have been also explored.