Assessing transformation processes of organic contaminants by compound-specific stable isotope analysis

The analysis of variations in stable isotope composition is becoming an essential approach for evaluating enzymatic and abiotic reactions of organic contaminants in soils and aquatic systems. Different, sometimes complementary analytical techniques are currently used and developed to determine stable isotope ratios in individual organic compounds. Anticipating an increasing demand for compound-specific isotope analysis, this survey compiles information for choosing the most promising analytical approach to an isotope-related problem. To this end, we review the principles of instrumentation for compound-specific isotope analysis and show how they can be exploited to assess contaminant transformation processes. Using chlorinated solvents and triazine herbicides as illustrative examples, we discuss how the isotope-sensitive techniques impact the investigation of stable isotope fractionation in environmental chemistry and microbiology.     

Bounds for solid angles of lattices of rank three

We find sharp absolute constants C₁ and C₂ with the following property: every well-rounded lattice of rank 3 in a Euclidean space has a minimal basis so that the solid angle spanned by these basis vectors lies in the interval [C₁,C₂]. In fact, we show that these absolute bounds hold for a larger class of lattices than just well-rounded, and the upper bound holds for all. We state a technical condition on the lattice that may prevent it from satisfying the absolute lower bound on the solid angle, in which case we derive a lower bound in terms of the ratios of successive minima of the lattice. We use this result to show that among all spherical triangles on the unit sphere in RN with vertices on the minimal vectors of a lattice, the smallest possible area is achieved by a configuration of minimal vectors of the (normalized) face centered cubic lattice in R³. Such spherical configurations come up in connection with the kissing number problem.     

Numerical simulation of forced convection over a periodic series of rectangular cavities at low Prandtl number

Convective heat transfer in laminar conditions is studied numerically for a Prandtl number Pr = 0.025, representative of liquid lead–bismuth eutectic (LBE). The geometry investigated is a channel with a periodic series of shallow cavities. Finite-volume simulations are carried out on structured orthogonal curvilinear grids, for ten values of the Reynolds number based on the hydraulic diameter between Re_m = 24.9 and Re_m = 2260. Flow separation and reattachment are observed also at very low Reynolds numbers and wall friction is found to be remarkably unequal at the two walls. In almost all cases investigated, heat transfer rates are smaller than the corresponding flat channel values. Low-Prandtl number heat transfer rates, investigated by comparison with Pr = 0.71 results, are large only for uniform wall temperature and very low Re. Influence of flow separation on local heat transfer rates is discussed, together with the effect of different thermal boundary conditions. Dependency of heat transfer performance on the cavity geometry is also considered.     

基于光腔衰荡光谱的便携式呼气异戊二烯分析仪性能评估

呼气异戊二烯是一种内源性代谢产物,其含量与人体血液中的胆固醇水平存在关联。但人体呼气影响因素众多,寻找其与胆固醇水平诊断参数的定量相关性,需要对选取的特定人群进行有效的呼吸气体分析（实时、在线、高灵敏度、高选择性、高精度的大量呼气数据获取）。光腔衰荡光谱（CRDS）是一种具有极高灵敏度、稳定性和选择性的光谱技术。采用目前市场在售的单波长紧凑型半导体紫外激光器,搭建了一套基于CRDS的呼气异戊二烯分析仪,该分析仪主要由激光系统、真空腔体、光电探测模块以及数据采集模块构成。线性拟合的结果显示所获得的衰荡信号接近单指数衰减（R2=0.998 39）,符合朗伯-比尔定律。探究了不同信号平均次数对衰荡信号稳定性的影响,综合考虑衰荡信号的稳定性和分析仪的响应时间,采用128次作为实验过程中的信号平均次数。对呼气异戊二烯分析仪的性能进行了测试,为了表征分析仪的稳定性,持续测量了分析仪16 min的真空衰荡时间。使用氮气、空气和呼吸样本,测量了呼气异戊二烯分析仪的重复性和响应速度。为了测试分析仪的线性度,测量了不同粒子数密度的异戊二烯标准气体（10×10-9,30×10-9,50×10-9,100×10-9,200×10-9）的衰荡时间。最后分析了在224 nm测量异戊二烯存在的光谱干扰问题（NO,N₂O和丙酮）。实验表明：分析仪具有高的灵敏度（检测极限为0.49×10-9）、良好的重复性、稳定性（0.48%）、近实时的响应速度（1秒测量一个数据）和良好的线性度（R2=0.993 13）,将检测极限提高至现有水平的1/1 000。研究证明基于CRDS的便携式呼气异戊二烯分析仪可实现对人体呼气异戊二烯的有效分析。     

在腔场与可移动镜子系统中对镜子位置的测量

描述利用腔场与镜的相互作用,对镜的位置坐标进行测量选域以使腔场变为Fock态的叠加态.并进一步讨论其压缩特性及相干性,通过数值计算,发现该腔场有非经典效应,即有一定的压缩效应和反聚束效应.     

正四面体套中心构型的存在唯一性

Two cases of the nested configurations in R^3 consisting of two regular quadrilaterals are discussed. One case of them do not form central configuration, the other case can be central configuration. In the second case the existence and uniqueness of the central configuration are studied. If the configuration is a central configuration, then all masses of outside layer are equivalent, similar to the masses of inside layer. At the same time the following relation between r(the ratio of the sizes) and mass ratio b = m/m must be satisfied b=24(3的立方根)（3r^2 2r 3）^-3/2-8（1-r）|1-r|^-3-3(6r的立方根)/24(3的立方根)（3 r）（3r^2 2r 3）^-3/2-8r（1-r）|1-r|^-3-3(6r^-2的立方根)in which the masses at outside layer are not less than the masses at inside layer, and the solution of this kind of central configuration is unique for the given ratio (b) of masses.     

On the Yang-Mills effective action with instanton gauge field

The calculation of the determinant for the second order covariant derivative operator is analyzed in the space R⁴ and for Yang-Mills instanton field configurations. The problems inherent to the ultraviolet ζ-function renormalization method and infrared divergencies of this operator are reviewed. A method for estimating the asymptotic coefficients of its determinant has been discussed and the failure of the general conformal invariance induced by any regularization technique has also been considered. A particular solution valid for a Yang-Mills multi instanton configuration, valuable in order to get the non conformally invariant piece of the general solution, is the main result obtained here. We give it as an integral equation, namely in a semi-explicit form.     

The effect of the counter-rotating terms on the Rabi oscillations and squeezing in the Dicke model with cavity losses

The effect of the counter-rotating terms in the Dicke model with cavity losses is examined. Numerical results for the time evolutions of the atomic population inversion, dipole moment, mean-photon number and atomic and field squeezing parameters for an initial coherent field are presented for various numbers of atoms and different cavity dampings. As a consequence of the counter-rotating terms, the appearance of new steady states for atomic population inversion and the mean-photon number is pointed out.     

Constitutive modeling of the mechanics associated with crystallizable shape memory polymers

Shape memory polymers are novel materials that can be easily formed into complex shapes, retaining memory of their original shape even after undergoing large deformations. The temporary shape is stable and return to the original shape is triggered by a suitable mechanism such as heating. In this paper, we develop constitutive equations to model the mechanical behavior of crystallizable shape memory polymers. Crystallizable shape memory polymers are called crystallizable because the temporary shape is fixed by a crystalline phase, while return to the original shape is due to the melting of this crystalline phase. The modeling is done using a framework that was developed recently for studying crystallization in polymers ([28], [25], [27], [31]) and is based on the theory of multiple natural configurations. In this paper we formulate constitutive equations for the original amorphous phase and the semi-crystalline phase that is formed after the onset of crystallization. In addition we model the melting of the crystalline phase to capture the return of the polymer to its original shape. The model has been used to simulate a typical uni-axial cycle of deformation, the results of this simulation compare very well with experimental data. In addition to this we also simulate circular shear of a hollow cylinder and present results for different cases in this geometry. Received: January 5, 2005     

Charge-conserving electron density averaging for a set of nuclear configurations

The Löwdin-Inverse Löwdin Transformation (LILT) technique combined with the Diophantine Density Matrix Purification (DDMP) method is suggested for a charge-preserving density averaging approach for a range of nuclear configurations.