Simultaneous determination of metabolites of trimethylbenzenes,dimethylbenzylmercapturicacid and dimethylhippuric acid,in human urine by solid-phase extraction followed by liquid chromatography tandem mass spectrometry

We describe a novel method for the determination of three kinds of dimethylbenzylmercapturic acids (DMM) and six kinds of dimethylhippuric acids (DMH), found in urine as metabolites of trimethylbenzenes, based on liquid chromatography/electrospray ionization tandem mass spectrometry. A solid-phase extraction procedure was used for the extractions of DMM and DMH from a urine sample, and the separation was performed on a reversed-phase C(30) column. The analytes were ionized by electrospray in the positive-ion mode. Operating in the multiple reaction monitoring mode, the linearity of the relative mass spectrometric responses to the internal standard versus analyte concentrations were established in the range 0.1-100 ng ml(-1). The extraction procedure was rapid and the relative standard deviations were below 5%. The detection limits of DMM and DMH in the urine by the proposed method were in the ranges 0.26-0.41 and 0.42-2.0 ng l(-1), respectively. Furthermore, DMM and DMH were detected in a urine sample from an individual who did not suffer from occupational exposure to trimethylbenzenes, by using this method.     

Synthesis of Both Enantiomers of Flavanone and 2‐Methylchromanone

An efficient enantiospecific synthesis of the (R)‐ and (S)‐enantiomers of flavanone and 2‐methylchromanone is described. The key steps are a C,C‐bond formation by ring opening of a chiral epoxide with a dithiane anion, followed by a Mitsunobu cyclization. The products obtained have high enantiomeric purity.     

Optical and time-resolved EPR studies of the excited states of azastilbenes and their protonated cations

The effects of protonation on the excited states oftrans-3-styrylpyridine (StP) andtrans-4,4′-dipyridylethylene (DPE) have been studied through measurements of the time-resolved electron paramagnetic resonance (EPR), ultraviolet absorption, and fluorescence spectra in methanol-water mixtures at 77 K. The assignment of the transient EPR signals was carried out with the aid of the stretched poly(vinyl alcohol) films method. From the analysis of these spectra it is concluded that the single protonation appears to have little effect on the zero-field splitting parameters and the anisotropy in the sublevel populating rates of the lowest excited triplet (T₁) states of StP and DPE. However, the decay rate constants of the fluorescent states decrease and fluorescence quantum yields increase on single protonation. These experimental results suggest that the single protonation causes a decrease in the intersystem crossing (ISC) rates for the three T₁ sublevels. These results are explained in terms of the vibronic mixing between the¹nπ* and¹ππ* states in the lowest excited singlet state. The assignment of StP to the specified conformer was carried out through the analysis of the anisotropic ISC processes.     

Numerical solution of singular integral equations in stress concentration problems under longitudinal shear loading

Summary The paper deals with numerical solutions of singular integral equations in stress concentration problems for longitudinal shear loading. The body force method is used to formulate the problem as a system of singular integral equations with Cauchy-type singularities, where unknown functions are densities of body forces distributed in the longitudinal direction of an infinite body. First, four kinds of fundamental density functions are introduced to satisfy completely the boundary conditions for an elliptical boundary in the range 0≤φ _k ≤2π. To explain the idea of the fundamental densities, four kinds of equivalent auxiliary body force densities are defined in the range 0≤φ _k ≤π/2, and necessary conditions that the densities must satisfy are described. Then, four kinds of fundamental density functions are explained as sample functions to satisfy the necessary conditions. Next, the unknown functions of the body force densities are approximated by a linear combination of the fundamental density functions and weight functions, which are unknown. Calculations are carried out for several arrangements of elliptical holes. It is found that the present method yields rapidly converging numerical results. The body force densities and stress distributions along the boundaries are shown in figures to demonstrate the accuracy of the present solutions. Received 26 May 1998; accepted for publication 27 November 1998     

Resonant neutral-particle emission after collisions of electrons with base-stacked oligonucleotide cations in a storage ring

Electron-cation collisions have been studied for various protonated deoxyoligonucleotide monocations, using an electrostatic storage ring equipped with a merged-electron-beam device. Resonant neutral-particle emissions have been observed at a collision energy of about 4.5 eV for some of them. The resonance parameters depend on DNA base composition and sequence, and the resonances occur only in oligonucleotide monocations with base stacking. The resonance widths increase with oligonucleotide length, but saturate at trimers. Quantum-chemical estimations performed here help explain these results.     

Miscibility and crystallization behavior of poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) and methoxy poly(ethylene glycol) blends

Poly(3‐hydroxybutyrate‐co‐3‐hydroxyhexanoate) (PHB‐HHx) and methoxy poly(ethylene glycol) (MPEG) blends were prepared using melt blending. The single glass transition temperature, T_g, between the T_gs of the two components and the negative χ value indicated that PHB‐HHx and MPEG formed miscible blends over the range of compositions studied. The Gordon–Taylor equation proved that there was an interaction between PHB‐HHx and MPEG in their blends. FTIR supported the presence of hydrogen bonding between the hydroxyl group of MPEG and the carbonyl group of PHB‐HHx. The spherulitic morphology and isothermal crystallization behavior of the miscible PHB‐HHx/MPEG blends were investigated at two crystallization temperatures (70 and 40 °C). At 70 °C, melting MPEG acted as a noncrystalline diluent that reduced the crystallization rate of the blends, while insoluble MPEG particles acted as a nucleating agent at 40 °C, enhancing the crystallization rate of the blends. However, no interspherulitic phase separation was observed at the two crystallization temperatures. The constant value of the Avrami exponent demonstrated that MPEG did not affect the three‐dimensional spherulitic growth mechanism of PHB‐HHx crystals in the blends, although the MPEG phase, such as the melting state or insoluble state, influenced the crystallization rate of the blends. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2852–2863, 2006     

Low temperature thermoelectric properties of Pb- or Sn-doped Bi-Sb alloys

Pb- or Sn-doped Bi₈₈Sb₁₂ alloys were prepared by direct melting, quenching, and annealing. The Bi-Sb alloy phase was predominant in all samples. Pb or Sn atoms were distributed almost uniformly in Bi₈₈Sb_12, while some segregation was confirmed at the grain boundaries when Pb or Sn was involved heavily. The thermoelectric properties of these doped materials were investigated by measuring the Hall coefficient, electrical resistivity, and Seebeck coefficient between 20 K and 300 K. The Hall and Seebeck coefficients of Pb- or Sn-doped samples were positive at low temperatures, indicating that the doping element acted as an acceptor. Temperatures resulting in positive Hall and Seebeck coefficients further increased with increasing doping amount and with respect to the annealing process. As a result, a large power factor of 1.2 W/mK² could be obtained in the 3-at% Sn-doped sample at 220 K, with a large positive Seebeck coefficient.     

Deviations of capacitive and inductive loops in the electrochemical impedance of a dissolving iron electrode.

The electrochemical impedance of an iron electrode often shows the capacitive and inductive loops on the complex plane. The capacitive loop originates from the time constant of the charge transfer resistance and the electric double layer capacitance. The inductive loop is explained by Faradaic processes involving the reaction intermediate. In some cases, these loops deviate from a true semicircle. In this paper, the origins and curve-fitting methods for the deviated loops of electrochemical impedance are discussed. The constant phase element (CPE) was used to present the deviation of the capacitive loop instead of electric double layer capacitance. The reaction rate constants, which are a function of the frequency, are proposed for the Faradaic impedance to present the deviated inductive loop.