Comparison of analytical strategies for the chromatographic and mass spectrometric measurement of brominated flame retardants: 1. Polybrominated diphenylethers

The chromatographic and mass spectrometric (MS) behaviors of 49 polybrominated diphenylether (PBDE) homologues toward various techniques is investigated. Special attention is paid to chromatographic separation, ionization processes, and signal acquisition modes. Different liquid chromatographic (LC) separation systems and gas chromatographic (GC) temperature program parameters are studied. For LC-MS experiments, the ionization efficiencies of electrospray, atmospheric pressure chemical ionization, and atmospheric pressure photoionization (APPI) are evaluated. For GC-MS experiments, negative chemical ionization with ammonia as reagent gas as well as negative and positive electron impact (EI) ionization are studied. Thus, fragmentation pathways of PBDEs are investigated, with the main objective being to determine the sensitivity/specificity balance of each tested technique with respect to their potential respective application (parent compound focusing, metabolite identification, and screening of analogue compounds). Finally, performances of the different tested techniques are compared and evaluated in terms of detection limits on standard solutions for each homologue group. In terms of ionization, EI remains the best compromise between sensitivity and specificity with possible complementary applications in MS-MS and high-resolution MS. Nevertheless, APPI appears to be a promising alternative.     

Heck, direct arylation, and hydrogenation: two or three sequential reactions from a single catalyst

Palladium-catalyzed tandem multifunctional reactions leading to the synthesis of substituted biaryl molecules have been developed including tandem Heck-direct arylation and tandem-sequential Heck-direct arylation-hydrogenation. These reactions occur in good yield and have been employed in the synthesis of a cytotoxic biaryl compound.     

Sub-ppm multi-gas photoacoustic sensor

A photoacoustic multi-gas sensor using tuneable laser diodes in the near-infrared region is reported. An optimized resonant configuration based on an acoustic longitudinal mode is described. Automatic tracking of the acoustic resonance frequency using a piezo-electric transducer and a servo electronics is demonstrated. Water vapour, methane and hydrogen chloride have been measured at sub-ppm level in different buffer gas mixtures. The importance of the system calibration in presence of several diluting gases is discussed. Finally, trace gas measurements have been assessed and detection limits (signal-to-noise ratio=3) of 80 ppb at 1651.0 nm for CH(4), 24 ppb at 1368.6 nm for H(2)O and 30 ppb at 1737.9 for HCl have been demonstrated.     

Analytic estimates and rigorous continuation for equilibria of higher-dimensional PDEs

In this paper we extend the ideas of the so-called validated continuation technique to the context of rigorously proving the existence of equilibria for partial differential equations defined on higher-dimensional spatial domains. For that effect we present a new set of general analytic estimates. These estimates are valid for any dimension and are used, together with rigorous computations, to construct a finite number of radii polynomials. These polynomials provide a computationally efficient method to prove, via a contraction argument, the existence and local uniqueness of solutions for a rather large class of nonlinear problems. We apply this technique to prove existence and local uniqueness of equilibrium solutions for the Cahn-Hilliard and the Swift-Hohenberg equations defined on two- and three-dimensional spatial domains.     

A characterization of semisimple plane polynomial automorphisms

It is well-known that an element of the linear group is semisimple if and only if its conjugacy class is Zariski closed. The aim of this paper is to show that the same result holds for the group of complex plane polynomial automorphisms.     

First synthesis of 2-aminosubstituted-2-perfluoroalkyl-3,6-dihydro-2H-thiopyrans by hetero-Diels-Alder reactions of fluorinated thioamides under microwave heating

This Letter presents the first examples of hetero-Diels-Alder reactions of polyfluoroalkanethiocarboxylic acid amides and 2,3-dimethylbutadiene under microwave heating. Cycloaddition reactions proved to be dependent on the nature of perfluoroalkyl chain and on the substituents attached to the nitrogen atom. Formation of ammonium salts was also performed by simple treatment of the corresponding cycloadducts with trifluoromethanesulfonic acid. In the case of octafluorobutyl-substituted derivative, one spontaneous desamination reaction took place leading to new 2H-thiopyran.     

Creating carbon-carbon bonds with samarium diiodide for the synthesis of modified amino acids and peptides

In this perspective, an overview of our experiences on the application of samarium diiodide in organic synthesis for the preparation of amino acid and peptide analogues is presented. Three different carbon-carbon bond forming reactions are discussed, including side chain introductions, gamma-amino acid synthesis and acyl-like radical additions for the construction of C-C mimics of the peptidic bonds.     

Probing microstructure evolution during the hardening of gypsum by proton NMR relaxometry

We report a comprehensive proton NMR relaxation study of the water confined in the evolving porous structure of hardened gypsum prepared with different water-to-plaster ratios (w/p) and increasing additions of crushed gypsum. This study gives some new information on the microstructure, the water distribution, and the hydration kinetics without any drying or perturbing preparation. The bi-exponential transverse magnetization decay reveals the existence of two water populations in slow exchange. However, the different behaviors of these populations during saturation and desaturation experiments show evidence of a fast exchange of each population with the surface. Two modes of organization of the microstructure of this material are identified through an original model of exchange as a function of the water-to-plaster ratio (0.4 < or = w/p < or = 0.6 and 0.7 < or = w/p < or = 1). A clear gap is shown in the exchange rate value above w/p = 0.6 that could be representative of a percolation threshold. Both the method and the theory presented can be applied more widely to other porous media with reactive surface areas.