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A method utilizing solvent extraction and analysis by gas chromatography-positive chemical ionization mass spectrometry (SE-GC-PCIMS) was developed for the analysis of three neutral hydrophobic perfluorooctanesulfonamide compounds [perfluorooctanesulfonamide (PFOSA), N-ethyl perfluorooctanesulfonamide (N-EtPFOSA), and N,N-diethyl perfluorooctanesulfonamide (N,N-Et2PFOSA)]. These compounds are suspected metabolic precursors of perfluorooctane sulfonate. The SE-GC-PCI-MS method was used to analyze all three perfluorooctanesulfonamides in fast food, fish, and Arctic marine mammal liver samples. The SE-GC-PCI-MS method produced relatively higher recoveries of the analytes (averaging 83 +/- 6%, 84 +/- 9%, and 89 +/- 19% for N,N-Et2PFOSA, N-EtPFOSA, and PFOSA, respectively) with lower coefficients of variation, and less susceptibility to matrix effects, than ion pair extraction-liquid chromatography-tandem mass spectrometric methods. Method detection limits (MDLs) were 100, 120, and 250 pg/g for N,N-Et2PFOSA, N-EtPFOSA, and PFOSA, respectively. The three compounds were found at concentrations ranging from below the MDL to 22 ng/g wet weight in fast food, fish, and Arctic marine mammal liver samples. 相似文献
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The responsivity at a constant detection area of non-steady-state photoinduced electromotive force (photo-emf) detectors is improved by a factor equal to the number of contact pairs contained in asymmetric interdigitated surface contacts. The polar nature of photo-emf current generation requires contact asymmetry in which one increases the total signal by blocking the illumination between alternate contact pairs, in distinct contrast to the behavior of conventional interdigitated contacts fabricated upon isotropic photoconductors. 相似文献
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Melanie J. M. Pepper Isaiah Shavitt Paul Von Ragu Schleyer Mikhail N. Glukhovtsev Rudolf Janoschek Martin Quack 《Journal of computational chemistry》1995,16(2):207-225
Large basis set ab initio calculations at correlated levels, including MP2, single reference, as well as multireference configuration interaction, carried out on the methane potential energy surface, have located and characterized a transition structure for stereomutation (one imaginary frequency). This structure is best described as a pyramidal complex between singlet methylene and a side-on hydrogen molecule with Cs symmetry. At the single reference CI level, it lies 105 kcal/mol above the methane Td-ground state but is stable relative to dissociation into CH2(1A1) and H2 by 13 kcal/mol at 0 K (with harmonic zero point energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endothermic (by 4 kcal/mol), but single bond rupture to give CH and H. is 3 kcal/mol exothermic. Thus, it does not appear likely that methane can undergo stereomutation classically beneath the dissociation limit. Confirming earlier conclusions, side-on insertion of 1A1 CH2 into H2 in a perpendicular geometry occurs without activation energy. Planar (D4h) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C2v structures with one three-center, two-electron bond and two two-electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D4h form lead to tetrahedral (Td) and pyramidal (C4v) methane. The latter has three negative eigenvalues in the force-constant matrix; one of these leads to the Td global minimum and the other to the Cs (parallel) stereomutation transition structure. Multireference CI calculations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the Cs stereomutation transition state 0.7 ± 0.5 kcal/mol higher than that of CH + H. dissociation products, and a ZPE-corrected energy which is 5 ± 1 kcal/mol higher. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the parity-violating weak nuclear interaction on the possibility of an experimental detection of stereomutation in methane. © 1995 by John Wiley & Sons, Inc. 相似文献
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Diesters obtained from diacids produced by oxidative ring cleavage of cycloadducts derived from acyl-nitroso compounds and cyclic 1,3-dienes undergo highly regioselective hydrolysis on reaction with lithium hydroperoxide, which allows for easy differentiation of the carboxyl groups leading to a new approach to polyoxamic acid. 相似文献
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MA Freitas SR Dillon RC Dougherty AG Marshall 《Rapid communications in mass spectrometry : RCM》1999,13(15):1622-1625
The self-chemical ionization of diethylzinc is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and semiempirical molecular orbital calculations. Electron impact of diethylzinc neutral produces the radical cation, C(4)H(15)Zn(+) (m/z x 122), which reacts further with the neutral (C(2)H(5))(2)Zn to give the following product ions: Zn(+) (m/z x 64), C(2)H(5)Zn(+) (m/z x 93), C(4)H(9)Zn(+) (m/z x 121), C(4)H(11)Zn(2)(+) (m/z x 187), and C(6)H(15)Zn(2)(+) (m/z x 215). To determine the structure and pathways for production of these ions, monoisotopic (12)C(4)H(15)(64)Zn(+), (64)Zn(+) and (12)C(2)H(5)(64)Zn(+) were individually isolated and reacted with the neutral background. We also performed semiempirical molecular orbital calculations (ZINDO/1). The molecular orbital calculations and experimental data are consistent in predicting that the ethyl group on the diethylzinc cation carries the positive charge. Copyright 1999 John Wiley & Sons, Ltd. 相似文献
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Ghosh A Wright MH Siegert C Pepper M Farrer I Ford CJ Ritchie DA 《Physical review letters》2005,95(6):066603
Nonequilibrium transport measurements in mesoscopic quasiballistic 2D electron systems show an enhancement in the differential conductance around the Fermi energy. At very low temperatures, such a zero-bias anomaly splits, leading to a suppression of linear transport at low energies. We also observed a scaling of the nonequilibrium characteristics at low energies which resembles electron scattering by two-state systems, addressed in the framework of two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an intrinsic phenomenon in unconfined 2D systems in the low electron-density regime. 相似文献