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41.
The methylene blue method has been widely used for analysis of sulfide for more than 100 years. Direct measurement of methylene
blue at nanomolar concentrations is impossible without a preconcentration step, however. In this study the response of LC–MS
with electrospray ionization (ESI) to methylene blue was evaluated. HPLC with simple isocratic elution was followed by ESI-MS
quantification, which was compared with traditional UV–visible detection. The limit of detection for sulfide was approximately
50 ng L−1, or 1.5 nmol L−1. Analysis time was substantially reduced by use of isocratic elution. Interfering compounds produced by side reactions can
be eliminated by use of the mass filter. A polysulfide sample was also analyzed to determine which products are formed and
whether or not polysulfides react stoichiometrically with methylene blue reagent. It seems that polysulfides do not react
quantitatively with methylene blue and so cannot be quantified reliably by use of this method. 相似文献
42.
We develop a correction for the coupled cluster version of the perfect pairing (PP) model. The correction is based on finding modified values of the PP amplitudes such that the second coupled cluster central moment defined in the space of all valence single and double substitutions vanishes and, subject to this constraint, minimizing the deviation between the modified and unmodified PP amplitudes with respect to a chosen metric. We discuss how this correction can be generalized to other constrained doubles models, such as local correlation and active-space models. While the correction is not strictly size consistent and retains some of the deficiencies of the PP model, numerical results indicate that much of the missing active-space coupled cluster singles and doubles correlation energy is recovered. 相似文献
43.
Schmiege BM Carney MJ Small BL Gerlach DL Halfen JA 《Dalton transactions (Cambridge, England : 2003)》2007,(24):2547-2562
This report describes the synthesis and characterization of metal halide complexes (M = Mn, Fe, Co) supported by a new family of pendant donor-modified alpha-diimine ligands. The donor (N, O, P, S) substituent is linked to the alpha-diimine by a short hydrocarbon spacer forming a tridentate, mer-coordinating ligand structure. The tridentate ligands are assembled from monoimine precursors, the latter being synthesized by selective reaction with one carbonyl group of the alpha-dione. While attempts to separately isolate tridentate ligands in pure form were unsuccessful, metal complexes supported by the tridentate ligand are readily synthesized in-situ, by forming the ligand in the presence of the metal halide, resulting in a metal complex which subsequently crystallizes out of the reaction mixture. Metal complexes with NNN, NNO, NNP and NNS donor sets have been prepared and examples supported by NNN, NNP and NNS ligands have been structurally characterized. In the solid state, NNN and NNP ligands coordinate in a mer fashion and the metal complexes possess distorted square pyramidal structures and high spin (S = 2) electronic configurations. Compounds with NNS coordination environments display a variety of solid state structures, ranging from those with unbound sulfur atoms, including chloride bridged and solvent ligated species, to those with sulfur weakly bound to the metal center. The extent of sulfur ligation depends on the donor ability of the crystallization solvent and the substitution pattern of the arylthioether substituent. 相似文献
44.
We explore the impact of awareness on epidemic spreading through a population represented by a scale-free network. Using a network mean-field approach, a mathematical model for epidemic spreading with awareness reactions is proposed and analyzed. We focus on the role of three forms of awareness including local, global, and contact awareness. By theoretical analysis and simulation, we show that the global awareness cannot decrease the likelihood of an epidemic outbreak while both the local awareness and the contact awareness can. Also, the influence degree of the local awareness on disease dynamics is closely related with the contact awareness. 相似文献
45.
A common task in microscopy is to fit an image of a fluorescent probe to a point spread function (PSF) in order to estimate the position of the probe. The PSF is often approximated as a Gaussian for mathematical simplicity. We show that the separable property of the Gaussian PSF enables a reduction of computational time from O(L2) to O(L), where L is the width (in pixels) of the image. When tested on realistic simulated data, our algorithm is able to localize the probes with precision close to the Cramér-Rao lower bound. 相似文献
46.
The complexes Ga2X4 · 2L (L = pyridine, 3-methylpyridine, 4-methylpyridine, morpholine, 1,4-thioxane, 1,4-dithiane, tetrahydropyran, tetrahydrofuran, tetrahydrothiophene and dimethylsulphide) have been prepared. Vibrational spectra indicate that they all contain GaGa bonds and this is confirmed for Ga2Cl4 · 2pyridine by a crystal structure determination which shows it to be isostructural with the bromide analogue. We were unable to synthesize complexes of stoichiometry Ga2X4 · 4L which had previously been reported. 相似文献
47.
Christopher G. Small 《Probability Theory and Related Fields》1983,63(4):517-527
Summary G.W. Brown, Yu.V. Prokhorov, L. Bondesson and others have developed characterization results for maximal invariant statistics. Such results show that it is possible to construct tests to distinguish between types of distributions. In this paper, these results are proved for a large number of cases that arise in particular in spatial statistics and directional statistics. The class of n-collected distributions, n3, is introduced. It is proved that if a distribution is n-collected then a sample of size n has sufficient information to distinguish the type of the distribution. In other words, the distribution of the maximal invariant characterizes type. 相似文献
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Quinidine, a useful antiarrhythmic compound, is usually contaminated with dihydroquinidine, a compound that itself shows potent antiarrhythmic activity. Complete hydrogenation of quinidine followed by conversion to dihydroquinidine derivatives was explored as a basis for eliminating the analytical problems inherent in the quality control of quinidine products and for determining their pharmacological potency and pharmacokinetic parameters without interfering impurities. Attempts to resolve the quinidine analogs, dihydrocupreidine, and its benzoyloxy ester failed with normal and reversed-phase chromatography. Ion-pairing chromatography using n-octanesulfonate in methanol:water proved successful. Using 9-hydroxy-4-methoxy acridine as internal standard, separation and quantitation of the dihydroquinidine analogs from spiked plasma samples was achieved with 92 to 95% efficiency. 相似文献