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211.
Stacy S. Vander Pol John R. Kucklick Stefan D. Leigh Barbara J. Porter Michele M. Schantz 《Analytical and bioanalytical chemistry》2010,397(2):483-492
Toxaphene is a complex technical mixture that has been found ubiquitously in the environment but has caused issues for analysis,
especially of individual congeners. This paper reports the elution order of 26 major toxaphene congeners on three gas chromatographic
columns. The three different stationary phases generally had similar elution orders for the toxaphene congeners, but fewer
co-elutions occurred on a low-bleed, low-polarity column. These congeners (except for two that co-eluted and were not added
to the calibration mixture) were examined in air particulate matter standard reference materials (SRMs), 1648a, 1649a, and
1649b as well as SRM 3067 toxaphene in methanol for assignment of reference values. SRM 3067 had mass fractions an order of
magnitude greater than the air particulate SRMs, which ranged from 0.568 ± 0.018 ng g−1 dry mass (B9-2006 in SRM 1648a) to 12.9 ± 0.20 ng g−1 dry mass (B9-715 (P 58) in SRM 1649a). The three air particulate SRMs all had different mass fractions and proportions of
congeners relative to the sum of the toxaphene congeners. SRM 3067 may be useful as a technical mixture toxaphene congener
calibrant. SRMs 1648a and 1649b will serve as reference materials for the analysis of 21 (three congeners were not included
due to values below the detection limit or a potential polychlorinated biphenyl co-elution) toxaphene congeners in atmospheric
particulate samples. 相似文献
212.
The detonation properties of liquid nitromethane [CH(3)NO(2)] are probably the most thoroughly studied of any condensed-phase explosive. Because it is homogeneous (i.e., lacks hot-spot phenomena), it provides a window into the underlying chemical processes induced by a passing shock or detonation wave-such information is submerged in the complex fluid mechanics when heterogeneous explosives are detonated. In this paper, we provide experimental data and data analysis of the effect that deuterating nitromethane's methyl group has on some aspects of the processes that occur in the detonating liquid material. In the experimental part of this study, we report diameter-effect curves (i.e., inverse charge internal radius vs steady detonation speed) for pure CH(3)NO(2) and pure CD(3)NO(2) confined in right-circular cylinders of C-260 brass. Large differences in the infinite-medium (i.e., plane wave) detonation speed and in the failure diameter of the two materials are observed. Interpretations of the observations based on physical and chemical theory are given. The observed large decrease in deuterated nitromethane's infinite-medium detonation speed, relative to the protonated material, is interpreted in terms of the Zeldovitch, von Neumann, and Doering theory of steady-state detonation. We also estimate the relative size of the steady plane-wave reaction-zone length of the two materials. We interpret the observed increases in NM's failure diameter and its steady one-dimensional chemical-reaction-zone length due to deuteration in terms of the quantity of NM aci ion present. The new results are placed in the context of earlier work on detonating liquid nitromethane. 相似文献
213.
Reeder BJ Svistunenko DA Cooper CE Wilson MT 《Journal of the American Chemical Society》2012,134(18):7741-7749
Tyrosine residues can act as redox cofactors that provide an electron transfer ("hole-hopping") route that enhances the rate of ferryl heme iron reduction by externally added reductants, for example, ascorbate. Aplysia fasciata myoglobin, having no naturally occurring tyrosines but 15 phenylalanines that can be selectively mutated to tyrosine residues, provides an ideal protein with which to study such through-protein electron transfer pathways and ways to manipulate them. Two surface exposed phenylalanines that are close to the heme have been mutated to tyrosines (F42Y, F98Y). In both of these, the rate of ferryl heme reduction increased by up to 3 orders of magnitude. This result cannot be explained in terms of distance or redox potential change between donor and acceptor but indicates that tyrosines, by virtue of their ability to form radicals, act as redox cofactors in a new pathway. The mechanism is discussed in terms of the Marcus theory and the specific protonation/deprotonation states of the oxoferryl iron and tyrosine. Tyrosine radicals have been observed and quantified by EPR spectroscopy in both mutants, consistent with the proposed mechanism. The location of each radical is unambiguous and allows us to validate theoretical methods that assign radical location on the basis of EPR hyperfine structure. Mutation to tyrosine decreases the lipid peroxidase activity of this myoglobin in the presence of low concentrations of reductant, and the possibility of decreasing the intrinsic toxicity of hemoglobin by introduction of these pathways is discussed. 相似文献
214.
Synthesis of unstable 2-aminothiophene by way of its hydrochloride salt has been achieved in excellent yield. The key reaction in the scheme involves a novel heterocyclization of benzylthionitriles, which would appear to have general utility. Spectral and physical properties are tabulated and a possible mechanism is proposed. 相似文献
215.
In this paper, we present Raman spectra of ZrS2, HfS2, MoS2 and WS2 using laser energies near the energies of the absorption edges. The Raman spectra probe the properties of the first-excited electronic state and the nature of the electron-phonon coupling. The spectra of the IVB disulfides are independent of the laser excitation energy, suggesting weak electron-phonon interaction. In contrast, additional Raman bands appear in the spectra of the VIB disulfides as the laser energy approaches the band gap energy. The new modes in the spectra of MoS2 and WS2 cannot be assigned as first-order processes nor as combination bands of the phonons with zero momentum. The resonance Raman scattering of MoS2 is analyzed in terms of second-order scattering due to the coupling of phonon modes of nonzero momentum with an electronic transition associated with excitonic states. 相似文献
216.
217.
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219.
Michelle M. Meighan Michael W. Keebaugh Alicia M. Quihuis Stacy M. Kenyon Mark A. Hayes 《Electrophoresis》2009,30(21):3786-3792
A novel method capable of differentiating and concentrating small molecules in bulk solution termed “electrophoretic exclusion” is described and experimentally investigated. In this technique, the hydrodynamic flow of the system is countered by the electrophoretic velocity to prevent a species from entering into the channel. The separation can be controlled by changing the flow rate or applied electric field in order to exclude certain species selectively while allowing others to pass through the capillary. Proof of principle studies employed a flow injection regime of the method and examined the exclusion of Methyl Violet dye in the presence of a neutral species. Methyl Violet was concentrated almost 40 times the background concentration in 30 s using 6 kV. Additionally, a threshold voltage necessary for exclusion was determined. The establishment of a threshold voltage enabled the differentiation of two similar cationic species: Methyl Green and Neutral Red. 相似文献
220.
In our simplicial version of lattice gauge theory, we approximate Euclidean path integrals by tiling space-time with simplexes
and by linearly interpolating the fields throughout each simplex from their values at the vertices. We compare this method
with Wilson's lattice gauge theory forU(1) in three dimensions. As a standard of comparison, we compute the exact values of Creutz ratios of Wilson loops in the
continuum theory. Monte Carlo computations using the simplicial method give Creutz ratios within a few percent of the exact
values for reasonably sized loop atβ=1, 2, and 10. Similar computations using Wilson's method give ratios that typically differ from the exact values by factors
of 2 or more for 1⩽β⩽3.5 and that have the wrongβ dependence. The better accuracy of the simplicial method is due to its use of the action and domain of integration of the
exact theory, unaltered apart from the granularity of the simplicial lattice. We also present data on the action density and
the mass gap.
Research supported by the U.S. epartment of Energy under grant DE-FG04-84ER40166. 相似文献