Thermal lens spectrometric determination of hexavalent chromium

The applicability of thermal lens spectrometry (TLS) for quantification and routine determination of hexavalent chromium was investigated by using a collinear dual beam thermal lens spectrometer. In aqueous solutions the LOD of 0.1 μg l⁻¹ was obtained for Cr(VI) by using 160 mW laser power. The performance of the technique was verified by the determination of hexavalent chromium in standard reference water samples (NIST SRM 1643a and NIST SRM 1643c) and comparing the results for Cr(VI) in CCA (Cr, Cu, As)-treated timber extracts to concentrations obtained by atomic absorption spectrometry (AAS). Good agreement between the TLS results and reported values for Cr(VI) in SRMs as well as AAS results for Cr(VI) in CCA-treated timbers confirmed that TLS is a reliable and accurate analytical technique applicable for the determination of Cr(VI) in aqueous solutions at concentration levels 0.5–100 μg l⁻¹.     

Studies on the extraction of chromium(VI) by ketones

A study of the extraction of chromium(VI) from aqueous media by ketones was made. Extraction of chromium was found to be most efficient from aqueous hydrochloric acid solutions. A mechanism for the extraction of chromium(VI) from aqueous hydrochloric acid solutions by methyl isobutyl ketone is proposed involving the formation of a receptor in the organic phase, the exchange of the chloride ion of the receptor for the anionic chromium(VI) species of the aqueous phase, and the solvation of the extracted chromium species. The differences in the abilities of various ketones to extract chromium(VI) from aqueous hydrochloric acid solutions, and the differences in the extraction of chromium (VI) from various aqueous acids by methyl isobutyl ketone are attributed to the differences in the formation of receptors.     

Interaction of polyvinylpyrrolidone and iodine

A nonsolvent preparation of the polyvinylpyrrolidone (PVP) complex provides new insight into the nature of the polymer-iodine interaction. The preparation is obtained by simply mixing PVP with crystalline iodine and is of interest because it provides a system in which no interfering ions are present and only one type of iodine is initially present. The iodine is shown to undergo hydrolysis with moisture in the polymer to give iodide and hypoiodite. The ionic forms of iodine appear to associate with the molecular iodine, resulting in the final stable PVP-iodine complex.     

Extremes of censored and uncensored lifetimes in survival data

Extremes - We consider a random censoring model for survival analysis, allowing the possibility that only a proportion of individuals in the population are susceptible to death or failure, and the...     

Immobilized Glucose Oxidase in the Potentiometric Detection of Glucose

Previous work has shown that glucose oxidase can be immobilized on platinum to give an electrode that responds potentiometrically to glucose over the clinically useful range of about 10-250 mg glucose/100 mL. The present studies were carried out with electrochemically pretreated platinum and with gold or porous graphite substituted for the platinum support. The presence of the enzyme gave a significantly enhanced potentiometric response over that obtained with the bare support for both the pretreated platinum and the porous graphite, but not with gold. However, with platinum the potentiometric response became more negative with increasing glucose concentration. With porous graphite, the potential changed in the positive direction as the glucose concentration was increased. Hysteresis was demonstrated for the platinum-enzyme electrode. Mass transfer measurements with a rotating ring-disc electrode (RRDE) showed measurable diffusional resistances to the transport of a model electroactive compound (potassium ferrocyanide) through a matrix of immobilized enzyme attached to the disc of the RRDE. These results are part of a larger study to define the source of the potentiometric response by examining the roles of the support and the mass transfer resistances through the immobilized enzyme matrix.     

Norms on Free Topological Groups

A norm on a group G is a function N mapping G into the set ofnon-negative real numbers such that for each x and y in G, N(xy^–1) N(x)+N(y) and N(e) = 0, where e is the identity element ofG. It is shown here that if F(X) is the free topological groupon any completely regular Hausdorff space X and H is a subgroupof F(X) generated by a finite subset of X, then any norm onH can be extended to a continuous norm on F(X).     

Length dependent folding kinetics of phenylacetylene oligomers: structural characterization of a kinetic trap

Using simulation to study the folding kinetics of 20-mer poly-phenylacetylene (pPA) oligomers, we find a long time scale trapped kinetic phase in the cumulative folding time distribution. This is demonstrated using molecular dynamics to simulate an ensemble of over 100 folding trajectories. The simulation data are fit to a four-state kinetic model which includes the typical folded and unfolded states, along with an intermediate state, and most surprisingly, a kinetically trapped state. Topologically diverse conformations reminiscent of alpha helices, beta turns, and sheets in proteins are observed, along with unique structures in the form of knots. The nonhelical conformations are implicated, on the basis of structural correlations to kinetic parameters, to contribute to the trapped kinetic behavior. The strong solvophobic forces which mediate the folding process and produce a stable helical folded state also serve to overstabilize the nonhelical conformations, ultimately trapping them. From our simulations, the folding time is predicted to be on the order of 2.5-12.5 mus in the presence of the trapped kinetic phase. The folding mechanism for these 20-mer chains is compared with the previously reported folding mechanism for the pPA 12-mer chains. A linear scaling relationship between the chain length and the mean first passage time is predicted in the absence of the trapped kinetic phase. We discuss the major implications of this discovery in the design of self-assembling nanostructures.     

Thiophene systems. 7. Pyrido[3,2-b]thieno[3,4-e][1,4]diazepine derivatives with potential cns activity

Pyrido[3,2-b]thieno[3,4-e][1,4]diazepines ( 1a-d ) were synthesized to investigate their potential CNS activity. Synthesis of the desired ring system was effected by condensation of 2,3-diaminopyridine ( 3 ) with methyl tetrahydro-4-oxo-3-thiophenecarboxylate ( 4 ). Structural assignment of the major condensation product 5 was determined by comparison of ¹H nmr absorptions of 5 with those of related methyl lactam derivatives 11 and 14. A discussion of the possible mechanism leading to 5 in preference to isomeric lactam 6 is presented. Biological evaluation of 1a-d revealed no interesting properties.     

Foldamer simulations: novel computational methods and applications to poly-phenylacetylene oligomers

We apply several methods to probe the ensemble kinetic and structural properties of a model system of poly-phenylacetylene (pPA) oligomer folding trajectories. The kinetic methods employed included a brute force accounting of conformations, a Markovian state matrix method, and a nonlinear least squares fit to a minimalist kinetic model used to extract the folding time. Each method gave similar measures for the folding time of the 12-mer chain, calculated to be on the order of 7 ns for the complete folding of the chain from an extended conformation. Utilizing both a linear and a nonlinear scaling relationship between the viscosity and the folding time to correct for a low simulation viscosity, we obtain an upper and a lower bound for the approximate folding time within the range 70 ns相似文献   

Quantum hair and quantum gravity

A black hole may carry quantum numbers that arenot associated with massless gauge fields, contrary to the spirit of the no-hair theorems. The quantum hair is invisible in the classical limit, but measurable via quantum interference experiments. Quantum hair alters the temperature of the radiation emitted by a black hole. It also induces non-zero expectation values for fields outside the event horizon; these expectation values are non-perturbative in , and decay exponentially far from the hole. The existence of quantum hair demonstrates that a black hole can have an intricate quantum-mechanical structure that is completely missed by standard semiclassical theory.Based on an essay which received second award from the Gravity Research Foundation, 1991