Application of high-performance liquid chromatography with ultraviolet diode array detection and refractive index detection to the determination of class composition and to the analysis of gasoline

Two approaches based on sorptive extraction, solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE), in combination with liquid chromatography (LC)-atmospheric pressure chemical ionization mass spectrometry (MS) have been assayed for analyzing chlorpyriphos methyl, diazinon, fonofos, phenthoate, phosalone, and pirimiphos ethyl in honey. In both, SPME and SBSE, enrichment was performed using a poly(dimethylsiloxane) coating. Significant parameters affecting sorption process such as sample volume, sorption and desorption times, ionic strength, elution solvent, and dilution (water/honey) proportion were optimized and discussed. Performance of both methods has been compared through the determination of linearity, extraction efficiencies, and limits of quantification. Relative standard deviations for the studied compounds were from 3 to 10% by SPME and from 5 to 9% by SBSE. Both methods were linear in a range of at least two orders of magnitude, and the limits of quantification reached ranging from 0.04 to 0.4 mg kg(-1) by SBSE, and from 0.8 to 2 mg kg(-1) by SPME. The two procedures were applied for analyzing 15 commercial honeys of different botanical origin. SPME and SBSE in combination with LC-MS enabled a rapid and simple determination of organophosphorus pesticides in honey. SBSE showed higher concentration capability (large quantities of sample can be handled) and greater accuracy (between 5 and 20 times) and sensitivity (between 10 and 50 times) than SPME: thus, under equal conditions, SBSE is the recommended technique for pesticide analysis in honey.     

Ballistic transport in PbTe-based nanostructures

We describe our study of ballistic transport in nanostructures of lead telluride, PbTe. Submicron devices have been fabricated by electron beam lithography and chemical etching of 50 nm wide PbTe single quantum wells embedded between Pb_0.92Eu_0.08Te barriers grown by MBE on BaF₂. The electron concentration in the devices was tuned by the gate voltage applied across an interfacial p–n junction. The most important observation was zero-magnetic field conductance quantization (in multiplies of 2e²/h) in narrow constrictions of dimensions comparable to electron mean free path calculated from transport mobility. This indicates considerable relaxation of requirements for quantum ballistic transport in comparison with other materials. We argue that the huge static dielectric constant of PbTe (₀=1350 at 4.2 K) leads to suppression of the long-range Coulomb potentials of charged impurities and, thus, provides favorable conditions for the conductance quantization.     

Mass spectrometric study of 2-acyloxy-4,6-dimethoxy-1,3,5-triazines

Electron impact ionization mass spectra of 2-acyloxy-4,6-dimethoxy-1,3,5-triazines were examined. Two general pathways of fragmentation were found: (i) formation of an acylium ion typical for all the compounds studied, and (ii) rearrangement followed by loss of carbon monoxide, characteristic of derivatives of aromatic carboxylic acids.     

Compensating defect centres in semi-insulating 6H-SiC

Photoinduced transient spectroscopy (PITS) has been applied to study electronic properties of point defects associated with charge compensation in semi-insulating (SI) 6H-SiC substrates. The photocurrent relaxation waveforms were digitally recorded in a wide temperature range of 20–800 K and in order to extract the parameters of defect centres, a two-dimensional analysis of the waveforms as a function of time and temperature has been implemented. As a result, the processes of thermal emission of charge carriers from defect centres were seen on the spectral surface as the folds, whose ridgelines depicted the temperature dependences of emission rate for detected defect centres. The new approach was used to compare the defect levels in vanadium-doped and vanadium-free (undoped) SI 6H-SiC wafers.     

Electron-positron pair creation by powerful laser-ion interaction

Presently available high-power laser pulses of ponderomotive energy U _p ? 2mc ² should permit the fundamental processes of quantum electrodynamics in such fields, in particular, the formation of electron-positron pairs in impacts of laser pulses with highly charged ions, to be observed. We evaluate the highly nonlinear production rates of this process and investigate the most favorable conditions of pair production, in particular, either along the direction of linear polarization or in the propagation direction of the laser pulse. For femtosecond radiation pulses, it is possible to represent the laser beam by a monochromatic and linearly polarized electromagnetic plane wave. This approximation considerably simplifies the calculations required.     

Cryoprotection in plant tissues related to reduced volume expansion of cryoprotectant solution

To clarify the mechanism of reduced volume expansion-related cryoprotection changes in solution volume during freezing using several types of cryoprotectant were investigated. The effect of each cryoprotectant solution on the survival of asparagus nodal segments cooled slowly (0.5 degrees C/min) to -40 degrees C was also examined. The ratio of the volume at -40 degrees C to the volume at +20 degrees C was used as an index for expansion, calculated as a ratio of the density at +20 degrees C to the density at -40 degrees C. Distilled, deionized water showed the largest volume change at a ratio of 1.094. The ratio gradually decreased with an increase in the molar concentration of cryoprotectant, with the magnitude of the change dependent on the nature of the cryoprotectant. Raffinose was the most effective in reducing volume expansion when compared with other cryoprotectants at a same concentration. Raffinose exhibited greatest cryoprotection in asparagus tissue at 0.6 M where the solution became saturated. Dimethyl sulfoxide (Me2SO) at 1.69 M had the largest effect on cryoprotecting asparagus tissue. Furthermore, Me2SO was also the most effective in reducing volume expansion among the group of cryoprotectants permeable to the plasma membrane. It is concluded that cryoprotection in tissues was closely related to reduced volume expansion especially at low concentration (< or = 1.0 M). Cryoprotectants of impermeable sugar group lost their cryoprotective effect at > 1.0 M, which may due to severe dehydration and cell damage occurred in hypertonic solution. Useful cryoprotectants should be furnished with high ability of reducing volume expansion during freezing as well as low toxicity and high permeability for cells.     

Minimal supersymmetric standard model with the gauge mediated supersymmetry breaking and the neutrinoless double-beta decay

Neutrinoless double-beta decay within Minimal Supersymmetric Standard Model with gauge mediated supersymmetry breaking is considered. Limits on R-parity breaking constant coming from non-observability of 0 in ⁷⁶Ge are found. The dependence of on different parameters at the messenger scale M are shown, with special attention paid to nuclear part of calculations. We have found that strongly depends on the effective supersymmetry breaking scale only and deduced limits imposed on this non-standard parameter by the germanium experiment.     

Differential pulse adsorptive stripping voltammetric determination of dinoseb and dinoterb at a modified electrode.

A sensitive adsorptive stripping voltammetric method for the determination of dinitrophenolic herbicides, dinoseb (DSB) and dinoterb (DTB) at a bare carbon paste electrode (CPE) and a clay modified carbon paste electrode (CMCPE) was developed. A systematic study of various experimental conditions, such as the pH, accumulation variables and composition of a modifier on the adsorptive stripping response, were examined by using differential pulse voltammetry. A significant improvement was observed in the sensitivity by using the present method with CMCPE. When CMCPE was used, a linear response was obtained over the concentration range 2 x 10(-10) to 3 x 10(-7) M and 6 x 10(-10) to 6 x 10(-7) M with lower detection limits of 1 x 10(-10) M and 5.4 x 10(-10) M for dinoseb and dinoterb, respectively, at an accumulation time of 100 s. The interference from other herbicides and ions on the stripping signals of both compounds was also evaluated. The described method was applied to estimate of the dinoseb and dinoterb in environmental samples.     

Topological and metric product\mathbb{Z}^d -actions and their applications-actions and their applications

We show that for a -action Ψ being the Kronecker sum of a symbolic strictly ergodic -actionT and a Chacon -actionS, the rank (covering number) of Ψ is the same as that forT. Using this result we construct, for a given natural numberr≥2 and a real numberb∈(0,1) withr\b≥1, a -action with rankr, covering numberb and a simple spectrum. On the other hand, for any positive integersr, m with 1≤m≤r≤∞ we construct a -action with rankr and spectral multiplicitym.     

A highly selective catalytic method for the oxidative functionalization of C-H bonds

This communication describes a new and highly practical Pd(II)-catalyzed method for the regio- and chemoselective oxidative functionalization of arenes and alkanes. Carbon-hydrogen bonds of substrates that contain a variety of directing groups (e.g., pyridine, azobenzene, pyrazole, and imine derivatives) are selectively transformed into esters, ethers, and aryl-halides under mild conditions. The scope of this reaction in terms of substrate, directing group, and oxidant is described, and a preliminary catalytic cycle is proposed.