NMR and EPR studies of the reaction of nucleophilic addition of (bi)sulfite to the nitrone spin trap DMPO

The reaction of the nitrone spin trap 5,5‐dimethylpyrroline‐N‐oxide (DMPO) with sodium (bi)sulfite in aqueous solutions was investigated using NMR and EPR techniques. Reversible nucleophilic addition of (bi)sulfite anions to the double bond of DMPO was observed, resulting in the formation of the hydroxylamine derivative 1‐hydroxy‐5,5‐dimethylpyrrolidine‐2‐sulfonic acid, with characteristic ¹H and ¹³C NMR spectra. The reaction mechanism was suggested and corresponding equilibrium constants determined. The mild oxidation of the hydroxylamine results in the formation of an EPR‐detected spectrum identical with that for the DMPO adduct with sulfur trioxide anion radical. The latter demonstrates that a non‐radical addition reaction of (bi)sulfite with DMPO may contribute to the EPR detection of SO₃^?? radical. This possibility must be taken into account in spin trapping analysis of sulfite radical. Copyright © 2003 John Wiley & Sons, Ltd.     

Radiative lifetimes of excited W II levels

Radiative lifetimes of 19 selected W II levels with energies between 36 000 cm^-1 and 55 000 cm^-1 have been measured with the time-resolved laser-induced fluorescence technique. The ions are generated in a hollow cathode discharge and stored in a linear Paul trap. Selected states are populated with tunable dye laser pulses and the subsequent fluorescence is measured by means of a 5 Gigasample transient digitizer and a fast photodetector with a risetime of 700 ps. By taking into account both the temporal profile of the laser pulses and the separately measured response function of the system, the lifetime can be determined from the full decay curve. A refined evaluation procedure, taking into account saturation effects in the signals, reduces the uncertainty in our data to around 1%. Received: 30 July 1998 / Revised: 18 August 1998     

非简并Λ型三能级原子的速度选择相干布居俘获

研究了非简并Λ型三能级原子速度选择相干布居俘获与原子能级结构的非对称性以及原子-激光失谐之间的关系.指出相对于简并、共振情况，原子动量分布的概率峰峰值降低、最可几动量发生变化，而且原子的俘获时间也变得很长.     

Production of a bose einstein condensate of metastable helium atoms

We recently observed a Bose-Einstein condensate in a dilute gas of ⁴He in the 2³S₁ metastable state. In this article, we describe the successive experimental steps which led to the Bose-Einstein transition at 4.7 μK: loading of a large number of atoms in a MOT, efficient transfer into a magnetic Ioffé-Pritchard trap, and optimization of the evaporative cooling ramp. Quantitative measurements are also given for the rates of elastic and inelastic collisions, both above and below the transition. Received 15 October 2001     

Analysis of Trapping Force and Torque for Fiber-optical Trap by the Finite-differential Time-domain (FDTD) Method

In this paper,the FDTD method is used to calculate the light field distribution in an uncoated fiber probe with a small dielectric particle.By integrating Maxwell′s stress tensor in the FDTD calculation,the time-averaged force exerted on the dielectric particles with different sizes and at different positions is obtained.The calculation results are also verified experimentally.The force near the axis in the vicinity of the tip is about 10-14 N for the incident 10 mW laser of 980 nm.Furthermore,the torque exerting on an ellipsoidal dielectric particle by focusing light field with two fiber probes is calculated and the dependence of this torque on both the declination angle of the ellipsoidal particle and the incident laser power ratio of the two probes is obtained.The detailed intensity map within the particle is also given.     

Energy transfer in hyperthermal Xe-graphite surface scattering

The scattering of a hyperthermal Xe from a graphite (0001) surface has been studied using a molecular beam-surface scattering technique and molecular dynamics (MD) simulations. The angular and velocity distributions of scattered Xe atoms were measured at incidence energies from 0.45 to 3.5 eV, three incidence angles of 15°, 35° and 60° and the surface temperatures of 300 K and 550 K. The observed time-of-flight spectra exhibit a sharp velocity distribution with only one velocity component, which is ascribed to the direct inelastic scattering process. The angle-resolved energy ratios of the mean final translational energy over the mean incidence energy E_f/E_i agree well with those predicted based on the assumption of the conservation of the momentum parallel to the surface. The Hard-Cube model, where the mass of the cube is approximately 310 u, has reproduced the angle-resolved flux distributions of scattered Xe atoms. In the Hard-Cube model almost 80% of the normal component of the incidence translational energy is found to be lost in collision. The MD simulations reproduce well the experimental results by using the Brenner potential for intralayer C atoms and a Lennard-Jones potential for interlayer C–C pair interactions.     

Delay Model for a Cycling Transport Through Porous Medium

A solute transport through a porous medium is examined provided that the fluid leaving the porous sample returns back in a continuous way. The porous medium is thus included into a closed hydrodynamic circuit. This cycling process is suggested as an experimental tool to determine porous medium parameters describing transport. In the present paper the mathematical theory of this method is developed. For the advective type of transport with solute retention and degradation in porous medium, the system of transport equations in a closed circuit is transformed to a delay differential equation. The exact analytical solution to this equation is obtained. The solute concentration manifests both the oscillatory and monotonous behaviors depending on system parameters. The number of oscillation splashes is shown to be always finite. The maximum/minimum points are determined as solutions of a polynomial equation whose degree depends on the unknown solution itself. The cyclic methods to determine porous medium parameters as porosity and retention rate are developed.     

Bubble clustering and trapping in large vortices. Part 1: Triggered bubbly jets investigated by phase-averaging

The characteristics of large coherent structures in turbulent, periodically-excited, as well as naturally-developing bubbly jets are investigated, and interactions between bubbles and large-scale vortices are studied quantitatively.     

Halogen‐bonded adduct of 1,2‐dibromo‐1,1,2,2‐tetrafluoroethane and 1,4‐diazabicyclo[2.2.2]octane

Halogen bonding is an intermolecular interaction capable of being used to direct extended structures. Typical halogen‐bonding systems involve a noncovalent interaction between a Lewis base, such as an amine, as an acceptor and a halogen atom of a halofluorocarbon as a donor. Vapour‐phase diffusion of 1,4‐diazabicyclo[2.2.2]octane (DABCO) with 1,2‐dibromotetrafluoroethane results in crystals of the 1:1 adduct, C₂Br₂F₄·C₆H₁₂N₂, which crystallizes as an infinite one‐dimensional polymeric structure linked by intermolecular N...Br halogen bonds [2.829 (3) Å], which are 0.57 Å shorter than the sum of the van der Waals radii.     

Particulate capillary precolumns with double-end polymer monolithic frits for on-line peptide trapping and preconcentration

In this work,a novel kind of particulate capillary precolumns with double-end polymer monolithic frits has been developed.Firstly,the polymer monolithic frit at one end was prepared via photo-initiated polymerization of a mixture of lauryl methacrylate and ethyleneglycol dimethacrylate with 1-propanol and 1,4-butanediol as porogens and 2,2-dimethoxy-2-phenylacetophenone as a photo-initiator in UV transparent coating capillary(100 μm i.d.).Subsequently,C18 particles(5 μm,100 A) were packed into the capillary,and sealed with the polymer monolithic frit at another end.To prevent the reaction of monomers and C18 particles,the packed C18 particles were masked during UV exposure.The loading capacity of such a precolumn was determined to be about 9 μg by frontal analysis with a synthetic peptide APGDR1 YVHPF as a model sample.Furthermore,two parallel precolumns were incorporated into a two-dimensional nano-liquid chromatography(2D nano-LC) system with dual capillary trap columns for peptide trapping and concentration.Compared to 2D nano-LC system with a single trap column,such two dimensional separations could be operated simultaneously to improve the analysis throughput.All these results demonstrated that such capillary precolumns with double frits would be promising for high-throughput proteome analysis.