Diffusion of Cesium Ions in Agar Gel Containing Alkali Metal Bromides

The present paper gives an account of different aspects of the tracer diffusion of Cs⁺ ions in alkali metal bromides. We have measured the diffusion coefficients, D, of cesium ions in 1% agar gel medium at 25 ^∘C using a zone-diffusion technique over a concentration range of 5 × 10⁻⁵ to 0.1 mol,dm⁻³. The values of the diffusion coefficients were found to deviate from theory, which are explained on the basis of different types of interactions occurring in the ion-gel-water system. The study is also focused on the effect of alkali metal bromides on the obstruction effect and activation energy for the tracer-diffusion of cesium ions in agar gel medium. It is observed that both parameters, extent of obstruction, ∝, and activation energy, E, decrease with increasing charge density of the cation of the supporting electrolyte. The influence of these trends is explained on the basis of competitive hydration between the ions and agar molecules, and the relative distortion in the water structure that is brought about by these different ions and agar molecules.     

用于示踪分析的固体汞合金电极线性扫描伏安法测痕量钼

研究了可用于示踪分析测定痕量钼的固体银汞合金电极催化伏安法.在H2SO4-苯羟乙酸-氯酸钾体系中,采用该电极于-0.165V(vs.SCE)处出现灵敏的还原峰,Mo的质量浓度在0～200 ng/mL范围内与峰高呈良好线性关系,检出限为0.42 ng/mL.该方法操作简便、快速,同时又可避免传统汞电极使用中带来的不便和毒性.对实际水样进行了测定.     

Analytical Treatment of a Push–Pull “Echo” Test

A semi-analytical streamline-based model, employing stratification and macro physics only, is developed and utilized to simulate injection/production phases of single-well push–pull tests. Modeling results are compared with experimental field data, giving an excellent match, without resorting to parameter fitting, simply by putting in known test-site properties, such as stratification data, hydraulic head gradients, and test parameters.     

Deconvolution with simple extrapolation for improved cerebral blood flow measurement in dynamic susceptibility contrast magnetic resonance imaging during acute ischemic stroke

Magnetic resonance (MR) perfusion imaging is a clinical technique for measuring brain blood flow parameters during stroke and other ischemic events. Ischemia in brain tissue can be difficult to accurately measure or visualize when using MR-derived cerebral blood flow (CBF) maps. The deconvolution techniques used to estimate flow can introduce a mean transit time-dependent bias following application of noise stabilization techniques. The underestimation of the CBF values, greatest in normal tissues, causes a decrease in the image contrast observed in CBF maps between normally perfused and ischemic tissues; resulting in ischemic areas becoming less conspicuous. Through application of the proposed simple extrapolation technique, CBF biases are reduced when missing high-frequency signal components in the MR data removed during deconvolution noise stabilization are restored. The extrapolation approach was compared with other methods and showed a statistically significant increase in image contrast in CBF maps between normal and ischemic tissues for white matter (P<.05) and performed better than most other methods for gray matter. Receiver operator characteristic curve analysis demonstrated that extrapolated CBF maps better-detected penumbral regions. Extrapolated CBF maps provided more accurate CBF estimates in simulations, suggesting that the approach may provide a better prediction of outcome in the absence of treatment.     

GC-R-CF-MS method to determine the 13C abundance of gaseous combustion products in cigarette smoke

To determine the ¹³C abundance of combustion and break down products formed in cigarette smoke, especially CO and CO₂, a simple and fast analytical method is needed. Taking into account the knowledge about the determination of the natural ¹³C abundance in air, an online method—based on gas chromatography-reaction-continuous flow mass spectrometry (GC-R-CF-MS)—has been developed, which enables the determination of the ¹³C abundance of CO and CO₂ in the vapour phase of cigarette smoke with a relative standard deviation of≤0.5% in one analytical run. Additionally, in a second step, the ¹³C abundance of total volatile carbon can be determined.     

Tracer diffusion in glassforming liquids

In the last decades, a wide collection of experimental evidence has been found in the study of supercooled glassformers on the existence of a crossover between two dynamical regimes at a temperature T_c. We discuss the validity of the Vogel-Fulcher-Tammann in both regions. The breakdown of the Stokes-Einstein relation below T_c is presented, indicating that the diffusion coefficient of a tracer becomes decoupled from the viscosity through an exponent ξ, and the diffusion process is intensified. We verify that a temperature shift on the diffusion coefficient introduces the same effect as the Stokes-Einstein breakdown equation. We present the dependence of this exponent on the ratio between the radii of the tracer and the host liquid molecule.     

Development of a method for assessing the relative contribution of waterborne and dietary exposure to zinc bioaccumulation in <Emphasis Type="Italic">Daphnia magna</Emphasis> by using isotopically enriched tracers and ICP–MS detection

In order to study the effect of anthropogenic substances on freshwater and marine ecosystems and to develop methods to derive water-quality criteria, ecotoxicological testing is required. While toxicity assessments are traditionally based on dissolved metal concentrations, assuming that toxicity is caused by waterborne metal only, it was recently pointed out that also the dietary exposure route should be carefully considered and interpreted in regulatory assessments of zinc. In this context, the aim of this experimental study was to develop a method which allows the uptake of waterborne and dietary zinc by Daphnia magna and the interaction between both exposure routes to be studied. Therefore, the setup of a dual isotopic tracer study was required. During several days, daphnids were exposed to ⁶⁷Zn and ⁶⁸Zn via the dietary and the waterborne routes, respectively, and after several time intervals the daphnids were sampled and subjected to isotopic analysis by means of inductively coupled plasma mass spectrometry (ICP–MS). In order to obtain reliable and accurate results for zinc, special care was taken to prevent contamination and to deal with the spectral interferences traditionally hindering the determination of zinc. The figures of merit of both a quadrupole-based ICP–MS instrument equipped with a dynamic reaction cell, and a sector field ICP–MS unit were studied, and it was concluded that by using a sector field mass spectrometer operated at medium mass resolution all interferences could be overcome adequately. Although the set-up of the exposure experiments seems to be rather simple at first sight, it was shown in this work that several (dynamic) variables can have an influence on the results obtained and on the subsequent data interpretation. The importance of these confounding factors was examined, and on the basis of preliminary calculations it became clear that not only the isotopic composition of the daphnids has to be studied—adequate monitoring of the isotopic composition of the dissolved phase and the algae during the exposure of the daphnids is also required to accurately discriminate between uptake from water and from food. Lieve I.L. Balcaen and Karel A.C. De Schamphelaere contributed equally to this study     

A solids mixing rate correlation for small scale fluidized beds

A new first degree solids mixing rate is proposed to evaluate the mixing of solids in small scale fluidized beds. Particle mixing experiments were carried out in a 2D fluidized bed with a cross-section of 0.02 m × 0.2 m and a height of 1 m. White and black particles with average diameters of 850 and 450 μm were used in our experiments. Image processing was used to measure the concentration of the tracers at different times. The effects of four representative operating parameters (superficial gas velocity, ratio of tracer particles to bed particles, tracer particle position, and particle size) on mixing are discussed with reference to the mixing index. We found that the Lacey index depends on the concentration of the tracers. The position of the tracers affects the initial mixing rate but not the final degree of mixing. However, the new mixing rate equation does not depend on the initial configuration of the particles because this situation is considered to be the initial condition. Using the data obtained in this work and that found in literature, an empirical correlation is proposed to evaluate the mixing rate constant as a function of dimensionless numbers (Archimedes, Reynolds, and Froude) in small scale fluidized beds. This correlation allows for an estimation of the mixing rate under different operating conditions and for the detection of the end point and/or the time of mixing.     

Lysozyme as diffusion tracer for measuring aqueous solution viscosity

Measuring tracer diffusion provides a convenient approach for monitoring local changes in solution viscosity or for determining viscosity changes in response to multiple solution parameters including pH, temperature, salt concentrations or salt types. One common limitation of tracer diffusion in biologically relevant saline solutions is the loss of colloidal stability and aggregation of the tracer particles with increasing ionic strength. Using dynamic light scattering to measure tracer diffusion, we compared the performance of two different types of tracer particles, polystyrene nanobeads vs. the small protein lysozyme, for viscosity measurements of saline solutions. Polystyrene beads provide reliable values for water viscosity, but begin flocculating at ionic strengths exceeding about 100 mM. Using lysozyme, in contrast, we could map out viscosity changes of saline solutions for a variety of different salts, for salt concentrations up to 1 M, over a wide range of pH values, and over the temperature range most relevant for biological systems (5–40 °C). Due to its inherently high structural and colloidal stability, lysozyme provides a convenient and reliable tracer particle for all these measurements, and its use can be readily extended to other optical approaches towards localized measurements of tracer diffusion such as fluorescence correlation spectroscopy.     

Positron emission particle tracking——Application and labelling techniques

The positron emission particle tracking （PEPT） technique has been widely used in science and engineering to obtain detailed information on the motion and flow fields of fluids or granular materials in multiphase systems, for example, fluids in rock cracks, chemical reactors and food processors; dynamic behaviour of granular materials in chemical reactors, granulators, mixers, dryers, rotating kilns and ball mills. The information obtained by the PEPT technique can be used to optimise the design, operational conditions for a wide range of industrial process systems, and to evaluate modelling work. The technique is based on tracking radioactively labelled particles （up to three particles） by detecting the pairs of backto-back 511 keV γ-rays arising from annihilation of emitted positrons. It therefore involves a positron camera, location algorithms for calculating the tracer location and speed, and tracer labelling techniques. This paper will review the particle tracking technique from tracking algorithm, tracer labelling to their application.