A method for self-attenuation and sample-height correction for counting efficiency of HPGe using Marinelli beaker geometry

A procedure for self-attenuation and sample height correction in HPGe gamma spectrometry efficiency has been presented. An MCNP model of an HPGe detector was used to calculate the full energy peak efficiency (FEPE) for a group of different samples with different heights in Marinelli beaker geometry. A proper function has been fitted to the simulation results to obtain the correction function. The function has been used to calculate the FEPE of a spiked soil sample in different sample heights by considering the experimentally known FEPE of another standard solution source. A good agreement between the experiments and calculations have been shown.     

A theoretical and experimental investigation of the source-sample-detector geometry for an annular type radioisotope excited XRF spectrometer

In this study, the dependence of the characteristic X-ray intensities on the counting geometry has been investigated for a radioisotope excited XRF spectrometer. The collimation factor for the source-sample-detector geometry, which was prepared for an annular type¹⁰⁹Cd radioisotope source, has been determined both theoretically and experimentally. The discrepancy between the theoretical and experimental results is discussed in terms of possible sources of errors.     

Scale-down of continuous protein purification by annular chromatography. Design parameters for the smallest unit

Theoretically, in fixed-bed chromatography an infinite separation time or volume is available. In annular chromatography the separation time is limited by the time required for a single rotation. In a chromatograph with a smaller annulus than the theoretical one, the equilibration zone will overtake the feed zone. The required separation time is determined by a number of steps such as equilibration, loading, washing, elution, regeneration and the retention of the solute. A mathematical model has been developed to estimate the minimum radius of an annular chromatograph. The minimal geometry of an annular chromatograph was calculated for a model system immunoglobulin and bovine serum albumin.     

Role of geometrical dimensions in electrophoresis applications with orthogonal fields

The role of geometrical dimensions in electrophoresis applications with axial and orthogonal (secondary) electric fields is investigated using a rectangular capillary channel. In particular, the role of the applied orthogonal electrical field in controlling key parameters involved in the effective diffusivity and effective (axial) velocity of the solute is identified. Such mathematically friendly relationships are obtained by applying the method of spatial averaging to the solute species continuity equation; this is accomplished after the role of the capillary geometrical dimensions on the applied electrical field equations has been studied. Moreover, explicit analytical expressions are derived for the effective parameters, i.e., diffusivity and convective velocity as functions of the applied (orthogonal) electric field. Previous attempts (see Sauer et al., 1995) have only led to equations for these parameters that require numerical solution and, therefore, limited the use of such results to practical applications. These may include, for example, the design of separation processes as well as environmental applications such as soil reclamation and wastewater treatment. An illustration of how a secondary electrical field can aid in reducing the optimal separation time is included.     

Generalized hybrid orbital for the treatment of boundary atoms in combined quantum mechanical and molecular mechanical calculations using the semiempirical parameterized model 3 method

The application of combined quantum mechanical (QM) and molecular mechanical methods to large molecular systems requires an adequate treatment of the boundary between the two approaches. In this article, we extend the generalized hybrid orbital (GHO) method to the semiempirical parameterized model 3 (PM3) Hamiltonian combined with the CHARMM force field. The GHO method makes use of four hybrid orbitals, one of which is included in the QM region in self-consistent field optimization and three are treated as auxiliary orbitals that do not participate in the QM optimization, but they provide an effective electric field for interactions. An important feature of the GHO method is that the semiempirical parameters for the boundary atom are transferable, and these parameters have been developed for a carbon boundary atom consistent with the PM3 model. The combined GHO-PM3/CHARMM model has been tested on molecular geometry and proton affinity for a series of organic compounds.Acknowledgement We thank the National Institutes of Health for support of this research.Contribution to the Jacopo Tomasi Honorary Issue     

Analysis of geometry effects on band spreading of microchip electrophoresis

The geometry and the flow field conditions in the separation microchannel of an electrophoresis chip system may have important impact on the system's separation efficiency. Understanding the geometry effect on the flow field physics in the separation microchannel is beneficial to the design or operation of an electrophoresis system. The turns in a microfabricated separation microchannel generally results in degraded separation quality. To avoid this limitation, channels are constructed with different types of turns to determine the optimum design that minimizes turn-induced band broadening. We have designed and tested various geometric bend ratios to greatly reduce this so-called "racetrack" effect. The effects of the separation channel geometry, fluid velocity profile and bend ratio on the band distribution in the detection area are discussed. Results show that the folded square U-shaped channel is better for miniaturization and simplification. The band tilting was corrected and the racetrack effect reduced in the detection area when the bend ratio is 4:1. The detection time obtained from the present numerical solution matches very well with the experimental data.     

Model for Plateau border drainage of power-law fluid with mobile interface and its application to foam drainage

A model for drainage of a power-law fluid through a Plateau border is proposed which accounts for the actual Plateau border geometry and interfacial mobility. The non-dimensionalized Navier-Stokes equations have been solved using finite element method to obtain the contours of velocity within the Plateau border cross section and average Plateau border velocity in terms of dimensionless inverse surface viscosity and power-law rheological parameters. The velocity coefficient, the correction for the average velocity through a Plateau border of actual geometry compared to that for a simplified circular geometry of the same area of cross section, was expressed as a function of dimensionless inverse surface viscosity and flow behavior index of the power-law fluid. The results of this improved model for Plateau border drainage were then incorporated in a previously developed foam drainage model [G. Narsimhan, J. Food Eng. 14 (1991) 139] to predict the evolution of liquid holdup profiles in a standing foam. Foam drainage was found to be slower for actual Plateau border cross section compared to circular geometry and faster for higher interfacial mobility and larger bubble size. Evolution of liquid holdup profiles in a standing foam formed by whipping and stabilized by 0.1% beta-lactoglobulin in the presence of xanthan gum when subjected to 16g and 45g centrifugal force fields was measured using magnetic resonance imaging for different xanthan gum concentrations. Drainage resulted in the formation of a separate liquid layer at the bottom at longer times. Measured bubble size, surface shear viscosity of beta-lactoglobulin solutions and literature values of power-law parameters of xanthan gum solution were employed in the current model to predict the evolution of liquid holdup profile which compared well with the experimental data. Newtonian model for foam drainage for zero shear viscosity underpredicted drainage rates and did not agree with the experimental data.     

Effect of capillary geometry on predicting electroosmotic volumetric flowrates in porous or fibrous media

Electrokinetic-based methods are used in a variety of applications including drug delivery and separation of biomolecules, among others. Many of these applications feature a fibrous or a porous medium that can be modeled by using capillary bundle models to predict the behavior of the electroosmotic flow within the particular system. The role of geometry in predicting volumetric flowrates in porous media is investigated by modeling the electroosmotic flow in idealized capillaries of rectangular, cylindrical, and annular geometries. This is achieved by the coupling of electrostatics and continuum hydrodynamics to obtain analytical expressions that govern the electrokinetically - driven volumetric flow within these idealized capillary geometries. A previous study developed a model to compare the cylindrical and annular capillary geometries by utilizing two methods that compare the areas of the two geometries. The methods used in this previous work will also be used in the present contribution to compare the volumetric flowrates in the cylindrical and annular capillaries with a rectangular capillary. Illustrative results will be presented to aid in the understanding of the influence of the various geometrical and electrostatic parameters that arise from the analysis of these volumetric flowrates. It was found that the electroosmotic volumetric flowrates are significantly affected by the capillary geometry.     

Computer simulation of the radiation field in a cobalt-60 irradiated volume

A computer model for calculating the characteristics of a radiation field in a cobalt-60 irradiated volume is presented. γ-Ray transmission by the source material and attenuation in the support structure is taken into account as are attenuation and scattering in the irradiated material. On the basis of the computer model, a FORTRAN program was written for an IBM-PC/XT and Macintosh personal computers, and used to calculate the radiation field for some common cobalt-60 arrangements used in the Cornell γ-cell. The calculated exposure rate values were compared with ionization chamber measurements for an annular geometry of Co⁶⁰ pencil sources. The agreement was to within 3%.     

毛细管电泳样品电堆积富集过程影响因素的数值分析

毛细管电泳样品电堆积富集是一种通过缓冲溶液浓度的差异在毛细管中形成电场强度梯度,从而对样品进行浓缩的富集技术。本文在已有数学模型的基础上,对影响毛细管电堆积富集过程的因素进行了分析。计算结果发现,样品粒子表面所带的电荷电性以及带电量会影响粒子的电泳速度,进而影响富集过程;外加电势的大小会影响样品粒子到达检测窗口的迁移时间;而样品塞的初始长度则会影响样品所能达到的最大富集浓度以及达到最佳的富集效果所需要的时间。所得到的结果对样品电堆积富集技术的进一步完善具有一定的理论指导意义。     

A visualization study of poly(ethylene terephthalate) flow using potential chain-ordering die geometries

Streak-photographic and stress birefringence techniques were used to analyze the flow of poly(ethylene terephthalate) through potential chain-ordering die geometries. The streak photographs were used to determine velocity distributions and streamlines in various convergent dies. The different contours were seen to have a significant effect on the polymer streamlines and velocity distributions. The measured velocities were used to develop empirical equations, specific for each geometry, which relate velocity to position within the die and to throughput rate. Flow birefringence was used to determine the extent of molecular ordering. The optimum chain-ordering die geometry was determined to be one which included a rapid initial decrease in cross-sectional area. Birefringence was also used to monitor polymer flow instability. An unusual mechanism for instability was observed at intermediate throughput rates.     

Effect of nonhomogeneous shear rates on the flow-induced crystallization of flexible macromolecules

Several observations are made regarding the role of nonhomogeneous shearing rates in the process of flow-induced fibrillar crystallization from solution. The flow geometry chosen in the present analysis is that occurring in the annular region between two concentric cylinders. Computational evaluation and discussion of the effect of stress-induced diffusion is presented in terms of the elastic dumbbell model. Significant variations in concentrations in the annular region are predicted at full development. Estimates of the dynamics of development of these profiles indicate that the length to capillary radii for full development depends upon the molecular weight of the polymer and the relative dimensions of the concentric cylinders. These results indicate the important role of stress-induced diffusion in the process of thickening of fibrillar crystals.     

Kinetics of the Electrons in Striations of Spherical Glow Discharges

Recent observations of spherical striations in large-volume nitrogen dcdischarges with a central anode have stimulated investigations of thenonlocal electron kinetics in these striations by solving the spatiallyinhomogeneous Boltzmann equation adapted to spherical geometry. Becausethe radial course of the electric potential is largely unknown in thisdischarge, different models concerning its radial course have been developedand used. These models are based on the measured radii of the striationsand the assumption that the potential drop between successive striationsdoes not change. As a consequence, with decreasing distance between thestriations the electric field strongly increases toward the centralanode. It has been found that spherical striations are only obtained ifthe electric field is strongly modulated. In this case, a highly nonlocalbehavior of the velocity distribution function and strongly modulatedradial courses of the macroscopic quantities have been obtained.     

Soil bacteria sensitivity towards heavy metals – Experimental system optimisation using chemical speciation calculations

The ecotoxicological relevance of many laboratory studies on soil bacteria sensitivity towards heavy metals is limited because culture conditions are chosen which do not adequately represent field conditions. The influence of the composition of culture media on the speciation of copper in an oligotrophic model soil system has been investigated. The expected chemical speciation has been calculated to obtain information on the concentration of the bioavailable fraction and the totally dissolved copper. For control measurements of totally dissolved copper, vacuum filtration has been used. The results of the measurements are in a good agreement with the calculations. Therefore, the use of speciation calculations is postulated as a useful tool for the assessment of free metal concentrations at higher pH, where the concentrations of dissolved metals are too low to be measured by simple methods.     

Flow rate influence on direct contact membrane distillation experiments: Different empirical correlations for Nusselt number

Direct contact membrane distillation (DCMD) experiments using distilled water are reported. Influence on the process of feed and permeate flow rates through the cell has been investigated in a wide flow range, from 2 to 8 l/min. Two main effects have been studied, its effect on the heat transfer coefficient and on the effective membrane thickness. An empiric dependence of the membrane thickness with linear velocity through the cell has been included in the equation for mass flux through the membrane obtained from the “Dusty-Gas” model with satisfactory results.     

Formation of soot clusters under the action of electric field of direct-current corona during combustion of natural gas

The influence of the electrode type and geometry, the interelectrode distance, and the electric field strength on the yield and distribution of soot particles during the combustion of natural gas in an electric field of direct-current corona discharge for positive and negative electrodes has been studied. The results are interpreted with allowance for the ion wind effect. The structure of soot clusters has been examined by electron microscopy.     

The Cotton-Mouton effect of neon and argon: a benchmark study using highly correlated coupled cluster wave functions

The Cotton-Mouton effect (magnetic field induced linear birefringence) has been studied for neon and argon using state-of-the-art coupled cluster techniques. The coupled cluster singles, doubles and triples (CCSDT) approach has been used to obtain static benchmark results and the CC3 model with an approximate treatment of triple excitations to obtain frequency-dependent results. In the case of neon the effect of excitations beyond triples has also been estimated via coupled cluster calculations including quadruple excitations (CCSDTQ), pentuple excitations (CCSDTQP), etc. up to the full configuration-interaction level. The results obtained for the anisotropy of the hypermagnetizability Deltaeta(omega), the molecular property that determines the magnetic field induced birefringence of spherically symmetric systems, are Deltaeta=2.89 a.u. for neon and Deltaeta=24.7 a.u. for argon, with a negligible effect of frequency dispersion. For neon we could estimate an absolute error on Deltaeta of 0.1 a.u. The accuracy of these results surpasses that of recently reported experimental data.