Trace analysis of polychlorinated dibenzo-p-dioxins, dibenzofurans and WHO polychlorinated biphenyls in food using comprehensive two-dimensional gas chromatography with electron-capture detection

Trace analysis of 2,3,7,8-polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs) and the 12 WHO-PCBs (four non-ortho and eight mono-ortho congeners that have been assigned toxic equivalence factors, TEFs, by the World Health Organisation) was conducted by comprehensive two-dimensional gas chromatography with a micro electron-capture detector (GC x GC-microECD). Four food matrices (fish oil from herring, spiked cows' milk, vegetable oil and an eel extract) were analysed by two GC x GC laboratories, and four GC-HRMS laboratories generated reference values. The two GC x GC laboratories used different column combinations for separating the target analytes. For the first dimension, non-polar DB-XLB and VF-1 columns were used, and for the second dimension, an LC-50 liquid crystalline column with unique selectivity for planar compounds. The congener-specific and total toxic equivalence (TEQ) data obtained using DB-XLB x LC-50 were in good agreement with results obtained by the GC-HRMS laboratories. The WHO-PCB data obtained with the VF-1 x LC-50 combination was also good, but the PCDD/F concentrations were sometimes overestimated due to matrix interferences. GC x GC-microECD using DB-XLB x LC-50 seems to fulfil the European Community requirements of a screening method for PCDD/F and WHO-PCB TEQ in food.     

In silico prediction of drug solubility: 1. Free energy of hydration

As a first step in the computational prediction of drug solubility the free energy of hydration, DeltaG*(vw) in TIP4P water has been computed for a data set of 48 drug molecules using the free energy of perturbation method and the optimized potential for liquid simulations all-atom force field. The simulations were performed in two steps, where first the Coulomb and then the Lennard-Jones interactions between the solute and the water molecules were scaled down from full to zero strength to provide physical understanding and simpler predictive models. The results have been interpreted using a theory assuming DeltaG*(vw) = A(MS)gamma + E(LJ) + E(C)/2 where A(MS) is the molecular surface area, gamma is the water-vapor surface tension, and E(LJ) and E(C) are the solute-water Lennard-Jones and Coulomb interaction energies, respectively. It was found that by a proper definition of the molecular surface area our results as well as several results from the literature were found to be in quantitative agreement using the macroscopic surface tension of TIP4P water. This is in contrast to the surface tension for water around a spherical cavity that previously has been shown to be dependent on the size of the cavity up to a radius of approximately 1 nm. The step of scaling down the electrostatic interaction can be represented by linear response theory.     

A tri-material elastodynamic solution for a transversely isotropic full-space

A linear elastic full-space composed of an upper half-space, a lower half-space and a layer of three different transversely isotropic materials under an internal load is considered. The axes of symmetry of the different regions are assumed to be normal to the planar interfaces of the regions and are thus parallel. An arbitrary load in the frequency domain is allowed on a finite patch located at the interface of the upper half-space and the adjacent layer. By means of the complete displacement potentials, the displacements and stresses in the three regions are determined in Fourier–Hankel space in the form of line integrals. The solution can be degenerated to the solution for (i) a full-space under an arbitrary buried load, (ii) a half-space contain a layer bonded to the top of it under an arbitrary surface force, (iii) a half-space under an arbitrary surface load, (iv) a two layer half-space under an arbitrary force applied at the interface of two regions, (v) a half-space under an arbitrary buried force, (vi) a layer of finite thickness fixed at the bottom and under an arbitrary surface load, and (vii) a bi-material full-space under an arbitrary load at the interface of two materials. Examples of the displacements and stresses are obtained numerically and compared to existing solutions.     

Finite element simulation of localized plastic deformation

Summary A rational finite element algorithm for capturing localized plastic deformation due to softening and/or non-associated plastic flow is devised. The incremental relations are based on implicit integration. In each increment it is important to use a carefully designed modified Newton iteration procedure in conjunction with a start solution whose calculation is based on a diagnostic bifurcation analysis. The algorithm performs successfully for a finite element mesh that is biased in the sense that the element sides are prealigned with the anticipated localization zone which is demonstrated for a slope stability problem.

---

Finite-Element-Berechnung von örtlichen plastischen Verformungen

Übersicht Die Konstruktion eines Algorithmus, der im Zusammenhang mit der Methode der Finiten Elemente örtliche plastische Verformungen als Folge von Materialentfestigung und/oder nichtassoziiertem plastischen Fließen erfaßt, wird beschrieben. Die inkrementellen Gleichungen ergeben sich aus der Verwendung eines impliziten Integrationsverfahrens. In jedem Inkrement wird ein sorgfältig entwickeltes, modifiziertes Newton-Iterationsverfahren verwendet, dessen erste Lösungsabschätzung auf einer diagnostischen Analyse des Verzweigungsproblems beruht. Am Beispiel einer Böschungsstabilitätsungtersuchung wird gezeigt, daß der Algorithmus erfolgreich ist, wenn das verwendete Finite-Elemente-Netz so ausgerichtet ist, daß die Elementseiten der erwarteten Versagenzone folgen.

---

---

Presented at the workshop on Numerical Methods for Localization and Bifurcation of Granular Bodies, held at the Technical University of Gdansk (Poland), September 25–30, 1989     

Melting of palladium clusters – density of states determination by Monte Carlo simulation

The density of states (DOS) has been calculated for the metal clusters Pd₁₃, Pd₅₅ and Pd₁₄₇ using the recently proposed reference system equilibration (RSE) method. The interaction within the clusters was described by a many-body alloy potential. Using this DOS, the caloric curve of Pd₁₃ has been calculated and excellent agreement with canonical Monte Carlo simulations is obtained. For Pd₅₅ and Pd₁₄₇, the solid and one molten isomers have been isolated in order to calculate the DOS for the isomers separately. The melting of the clusters occurs when the DOS for the solid and the molten isomers are equal. Comparison with previous microcanonical Monte Carlo simulations shows that the number of statistically equivalent molten isomers are 1.1×10¹⁸ for Pd₅₅ and 4.1×10⁴¹ for Pd₁₄₇.     

Impact against X65 steel pipes – An experimental investigation

Offshore pipelines subjected to accidental loads, such as impacts from trawl gear or anchors, may experience large global displacements from its initial position and large local strains. The axial forces set up during deformation move the pipeline back towards its initial position, thereby creating a complex local stress and strain history that may lead to fracture in the pipeline. In this study, material and component tests have been carried out on an X65 offshore pipeline material to investigate the behaviour during impact, and to observe if fracture occurs. Pipes were first impacted in a pendulum accelerator at varying velocities before they were pulled straight in a tension machine. Fracture was found in the impacted area of all the pipes. Material tests were carried out to determine the characteristics of the X65 pipeline material. A metallurgical investigation was also conducted, revealing that fracture initiated both inside the pipe wall and on the surface.     

Theory of solvent effects on the equilibrium properties of a diatomic guest molecule

The medium effects on the equilibrium properties of a diatomic molecule in a simple fluid medium have been simulated using a mixed ensemble Monte Carlo method in the special case of bromine in argon. The change in the bond length distribution has been obtained over a range of densities and temperatures of the medium. A statistical theory based on a treatment of the medium as a van der Waals fluid is developed and used to predict the apparent change in the vibrational potential due to the medium. Good agreement is obtained between theory and simulation for low to moderate fluid densities but the effects are overestimated for highly dense fluids.     

A systematic derivation of exact generalized Brownian motion theory

We present here a simple unified derivation of the exact Fokker-Planck equation obtained earlier by Zwanzig and the exact Langevin and transport equations derived by Mori. The derivation, based on the use of a Hilbert space formulation of the dynamics, leads to substantial generalizations of these results in a straightforward manner. We obtain nonlinear Langevin equations for classical systems and discuss the extension of the theory to driven transport and to quantum dynamics based either on the use of density matrices or Γ-space densities as suggested by Wigner. Remaining limitations of the theory are pointed out.     

Generalized van der Waals theory VIII. An improved analysis of the liquid/gas interface

We extend a preceding application of the GvdW theory to the prediction of interface profiles and surface tension in simple fluids by incorporating a variational determination of the effective hard-sphere diameter. This has previously been found to improve the predicted equation of state. Here we find that it also improves the prediction of interface profiles and surface tension in LJ(12-6) fluids. The agreement with experiment and simulation when these quantities are considered as functions ofT/T _c is to within about 5%. As in our earlier calculations the nonlocal entropie effects are found to reduce the surface tension by 5%–10%. In the present conceptually more accurate theory this significantly improves the agreement with experiment and simulation.