First-encounter time of two diffusing particles in two- and three-dimensional confinement.

The statistics of the first-encounter time of diffusing particles changes drastically when they are placed under confinement. In the present work, we make use of Monte Carlo simulations to study the behavior of a two-particle system in two- and three-dimensional domains with reflecting boundaries. Based on the outcome of the simulations, we give a comprehensive overview of the behavior of the survival probability S(t) and the associated first-encounter time probability density H(t) over a broad time range spanning several decades. In addition, we provide numerical estimates and empirical formulas for the mean first-encounter time 〈T〉, as well as for the decay time T characterizing the monoexponential long-time decay of the survival probability. Based on the distance between the boundary and the center of mass of two particles, we obtain an empirical lower bound t_{B} for the time at which S(t) starts to significantly deviate from its counterpart for the no boundary case. Surprisingly, for small-sized particles, the dominant contribution to T depends only on the total diffusivity D=D_{1}+D_{2}, in sharp contrast to the one-dimensional case. This contribution can be related to the Wiener sausage generated by a fictitious Brownian particle with diffusivity D. In two dimensions, the first subleading contribution to T is found to depend weakly on the ratio D_{1}/D_{2}. We also investigate the slow-diffusion limit when D_{2}≪D_{1}, and we discuss the transition to the limit when one particle is a fixed target. Finally, we give some indications to anticipate when T can be expected to be a good approximation for 〈T〉.

Pseudo-two-dimensional dynamics in a system of macroscopic rolling spheres.

We study in this work the dynamics of a collection of identical hollow spheres (ping-pong balls) that rest on a horizontal metallic grid. Fluidization is achieved by means of a turbulent air current coming from below. The upflow is adjusted so that the particles do not levitate over the grid, resulting in quasi-two-dimensional dynamics. We show that the behavior of diffusion and correlations in this system is particularly rich. Noticeably as well (and related to the complex dynamical behavior), a variety of phases appear, with important peculiarities with respect to analogous setups. We observe gas, liquid, glass, and hexagonal crystal phases. Most notably, we show that the melting of the hexagonal crystal occurs in coexistence with a liquid phase. This strikingly differs from the corresponding transition in a purely two-dimensional systems of air-fluidized disks, for which no phase coexistence has been reported in the literature.

Reaction-diffusion and reaction-subdiffusion equations on arbitrarily evolving domains.

Reaction-diffusion equations are widely used as the governing evolution equations for modeling many physical, chemical, and biological processes. Here we derive reaction-diffusion equations to model transport with reactions on a one-dimensional domain that is evolving. The model equations, which have been derived from generalized continuous time random walks, can incorporate complexities such as subdiffusive transport and inhomogeneous domain stretching and shrinking. Inhomogeneously growing domains are frequently encountered in biological phenomena involving stochastic transport, such as tumor growth and morphogen gradient formation. A method for constructing analytic expressions for short-time moments of the position of the particles is developed and moments calculated from this approach are shown to compare favorably with results from random walk simulations and numerical integration of the reaction transport equation. The results show the important role played by the initial condition. In particular, it strongly affects the time dependence of the moments in the short-time regime by introducing additional drift and diffusion terms. We also discuss how our reaction transport equation could be applied to study the spreading of a population on an evolving interface. From a more general perspective, our findings help to mitigate the scarcity of analytic results for reaction-diffusion problems in geometries displaying nonuniform growth. They are also expected to pave the way for further results, including the treatment of first-passage problems associated with encounter-controlled reactions in such domains.

First-encounter time of two diffusing particles in confinement.

We investigate how confinement may drastically change both the probability density of the first-encounter time and the associated survival probability in the case of two diffusing particles. To obtain analytical insights into this problem, we focus on two one-dimensional settings: a half-line and an interval. We first consider the case with equal particle diffusivities, for which exact results can be obtained for the survival probability and the associated first-encounter time density valid over the full time domain. We also evaluate the moments of the first-encounter time when they exist. We then turn to the case with unequal diffusivities and focus on the long-time behavior of the survival probability. Our results highlight the great impact of boundary effects in diffusion-controlled kinetics even for simple one-dimensional settings, as well as the difficulty of obtaining analytic results as soon as the translational invariance of such systems is broken.

Efectos de la aplicación del Programa de Enriquecimiento Instrumental sobre un grupo de jóvenes infractores / The Effects of Applying the Instrumental Enrichment Program to a Group of Juvenile Offenders / Efeitos da implementai em um grupo de jovens infratores

Resumen Esta investigación tuvo como objetivo analizar los efectos de la aplicación del Programa de Enriquecimiento Instrumental (PEI) de Reuven Feuerstein sobre un grupo de jóvenes infractores en proceso de resocialización de la Fundación El Faro del municipio de San José de Cúcuta, Colombia. Corresponde a un estudio cualitativo de nivel descriptivo enmarcado en la orientación epistemológica del paradigma interpretativo. En el estudio participaron diez jóvenes en proceso de resocialización con diversas características y limitaciones; para la intervención y recolección de información se utilizaron los instrumentos de relaciones virtuales, comparaciones y relaciones espaciales del PEI junto al diario de campo. Los resultados presentados desde la perspectiva cualitativa muestran un conjunto de cambios en las estructuras cognitivas de los jóvenes sujetos a intervención, es decir, sobre las funciones cognitivas y las operaciones mentales, aunque algunos factores limitaron la mediación como el consumo de sustancias psicoactivas y el ambiente del centro de rehabilitación. La evidencia permite concluir que son significativas las potencialidades del programa cognitivo y la Experiencia de Aprendizaje Mediado -EAM- como se señalan en otros estudios sobre los diferentes ámbitos de aplicación de la teoría de la Modificabilidad Estructural Cognitiva.

Abstract The objective of this research was to analyze the effects of applying Reuven Feuerstein's Instrumental Enrichment (FIE) Program to a group of juvenile offenders during a resocialization process by Fundación El Faro in the municipality of San José of Cucuta, Colombia. It corresponds to a qualitative, descriptive study framed within the epistemological orientation of the interpretative paradigm. Ten young people with various characteristics and limitations participated in the resocialization process. Virtual relationship instruments, comparisons and FIE spatial relationships were used in the intervention and for information collection, along with the field log. The results presented from a qualitative perspective show a set of changes in the cognitive structures of young people subject to intervention. That is, cognitive functions and mental operations changed, although some factors limited mediation, such as psychoactive substance consumption and the environment within the rehabilitation center. The evidence allows concluding that the Mediated Learning Experience (MLE) and the program's cognitive potentials are significant, as indicated in other studies on the different environments in which the Structural Cognitive Modifiability Theory is applied.

Resumo Esta pesquisa teve como objetivo analisar os efeitos da aplicação do Programa de Enriquecimento Instrumental (PEI) de Reuven Feuerstein num grupo de jovens infratores em processo de ressocialização da Fundação El Faro do município de San José de Cúcuta, Colômbia. Trata-se de um estudo qualitativo de nível descritivo enquadrado na orientação epistemológica do paradigma interpretativo. Participaram do estudo dez jovens em processo de ressocialização, com diversas características e limitações. Para a intervenção e coleta de informações, foram utilizados os instrumentos de relações virtuais, comparações e relações espaciais do PEI, juntamente com o diário de campo. Os resultados apresentados na perspectiva qualitativa mostram um conjunto de mudanças nas estruturas cognitivas dos jovens sujeitos à intervenção, ou seja, nas funções cognitivas e nas operações mentais, embora alguns fatores tenham limitado a mediação, como o consumo de substâncias psicoativas e o ambiente do centro de reabilitação. As evidências permitem concluir que as potencialidades do programa cognitivo e da Experiência de Aprendizagem Mediada -EAM- são significativas, conforme apontado em outros estudos nas diferentes áreas de aplicação da teoria da Modificabilidade Estrutural Cognitiva.

Feasibility of gas chromatography-atmospheric pressure photoionization-high-resolution mass spectrometry for the analysis of polychlorinated dibenzo-p-dioxins, dibenzofurans, and dioxin-like polychlorinated biphenyls in environmental and feed samples.

In this work, the suitability of atmospheric pressure photoionization (APPI) has been assessed for the determination of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (dl-PCBs) by gas chromatography-high-resolution mass spectrometry (GC-HRMS). The APPI of target compounds has been tested in both positive and negative ion modes. Under positive ion mode, the analytes generated the molecular ion, which was favoured using dopants that promote charge exchange gas-phase reactions (i.e., benzene), while in negative ion mode, the ion [M-Cl+O]- for PCDFs and dl-PCBs were mainly formed, providing the best results using benzene and diethyl ether as dopants, respectively. Concerning PCDDs, highly chlorinated congeners were mainly ionized by means of the [M-Cl]- ion, whereas [M-Cl+O2]- was the base peak for tetraCDD and [M-Cl+O]- for penta- and hexaCDDs. Method quality parameters, in accordance with the current EU Regulation guidelines for food and feed analysis, showed the good performance of the two GC-APPI-HRMS (Orbitrap) methods since they provided high detection capability (low fg levels), good linearity, and satisfactory precision (RSD% < 9%). In addition, the GC-APPI-HRMS (Orbitrap) methods were validated by analysing selected environmental and feed samples and the results were compared to those obtained using conventional GC-EI-HRMS, demonstrating the good performance in the analysis of the target compounds. Hence, the GC-APPI-HRMS technique can be proposed as alternative to the conventional methods for the determination of PCDD/Fs and dl-PCBs in environmental and feed matrices.

Standard and fractional Ornstein-Uhlenbeck process on a growing domain.

We study normal diffusive and subdiffusive processes in a harmonic potential (Ornstein-Uhlenbeck process) on a uniformly growing or contracting domain. Our starting point is a recently derived fractional Fokker-Planck equation, which covers both the case of Brownian diffusion and the case of a subdiffusive continuous-time random walk (CTRW). We find a high sensitivity of the random walk properties to the details of the domain growth rate, which gives rise to a variety of regimes with extremely different behaviors. At the origin of this rich phenomenology is the fact that the walkers still move while they wait to jump, since they are dragged by the deterministic drift arising from the domain growth. Thus, the increasingly long waiting times associated with the aging of the subdiffusive CTRW imply that, in the time interval between two consecutive jumps, the walkers might travel over much longer distances than in the normal diffusive case. This gives rise to seemingly counterintuitive effects. For example, on a static domain, both Brownian diffusion and subdiffusive CTRWs yield a stationary particle distribution with finite width when a harmonic potential is at play, thus indicating a confinement of the diffusing particle. However, for a sufficiently fast growing or contracting domain, this qualitative behavior breaks down, and differences between the Brownian case and the subdiffusive case are found. In the case of Brownian particles, a sufficiently fast exponential domain growth is needed to break the confinement induced by the harmonic force; in contrast, for subdiffusive particles such a breakdown may already take place for a sufficiently fast power-law domain growth. Our analytic and numerical results for both types of diffusion are fully confirmed by random walk simulations.

First-passage properties of mortal random walks: Ballistic behavior, effective reduction of dimensionality, and scaling functions for hierarchical graphs.

We consider a mortal random walker on a family of hierarchical graphs in the presence of some trap sites. The configuration comprising the graph, the starting point of the walk, and the locations of the trap sites is taken to be exactly self-similar as one goes from one generation of the family to the next. Under these circumstances, the total probability that the walker hits a trap is determined exactly as a function of the single-step survival probability q of the mortal walker. On the nth generation graph of the family, this probability is shown to be given by the nth iterate of a certain scaling function or map q→f(q). The properties of the map then determine, in each case, the behavior of the trapping probability, the mean time to trapping, the temporal scaling factor governing the random walk dimension on the graph, and other related properties. The formalism is illustrated for the cases of a linear hierarchical lattice and the Sierpinski graphs in two and three Euclidean dimensions. We find an effective reduction of the random walk dimensionality due to the ballistic behavior of the surviving particles induced by the mortality constraint. The relevance of this finding for experiments involving travel times of particles in diffusion-decay systems is discussed.

Long-term monitoring programme of polychlorinated dioxins and polychlorinated furans in ambient air of Catalonia, Spain (1994-2015).

Reductions in concentrations of persistent organic pollutants were observed for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in a comparison of two consecutive decades ending in December 31, 2015 (n = 413) and compiled from a 48-station long-term air-monitoring network in Catalonia, Spain. Reductions were observed in geometric mean concentration of total PCDD/Fs, decreasing at a rate of 11% yr-1. Results reflect the relative success of improved pollution control measures at industry and traffic locations. At traffic and industrial locations, PCDD/Fs were reduced by around 68% (from 70 to 22 fg I-TEQ/m3) and 66% (from 57 to 19 fg I-TEQ/m3), respectively. The highest concentration of 1196 fg I-TEQ/m3 was observed during the first decade at an industrial location. The highest concentration over the second decade was at a background location during winter (674 fg I-TEQ/m3). Winter observations are generally higher than those recorded in summer, as evident in an 8-year period of moderate PCDD/F levels (<250 fg I-TEQ/m3, n = 143). PCDD/F congener profiles were identified has having less chlorinated PCDD/F, indicating the presence of nearby combustion sources at selected locations. More long-term sampling campaigns, combined with network optimization and integration into a meteorological model, can offer a tool for future PCDD/Fs control measures in not only Catalonia - but also applicable to other areas.

Retention and transport of mecoprop on acid sandy-loam soils.

Interaction with soil components is one of the key processes governing the fate of agrochemicals in the environment. In this work, we studied the adsorption / desorption and transport of mecoprop (methylchlorophenoxypropionic acid or MCPP) in four acid sandy-loam soils with different organic matter contents. Kinetics of adsorption and adsorption/desorption at equilibrium were studied in batch experiments, whereas transport was studied in laboratory columns. Adsorption and desorption were found to be linear or nearly-linear. The kinetics of mecoprop adsorption were relatively fast in all cases (less than 24h). Adsorption and desorption were adequately described by the linear and Freundlich models, with KF values that ranged from 0.7 to 8.8Ln µmol1-nkg-1 and KD values from 0.3 to 3.6Lkg-1. The results of the transport experiments showed that the retention of mecoprop by soil was very low (less than 6.2%). The retention of mecoprop by the soils in all experiments increased with organic matter content. Overall, it was observed that mecoprop was weakly adsorbed by the soils, what would result in a high risk of leaching of this compound in the environment.

Analysis of polychlorinated dibenzo-p-dioxins and dibenzofurans in stack gas emissions by gas chromatography-atmospheric pressure chemical ionization-triple-quadrupole mass spectrometry.

A gas chromatography coupled to triple-quadrupole mass spectrometry (GC-(QqQ)MS/MS) including a soft-ionization through an atmospheric pressure chemical ionization (APCI) source based method was compared with the high resolution mass spectrometry (HRMS) standard reference method EN1948, for the analysis of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) in stack gas emissions. The stack emission samples were collected, both, by manual method sampling (from 6 to 8h) and by long-term sampling systems (sampling time of several weeks). This work presents the first comparison of GC-(QqQ)MS/MS with APCI source with the European Standard EN1948 technique for stack gas emissions. Sample concentrations ranged from 0.5 to 596pg I-TEQ/Nm3. Comparative results in all investigated samples showed relative errors that were within ±15%. These results make GC-(QqQ)MS/MS with APCI suitable for the quantitative analysis of dioxins in the studied samples and create a real alternative tool to the reference sector GC-HRMS instruments.

Continuous-time random-walk model for anomalous diffusion in expanding media.

Expanding media are typical in many different fields, e.g., in biology and cosmology. In general, a medium expansion (contraction) brings about dramatic changes in the behavior of diffusive transport properties such as the set of positional moments and the Green's function. Here, we focus on the characterization of such effects when the diffusion process is described by the continuous-time random-walk (CTRW) model. As is well known, when the medium is static this model yields anomalous diffusion for a proper choice of the probability density function (pdf) for the jump length and the waiting time, but the behavior may change drastically if a medium expansion is superimposed on the intrinsic random motion of the diffusing particle. For the case where the jump length and the waiting time pdfs are long-tailed, we derive a general bifractional diffusion equation which reduces to a normal diffusion equation in the appropriate limit. We then study some particular cases of interest, including Lévy flights and subdiffusive CTRWs. In the former case, we find an analytical exact solution for the Green's function (propagator). When the expansion is sufficiently fast, the contribution of the diffusive transport becomes irrelevant at long times and the propagator tends to a stationary profile in the comoving reference frame. In contrast, for a contracting medium a competition between the spreading effect of diffusion and the concentrating effect of contraction arises. In the specific case of a subdiffusive CTRW in an exponentially contracting medium, the latter effect prevails for sufficiently long times, and all the particles are eventually localized at a single point in physical space. This "big crunch" effect, totally absent in the case of normal diffusion, stems from inefficient particle spreading due to subdiffusion. We also derive a hierarchy of differential equations for the moments of the transport process described by the subdiffusive CTRW model in an expanding medium. From this hierarchy, the full time evolution of the second-order moment is obtained for some specific types of expansion. In the case of an exponential expansion, exact recurrence relations for the Laplace-transformed moments are obtained, whence the long-time behavior of moments of arbitrary order is subsequently inferred. Our analytical and numerical results for both Lévy flights and subdiffusive CTRWs confirm the intuitive expectation that the medium expansion hinders the mixing of diffusive particles occupying separate regions. In the case of Lévy flights, we quantify this effect by means of the so-called "Lévy horizon."

Construcción de ciudadanía: un modelo para su desarrollo en la escuela / The construction of citizenship: a model for its development in the school

Se analiza las relaciones entre ciudadanía, política y educación, y a partir de ello, se propone un modelo para la formación de ciudadanos en la escuela, para la formación de la ciudadanía que incluye: propósitos de la enseñanza, papel de los contenidos en este proceso, formas para llevar a cabo enseñanza - aprendizaje y evaluación del aprendizaje.

The relationship among citizens, politics, and education is discussed, and based on that, a model for the training of citizens in the school, for the formation of citizenship is proposed, which includes purposes of education, the role of the contents in this process, ways to carry out the teaching - learning process, and learning assessment.

Diffusion in an expanding medium: Fokker-Planck equation, Green's function, and first-passage properties.

We present a classical, mesoscopic derivation of the Fokker-Planck equation for diffusion in an expanding medium. To this end, we take a conveniently generalized Chapman-Kolmogorov equation as the starting point. We obtain an analytical expression for the Green's function (propagator) and investigate both analytically and numerically how this function and the associated moments behave. We also study first-passage properties in expanding hyperspherical geometries. We show that in all cases the behavior is determined to a great extent by the so-called Brownian conformal time τ(t), which we define via the relation τ[over ̇]=1/a^{2}, where a(t) is the expansion scale factor. If the medium expansion is driven by a power law [a(t)∝t^{γ} with γ>0], then we find interesting crossover effects in the mixing effectiveness of the diffusion process when the characteristic exponent γ is varied. Crossover effects are also found at the level of the survival probability and of the moments of the first passage-time distribution with two different regimes separated by the critical value γ=1/2. The case of an exponential scale factor is analyzed separately both for expanding and contracting media. In the latter situation, a stationary probability distribution arises in the long-time limit.

Evaluation of the capabilities of atmospheric pressure chemical ionization source coupled to tandem mass spectrometry for the determination of dioxin-like polychlorobiphenyls in complex-matrix food samples.

The use of the novel atmospheric pressure chemical ionization (APCI) source for gas chromatography (GC) coupled to triple quadrupole using tandem mass spectrometry (MS/MS) and its potential for the simultaneous determination of the 12 dioxin-like polychlorobiphenyls (DL-PCBs) in complex food and feed matrices has been evaluated. In first place, ionization and fragmentation behavior of DL-PCBs on the APCI source under charge transfer conditions has been studied followed by their fragmentation in the collision cell. Linearity, repeatability and sensitivity have been studied obtaining instrumental limits of detection and quantification of 0.0025 and 0.005 pg µL(-1) (2.5 and 5 fg on column) respectively for every DL-PCB. Finally, application to real samples has been carried out and DL-PCB congeners (PCB 77, 81, 105, 114, 118, 123, 126, 156, 157, 167, 169, 189) have been detected in the different samples in the range of 0.40-10000 pg g(-1). GC-(APCI)MS/MS has been proved as a suitable alternative to the traditionally accepted confirmation method based on the use of high resolution mass spectrometry and other triple quadrupole tandem mass spectrometry techniques operating with electron ionization. The development of MS/MS methodologies for the analysis of dioxins and DL-PCBs is nowadays particularly important, since this technique was included as a confirmatory method in the present European Union regulations that establish the requirements for the determination of these compounds in food and feed matrices.

Anomalous diffusion and dynamics of fluorescence recovery after photobleaching in the random-comb model.

We address the problem of diffusion on a comb whose teeth display varying lengths. Specifically, the length ℓ of each tooth is drawn from a probability distribution displaying power law behavior at large ℓ,P(ℓ)∼ℓ^{-(1+α)} (α>0). To start with, we focus on the computation of the anomalous diffusion coefficient for the subdiffusive motion along the backbone. This quantity is subsequently used as an input to compute concentration recovery curves mimicking fluorescence recovery after photobleaching experiments in comblike geometries such as spiny dendrites. Our method is based on the mean-field description provided by the well-tested continuous time random-walk approach for the random-comb model, and the obtained analytical result for the diffusion coefficient is confirmed by numerical simulations of a random walk with finite steps in time and space along the backbone and the teeth. We subsequently incorporate retardation effects arising from binding-unbinding kinetics into our model and obtain a scaling law characterizing the corresponding change in the diffusion coefficient. Finally, we show that recovery curves obtained with the help of the analytical expression for the anomalous diffusion coefficient cannot be fitted perfectly by a model based on scaled Brownian motion, i.e., a standard diffusion equation with a time-dependent diffusion coefficient. However, differences between the exact curves and such fits are small, thereby providing justification for the practical use of models relying on scaled Brownian motion as a fitting procedure for recovery curves arising from particle diffusion in comblike systems.

A four-year survey in the farming region of Chile, occurrence and human exposure to polychlorinated dibenzo-p-dioxins and dibenzofurans, and dioxin -like polychlorinated biphenyls in different raw meats.

For the first time in South America, a four-year survey (2011-2014) was conducted to assess the occurrence of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) and dioxin-like polychlorinated biphenyls (dl-PCBs) in different raw meats (bovine, pork, ovine, chicken, and turkey) sampled from ten of the fifteen regions of Chile. When expressed as pg World Health Organization Toxic Equivalent (WHO-TEQ2005)g-1 fat, the highest PCDD/F values for each species were 0.54 (bovine-2012), 0.27 (pork-2013), 0.23 (ovine-2011), 0.61 (chickens-2013), and 0.34 (turkey-2012). The highest mean dl-PCBs levels were 0.18 (bovine-2011), 0.05 (pork-2014), 0.13 (ovine-2011), 0.1 (chicken-2014), and 0.21 (turkey-2013). Penta- and tetra-chlorinated congeners dominated PCDD/F WHO-TEQ2005 profiles during the survey, while PCB 126 dominated dl-PCBs profiles. Statistically significant interspecies differences were found. Dietary intake was also estimated, and the highest total PCDD/F and dl-PCBs values, found in poultry meat, were 0.09pgWHO-TEQ2005kg-1bwd-1 (2013) for adults and 0.36pgWHO-TEQ2005kg-1bwd-1 (2013) for children. The concentrations and dietary intakes for the studied compounds in raw meat were below international and national maximum permitted limits.

Polychlorinated dibenzo-p-dioxins, dibenzofurans and dioxin-like PCBs in commercialized food products from Colombia.

Polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and dioxin-like polychlorinated biphenyls (dl-PCBs) are commonly known as dioxins and are the most toxic members of persistent organic pollutants (POPs) because present a variety of health effects especially as promoting agent of growing and transformation of cancer cells. They are bio-accumulate in humans primarily via the diet, specifically by ingestion of foods that have high lipid content which are generally associated with foods of animal origin such as oils and fats and with fishery and dairy products. In Colombia the Ministry of Health and Social Protection which is the entity responsible for surveillance food conditions, has established maximum levels for dioxins and dl-PCBs in oils from animal and vegetable origins. Oils of vegetable and animal origin represent an appreciable intake in the country thus the presence of dioxins and dl-PCBs in these materials is a matter of concern because they can bioaccumulate in fat. In this contribution the levels of PCDD/Fs and dl-PCBs in olive, soybean, fish oil, butter and shrimp consumed in Colombia were determined using HRGC-HRMS and were compared with the maximum levels permitted in oil samples according to both the Colombian and European regulations. WHO-TEQ concentrations for PCDD/Fs and dioxin like PCBs ranged from 0.24 to 1.710pgWHO-TEQ PCDD/Fg(-1) of fat and from 0.050 to 3.000pgWHO-TEQ PCBg(-1) of fat, respectively. As expected, fish oils and shrimp present the highest WHO-TEQ PCDD/Fs and dl-PCBs values followed by butter and soybean oil sample, while the olive oil shows the lowest levels. In general, the vegetable oils show levels below the limits established by both the Colombian and European regulations. The levels from soybean oil found in this study were slightly higher than the threshold established both by the Commission Regulation European Union (EU) and the Colombian legislation, while fish oils showed concentrations above the European regulations. Furthermore, shrimp exhibited values below the maximum concentration levels established by the EU and Colombian regulation.

Dioxins, furans, biphenyls, arsenic, thorium and uranium in natural and anthropogenic sources of phosphorus and calcium used in agriculture.

The aim of this study was to assess the presence of dioxins, furans and biphenyls, and the inorganic contaminants such as arsenic (As), thorium (Th) and uranium (U) in three main products used in Agriculture in Brazil: feed grade dicalcium phosphate, calcined bovine bone meal and calcitic limestone. The first two are anthropogenic sources of phosphorus and calcium, while calcitic limestone is a natural unprocessed mineral. Regarding to dioxin-like substances, all samples analyzed exhibited dioxins (PCDD) and furans (PCDF) and dioxin-like polychlorinated biphenyls (dl-PCBs) concentrations below limit of detection (LOD). In general, achieved is in accordance with regulation in Brazil where is established a maximum limit in limestone used in the citric pulp production (0.50pg WHO-TEQ g(-1)). In addition, reported data revealed very low levels for limestone in comparison with similar materials reported by European legislation. As result for toxic metals, achieved data were obtained using Instrumental Neutron Activation Analysis (INAA). On one hand, limestone sample exhibits the largest arsenic concentration. On another hand, dicalcium phosphate exhibited the largest uranium concentration, which represents a standard in animal nutrition. Therefore, it is phosphorus source in the animal feed industry can be a goal of concern in the feed field.

Theoretical study of carbon-based tips for scanning tunnelling microscopy.

Motivated by recent experiments, we present here a detailed theoretical analysis of the use of carbon-based conductive tips in scanning tunnelling microscopy. In particular, we employ ab initio methods based on density functional theory to explore a graphitic, an amorphous carbon and two diamond-like tips for imaging with a scanning tunnelling microscope (STM), and we compare them with standard metallic tips made of gold and tungsten. We investigate the performance of these tips in terms of the corrugation of the STM images acquired when scanning a single graphene sheet. Moreover, we analyse the impact of the tip-sample distance and show that it plays a fundamental role in the resolution and symmetry of the STM images. We also explore in depth how the adsorption of single atoms and molecules in the tip apexes modifies the STM images and demonstrate that, in general, it leads to an improved image resolution. The ensemble of our results provides strong evidence that carbon-based tips can significantly improve the resolution of STM images, as compared to more standard metallic tips, which may open a new line of research in scanning tunnelling microscopy.