Dosage du carbone organique dissous dans les eaux douces naturelles. Intérêt, Principe, Mise en Oeuvre et Précautions Opératoires

International audienceCe mémoire présente un travail de validation du dosage du carbone organique dissous(COD) et du carbone inorganique (CI) contenus dans les eaux douces naturelles développé surl'analyseur Shimadzu©, modèle TOC 5050A.Les limites de détection calculées pour cet analyseur avec un catalyseur de sensibilitédite normale sont de 0.31 ppm pour le COD et de 0.10 ppm pour le CI, respectivement. Leslimites de quantification sont logiquement plus élevées: 0.41 ppm (COD) et 0.13 ppm (CI).Ces résultats permettent de conclure que le catalyseur dit de "sensibilité normale" qui équipeen routine cet analyseur est adapté à l'analyse de la plupart des eaux douces naturelles.Les tests d'étalonnage effectués ont montré que l'appareil Shimadzu© TOC 5050A étaittrès stable dans le temps, les dérives constatées étant inférieures à 5%. De ce fait, uneprocédure allégée d'étalonnage comprenant l'injection d'un seul point de gamme en début dechaque série d'échantillons suffit à garantir une bonne justesse des résultats, même si celle-ciest difficile à quantifier du fait de l'absence de solutions standard certifiées.Les trois méthodes susceptibles d'être mises en oeuvre par l'analyseur Shimadzu©, TOC5050A (COD = CT – CI; NPOC, NPIW) ont été testées. Seules la méthode COD = CT – CIpermet de doser la totalité du compartiment "matière organique dissoute" des eaux doucesnaturelles. Les deux autres ne permettent pas d'appréhender les molécules les plus volatiles,les écarts entre valeurs "vraies" et valeurs "mesurées" pouvant aller jusqu'à 25%.Des expériences visant à tester les modalités de préparation et de conservation deséchantillons ont également été effectuées. Les résultats montrent que pour des eaux peuchargées en matière organique (COD 5%) entre valeurs mesurées et valeurs vraies. Dans ce cas,l'opérateur soucieux de produire des résultats justes sera conduit à filtrer les eaux directementsur le terrain et à réduire au maximum l'intervalle de temps entre le prélèvement et l'analyse

Higher-order Kerr terms allow ionization-free filamentation in gases

We show that higher-order nonlinear indices ($n_4$, $n_6$, $n_8$, $n_{10}$) provide the main defocusing contribution to self-channeling of ultrashort laser pulses in air and Argon at 800 nm, in contrast with the previously accepted mechanism of filamentation where plasma was considered as the dominant defocusing process. Their consideration allows to reproduce experimentally observed intensities and plasma densities in self-guided filaments.Comment: 11 pages, 6 figures (11 panels

Mobile source of high-energy single-cycle terahertz pulses

The Teramobile laser facility was used to realize the first mobile source of high-power THz pulses. The source is based on a tilted-pulse-front pumping THz generation scheme optimized for application of terawatt laser pulses. Generation of 50-μJ single-cycle electromagnetic pulses centered at 0.19 THz with a repetition rate of 10Hz was obtained for incoming 700-fs 120-mJ near-infrared laser pulses. The corresponding laser-to-THz photon conversion efficiency is approximately 100

Age and sex affect intersubject correlation of EEG throught development

Recent efforts have aimed to characterize clinical pediatric populations by using neurophysiological tests in addition to behavioral assays. Here we report on a data collection effort in which electroencephalography (EEG) was recorded in both juveniles and adults (N=114 participants, ages 6-44 years of age) during various stimulation protocols. The present analysis focuses on how neural responses during passive viewing of naturalistic videos vary with age and sex, and in particular, how similar they are within developmental groups. Similarity of neural responses was measured as the inter-subject correlation of the EEG. Stimulus-evoked neural responses are more similar among children and decrease in similarity with age. Among children, males respond more similarly to each other than females. This was uniformly true for a variety of videos. The decrease in group similarity with age may result from an overall decline in the magnitude of evoked responses, but this cannot explain the sex differences found in the young. We therefore propose that as children mature, neural function may become more variable

Learning hierarchical sequence representations across human cortex and hippocampus

Sensory input arrives in continuous sequences that humans experience as segmented units, e.g., words and events. The brain’s ability to discover regularities is called statistical learning. Structure can be represented at multiple levels, including transitional probabilities, ordinal position, and identity of units. To investigate sequence encoding in cortex and hippocampus, we recorded from intracranial electrodes in human subjects as they were exposed to auditory and visual sequences containing temporal regularities. We find neural tracking of regularities within minutes, with characteristic profiles across brain areas. Early processing tracked lower-level features (e.g., syllables) and learned units (e.g., words), while later processing tracked only learned units. Learning rapidly shaped neural representations, with a gradient of complexity from early brain areas encoding transitional probability, to associative regions and hippocampus encoding ordinal position and identity of units. These findings indicate the existence of multiple, parallel computational systems for sequence learning across hierarchically organized cortico-hippocampal circuits

The Variability of Neural Responses to Naturalistic Videos Change with Age and Sex

Neural development is generally marked by an increase in the efficiency and diversity of neural processes. In a large sample (n=114) of human children and adults with ages ranging from 5 to 44 yr, we investigated the neural responses to naturalistic video stimuli. Videos from both real-life classroom settings and Hollywood feature films were used to probe different aspects of attention and engagement. For all stimuli, older ages were marked by more variable neural responses. Variability was assessed by the intersubject correlation of evoked electroencephalographic responses. Young males also had less-variable responses than young females. These results were replicated in an independent cohort (n = 303). When interpreted in the context of neural maturation, we conclude that neural function becomes more variable with maturity, at least during the passive viewing of real-world stimuli.Fil: Petroni, Agustín. City University of New York; Estados Unidos. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Cohen, Samantha S.. City University of New York; Estados UnidosFil: Ai, Lei. City University of New York; Estados UnidosFil: Langer, Nicolas. City University of New York; Estados UnidosFil: Henin, Simon. City University of New York; Estados UnidosFil: Vanderwal, Tamara. City University of New York; Estados UnidosFil: Milham, Michael P.. City University of New York; Estados UnidosFil: Parra, Lucas C.. City University of New York; Estados Unido

On negative higher-order Kerr effect and filamentation

As a contribution to the ongoing controversy about the role of higher-order Kerr effect (HOKE) in laser filamentation, we first provide thorough details about the protocol that has been employed to infer the HOKE indices from the experiment. Next, we discuss potential sources of artifact in the experimental measurements of these terms and show that neither the value of the observed birefringence, nor its inversion, nor the intensity at which it is observed, appear to be flawed. Furthermore, we argue that, independently on our values, the principle of including HOKE is straightforward. Due to the different temporal and spectral dynamics, the respective efficiency of defocusing by the plasma and by the HOKE is expected to depend substantially on both incident wavelength and pulse duration. The discussion should therefore focus on defining the conditions where each filamentation regime dominates.Comment: 22 pages, 11 figures. Submitted to Laser physics as proceedings of the Laser Physics 2010 conferenc