Resolver el problema abierto : teñir lanas a partir de productos colorantes naturales : una actividad investigativa para la enseñanza secundaria obligatoria

We present in this paper a class work for solving investigative problems, inserted into the theoretical constructivist context. We have chosen the topic «Dye wool with natural dyers», for its relevance in Organic Chemistry and also for its cultural and social interest, showing an actual perspective of Science. On the other hand, we develop the class performance with emphasis in assessment

Excitability in a nonlinear magnetoacoustic resonator

We report a nonlinear acoustic system displaying excitability. The considered system is a magnetostrictive material where acoustic waves are parametrically generated. For a set of parameters, the system presents homoclinic and heteroclinic dynamics, whose boundaries define a excitability domain. The excitable behaviour is characterized by analyzing the response of the system to different external stimuli. Single spiking and bursting regimes have been identified.Comment: 4 pages, 5 figure

Preventing false negatives with high-resolution mass spectrometry: the benzophenone case

Benzophenone (BP) is one of the many contaminants reported as present in foodstuff due to its migration from food packaging materials. Liquid chromatography tandem mass spectrometry (LC-MS/MS) is acknowledged in the literature as the method of choice for this analysis. However, cases have been reported where the use of this methodology was not enough to unambiguously confirm the presence of a contaminant. In previous work performed by the authors, the unequivocal identification of BP in packaged foods was not possible even when monitoring two m/z transitions, since ion ratio errors higher than 20% were obtained. In order to overcome this analytical problem a fast, sensitive and selective liquid chromatography-high resolution-mass spectrometry (LC-HRMS) methodology has been developed and applied to the analysis of BP in packaged foods. A direct comparison between liquid chromatography high resolution mass spectrometry (LC-HRMS) and LC-MS/MS data indicated better selectivity when working with LC-HRMS at a resolving power of 50,000 FWHM than when monitoring two m/z transitions by LC-MS/MS. The resolving power used enabled the detection and identification of Harman as the compound impeding the confirmation of BP by LC-MS/MS. Similar quantitative results were obtained by an Orbitrap mass analyser (Exactive ¿) and a triple quadrupole mass analyser (TSQ Quantum Ultra AM ¿)

Atmospheric pressure photoionization mass spectrometry of fullerenes

Atmospheric pressure photoionization (APPI) was evaluated for the analysis of fullerenes. An important response improvement was found when using toluene mediated APPI in negative mode if compared with other API sources (electrospray and atmospheric pressure chemical ionization). Fullerene APPI negative mass spectra were dominated by the isotopic cluster of the molecular ion, although isotopic patterns for M+1, M+2 and M+3 ions showed higher than expected relative abundances. These discrepancies are explained by the presence of two isobaric ions, one due to 13C and the other to the addition of hydrogen to a double bond of the fullerene structure. Triple quadrupole tandem mass spectrometry and ultra-high resolution mass spectrometry and accurate mass measurements were used to confirm these assignments. Additionally, cluster ions M+16 and M+32 were characterized following the same strategy. Ions due to the addition of oxygen and alkyl additions were attributed to the presence of methanol in the mobile phase. For the fast chromatographic separation of fullerenes (less than 3.5 min) a sub-2 µm C18 column and isocratic elution (toluene:methanol 45:55 v/v) was used. Highly selective-selected ion monitoring (H-SIM) mode (mass resolving power >12,500 FWHM) was proposed monitoring the two most intense isotope ions in the [M]-¿ cluster. Method limits of quantitation down to 10 pg L-1 for C60 and C70 fullerenes and between 0.75-5.0 ng L-1 for larger fullerenes were obtained. Finally, the UHPLC-APPI-MS method was used to analyze fullerenes in river and pond water samples

Identificación de competencias y características deseables en el profesorado de ciencias de EGB

This article offers the vision held by some groups of teachers on various qualities taken as important for a Science Teacher of Primary Education, as well as the opinion on to what extend those qualities are found in this type of teachers

Self-organization of ultrasound in viscous fluids

We report the theoretical and experimental demonstration of pattern formation in acoustics. The system is an acoustic resonator containing a viscous fluid. When the system is driven by an external periodic force, the ultrasonic field inside the cavity experiences different pattern-forming instabilities leading to the emergence of periodic structures. The system is also shown to possess bistable regimes, in which localized states of the ultrasonic field develop. The thermal nonlinearity in the viscous fluid, together with the far-from-equilibrium conditions, are is the responsible of the observed effects

Two and three-dimensional oscillons in nonlinear Faraday resonance

We study 2D and 3D localised oscillating patterns in a simple model system exhibiting nonlinear Faraday resonance. The corresponding amplitude equation is shown to have exact soliton solutions which are found to be always unstable in 3D. On the contrary, the 2D solitons are shown to be stable in a certain parameter range; hence the damping and parametric driving are capable of suppressing the nonlinear blowup and dispersive decay of solitons in two dimensions. The negative feedback loop occurs via the enslaving of the soliton's phase, coupled to the driver, to its amplitude and width.Comment: 4 pages; 1 figur

Lasing without inversion in three-level systems : self-pulsing in the cascade schemes

Lasing without inversion (LWI) in specific models of closed three-level systems is analyzed in terms of nonlinear dynamics. From a linear stability analysis of the trivial nonlasing solution of the homogeneously broadened systems with on-resonance driving and laser fields, we find that, near lasing threshold, resonant closed Λ and V schemes yield continuous-wave LWI while resonant cascade schemes can give rise to self-pulsing LWI. The origin of this different behavior is discussed. For parameters of a real cascade system in atomic 138Ba we check numerically that the self-pulsing solution is stable in a broad range of nonzero detunings. It is shown that the self-pulsing emission can still be observed when the typical residual Doppler broadening of an atomic beam is taken into account