A method based on laser induced breakdown spectroscopy (LIBS) for monitoring lead and copper accumulation in edible fish, particularly "tilapia del Nilo" (Oreochromis niloticus) is presented. The capability of this analytical method is compared with results obtained by atomic absorption spectrometry. Detection limits by LIBS are 25 parts per million (ppm) for Pb and 100 ppm for Cu, values that are below the maximum permissible levels of some international standards. Application of LIBS detection allows the development of portable instruments for contamination control of edible fish.

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TiO 2 films were deposited onto glass substrates by pulsed laser deposition (PLD) in high vacuum under monopulse and multipulse excitation. After deposition, a thermal treatment on air atmosphere was done to promote crystallization. Films were studied by x-ray diffraction, energy dispersive spectroscopy, scanning electron, atomic force microscopy, UV-vis spectroscopy, and ellipsometry. After air annealing, films gain a different amount of oxygen: TiO 1.9 in multipulse regime versus TiO 1.7 in the monopulse one. Splashing is observed in both regimes although in the multipulse mode greater particles are found, that derived in a less compact film after annealing which could be the cause of the better oxygen diffusion. The optical band gap of the film prepared with monopulse excitation is 3.09 eV. This value increased to 3.34 eV with annealing, corresponding to that of anatase. The film made with multipulses has an E g =3.12 eV which was invariant upon annealing. The difference in the properties of the films grown in the different regimes was attributed to the re-excitation of the plasma during the ablation process in the multipulse ablation that leads to an increased splashing density and thereafter a less compact film and the presence of off-stoichiometry inclusions within the film bulk.

Análisis de plasmas inducidos por láser con excitación multipulso para el proceso de ablación de areolas de TunaLaser-induced breakdown spectroscopy analysis with multi-pulse excitation for ablation process of tuna areolas seguimiento en tiempo real de un proceso de ablación láser, incluso para muestras de composición compleja. Para el monitoreo de un proceso industrial, esta herramienta sería más adecuada, si se cuenta con una calibración del equipo y un protocolo particular definido. En el trabajo, se ejemplifica esta posibilidad para el caso de la ablación láser de areolas de Tuna (fruto de el Opuntia ficus-indica).Método: Se realiza el análisis LIBS de muestras de tuna, tanto en las areolas como en la corteza.Para la excitación del plasma se emplea un láser de Nd:YAG en régimen de Q:Switch con tren de pulsos. Con el objeto de asegurar el reconocimiento de las líneas en los espectros LIBS, se realizaron análisis en patrones de composición conocida, estableciéndose un protocolo para la interpretación de los espectros.Resultados: Se logró una clara lectura de la línea de H ubicada en 656.2 nm. Se estableció un protocolo para la interpretación de los espectros, que permitió la identificación de 49 picos característicos. Finalmente, se determina la composición para la corteza y la areola y se lleva a cabo una comparación cualitativa entre éstas.Discusión: Se muestra que la excitación láser en forma de tren de pulsos, entendido este régimen como modalidad particular del Q:Switch, permite la determinación eficiente de elementos, con una adecuada relación señal-ruido. Por otra parte, que es posible determinar diferencias composicionales evidentes entre corteza y la areola, lo cual permite monitorear un proceso de ablación de areolas con láser de manera simple.Palabras claves: Opuntia, plasma, láser, LIBS. Recepción: 16-02-10Aceptación: 04-03-10 Method:The LIBS analysis of tuna samples takes place, both in the areola and in the cortex. For the plasma excitation we uses a Nd: YAG laser with special Q:Switch scheme: The pulse train emission regime. In order to guarantee the lines recognition in the LIBS spectra, before experiments widh samples, we takes the spectrum of well known composition standarts, establishing a protocol for the interpretation of spectra.Results: A clear reading for the H line located at 656.2 nm was obtained. A simple protocol was established for the spectra interpretation, which allowed the identification of 49 characteristic peaks. Finally, the composition to the cortex and the areola were determined and carried out a qualitative comparison between them. Discussion or Conclusion:The results shows that the laser excitation in pulse train form, understood as a particular form of Q: Switch regime, provides an intense plasma emission and efficient elements determination, with adequate signal to noise ratio. Moreover, it is possible to identify compositional differences between crust and areola, which can permits to track the laser ablation of areolas process in a simple way.