Héctor Gutiérrez

Effects of atomization surfaces and modifiers on the electrothermal atomization of cadmium

Abstract Atomization of cadmium on tungsten carbide, molybdenum carbide and metallic palladium coated platforms and on pyrolytic graphite coated platforms with and without tungsten, molybdenum and palladium modifiers has been studied. A physical vapour deposition method is used for coating. A high thermal stability, a greater appearance temperature and a decrease in sensitivity are found when compared with atomization on conventional platforms. Diffusion and kinetic parameters are obtained using different models. More than one activation energy (Ea) and frequency factor (v) are obtained depending on the atomization temperature while the atomic vapour appears. These results may indicate that more than one formation process is simultaneously present or that the evolution from one formation process to another is temperature dependent. Greater values of Ea and v are found during atomization on the molybdenum carbide coated platform and when molybdenum or palladium modifiers are used. This may suggest that strong cadmium-metal interaction is present. This strong interaction is not present when palladium is used as a metallic coating, although a similar trend in thermal stability is observed. The contribution of the expansion to the atomic vapour dissipation is also evaluated.

Effects of atomization surfaces and modifiers on the kinetics of copper atomization in electrothermal atomic absorption spectrometry

Abstract Atomization of copper from different atomization surfaces and in the presence of chemical modifiers has been investigated. The atomization surfaces are molybdenum and tungsten carbide-coated platforms obtained by physical vapour deposition and pyrolytic graphite coated platforms. Molybdenum, tungsten and palladium have been used as chemical modifiers. The kinetic parameters of copper atomization in electrothermal atomic absorption spectrometry have been determined by three different methods. Although in all cases, differences in the atomization profiles are observed, the appearance temperatures when the carbide-coated platform and chemical modifiers are used, are not significantly different from those observed with conventional pyrolytic graphite platforms. Nevertheless, a longer tailing of the atomization profile and a slight thermal stabilization of the copper atomic vapour are found when tungsten and palladium modifiers are employed. In these later cases, two different activation energies are determined for each absorbance profile by the different models employed, indicating the presence of two possible precursors in the atomization mechanism. The activation energy ( E a ) values calculated in these cases are 75–104 kJ mol −1 when the atomic vapour starts to appear and 170–300 kJ mol −1 at high temperatures. The lower E a values are comparable to the single E a value obtained with pyrolytic graphite platform and the other cases studied, in which a single precursor mechanism is proposed. In these cases, the copper atomic vapour seems to desorb from the surface as highly dispersed atoms. When the palladium modifier is employed, the high E a and changes in the reaction order found at high temperatures, would indicate that the atomic vapour is additionally originating from the evaporation of aggregates. These aggregates could be formed by redistribution and agglomeration at higher temperatures and possibly trapping by the modifier. In the case of the tungsten modifier, dispersed atoms present in the cooler places of the platform could be responsible for the apparent increase of the E a value found. This second release could provide the longer tailing observed in the absorbance profiles when palladium and tungsten modifiers are used. A first-order kinetic release found in some of the cases studied could suggest a strong interaction with the surface; however, the low frequency factor value found gives no evidence for any substrate-analyte interaction in any of the cases studied.

Development and testing of a multiwell plates absorbance reader for clinical analysis using inexpensive webcam

Biochemical analysis and clinical tests like glucose, hemoglobin, cholesterol, iron, etc. are crucial for early illness diagnosis like diabetes, anemia and coronary deceases. These tests usually are done in state of the art instruments in well equipped laboratories in health centers. In some cases, these instruments are not portable, so they are not recommended for clinical field studies in remote areas. The present work shows a portable low-cost prototype of multi-well plates reader designed for clinical analysis. A Light Emission Diodes (LEDs) array is used as excitation source and an inexpensive webcam as detector. The light source illuminates the 96 well plates and the webcam take the image with 640x480 pixels. The data is acquired and processed by using a portable computer. 96 samples can be read including blanks and calibration standards simultaneously. Light absorption data are processed using a MatLab software designed in our laboratory to obtain calibration curves, standards lectures and samples concentration. The system was evaluated using different analytes series solutions: Neutral Red, Cooper (II) Ammonia Complex and Methyl Orange. The results shows that it is possible to measure few micro liters of solutions with adequate exactitude and precision of less than 3%. As possible analytical clinical application, iron determination was performed using Fe(III) Thiocyanate complex. This method is usually applied in serum samples analysis. The sensibility achieved with the proposed instrumentation configurations allows the analysis of iron in serum samples in the references values normal range (0.75 - 1.5 mg/L) in human.

Inexpensive Wilhelmy balance based in a fiber optic sensor for the study of Langmuir films

An inexpensive Wilhelmy balance based on a fiber optic sensor capable of sensitive surface tension measurements has been designed and implemented. The system consists of a leaf spring conforming a cantilever structure and a bifurcated optical fiber acting as a laser beam deflection detector. Operated in a static way, it achieves a force measurement sensitivity of 0.154 V by N and a tension surface resolution of 0.1 mN/m. π-A isotherms of Langmuir films from insoluble amphiphiles 5 hexadecanoylaminofluorescein (fluorescein H-110) in water, were followed as a model system to characterize this instrument.

Concentration dependence of surface nonlinear susceptibility of vanadyl-porphyrin at silica surfaces

Resonant molecular optical second harmonic generation (SHG) has been obtained from [2, 3, 7, 8, 12, 13, 17, 18- Octaethyl-porphinato] vanadyl, (OEP)V=O, adsorbed on the hypotenuse face of a glass prism. Rotational intensity patterns are showed by plotting the second harmonic intensity as a function of the incident polarization angle of the fundamental wave, at a fixed angle of incidence. Evaluation of these patterns by Fourier analysis allowed the determination of the second order susceptibilities components of the adsorbed layers. Assuming a delta distribution function, the mean orientation angles from normal to the surface plane, were obtained for different solution concentrations. Our results indicate an abrupt change in the angle for two concentration ranges.

Comparative study of the nonlinear optical surface response of metal-substituted porphyrins adsorbed on glass

A comparative study of the second harmonic generation at the M(H2,Ni(II),V=O)-porphyrin/glass interface in a total reflection geometry is presented. The state of polarization of the incident pulses was changed by a computer controlled rotation of a quarter wave plate. The rotation patterns obtained from the experimental data were fit to a Fourier series based upon theoretical Fourier coefficients. Complex valued components of the superficial susceptibility tensor are showed. From these values, average tilt angles concentration dependent were obtained. A different behavior for each Metalloporphyrin was observed.

Spectroscopic studies of asphaltenes

Molecular absorption and fluorescence studies of Asphaltenes in toluene solution and thin films absorbed onto glass plates were carried out. The fluorescence was induced by two different type of laser: a CW Ar+ and Nd-Yag laser. A red shifted was observed in both: fluorescence and absorption spectra, which can be attributed to asphaltenes aggregation. Experiments made with sample films in glass plate with the 532 nm Nd-Yag laser, shows a very narrow band at 566.1 nm (1126 cm-1) but with higher relative intensity compare to those obtained with CW laser, which is attributed to a Raman line. This signal can be employed to perform adsorption studies onto solid surfaces.

Application of forward-angle light scattering to measured flocculation point of asphaltenes and evaluation of flocks dimensions

In this work we present an application of the forward-angle light scattering (FAST) for the flocculation point determination in asphaltene-toluene solutions. This technique allows to obtain an accurate and precise flocculation point determination compare to those obtained by traditional techniques. Estimations of flock dimensions based in the FAST measurements are presented.

Trace Level Vanadium Determination Using Thermal Lens Spectrophotometry

Trace level vanadium determination is reported using a dual beam thermal lens spectrometer. The thermal lens was generated using an argon ion beam laser (pump beam) which was focused into a sample cuvette. The thermal lens signal (TLs) was monitored with a He-Ne laser beam and a photodiode detector. Multichannel averager software was developed for processing the transient TLs. The optimal set up, ensuring maximum sensitivity and linear calibration graphs was obtained using experimental design techniques. Under optimized conditions, the detection limits for aqueous and ethanol-water (2+3 v/v) and (4+1 v/v) vanadium complex solutions were, respectively, 0.0071 mg/l, 0.0065 and 0.0039 mg/l.