Determination of iridium concentration in sedimentary rocks and in the geochemical standard PCC-1 by radiochemical neutron activation analysis

A radiochemical neutron activation analysis procedure is described for the determination of concentration levels of iridium in sedimentary rocks and in the geochemical standard PCC-1. After irradiation, the powdered rock samples and standard are dissolved with a mixture of HF, HNO₃ and HClO₄ in a teflon bomb. The final solution obtained, in dilute HCl, is passed through a column containing the cationic resin Bio Rad AG 50W-X8. The interfening radionuclides are sorbed by the resin. The effluent solution containing iridium is concentrated for counting by evaporation. Experiments with radioactive tracer for checking radiochemical separation yield are carried out. The accuracy of the method is evaluated by means of analysis of the USGS “standard rock” peridotite, PCC-1. The precision is about 25% in the ppb range. The method is used for the determination of iridium in 16 samples of sedimentary rocks collected at different depths in the “Campos” basin-RJ (BRAZIL).     

Mössbauer spectroscopy on the double substituted lithium ferrite Li[Fe0.9(AlxGa1−x)0.1]5O8

Room temperature Mössbauer spectra of Ga and Al substituted lithium ferrite Li[Fe_0.9(Al_xGa_1?x)_0.1]₅O₈ with x = 0.00, 0.25, 0.50, 0.75 and 1.00 are reported. It is shown that the varying covalence of bonds caused by gallium and aluminium ions can explain the observed values of hyperfine fields and isomer-shifts.     

Fixed points of the dissipative Hofstadter model

The phase diagram of a dissipative particle in a periodic potential and a magnetic field is studied in the weak barrier limit and in the tight binding regime. For the case of half flux per plaquette, and for a wide range of values of the dissipation, the physics of the model is determined by a nontrivial fixed point. A combination of exact and variational results is used to characterize this fixed point. Finally, it is also argued that there is an intermediate energy scale that separates the weak coupling physics from the tight binding solution.     

U and Th thin film neutron dosimetry for fission-track dating: application to the age standard Moldavite

Neutron dosimetry based on U and Th thin films was used for fission-track dating of the age standard Moldavite, the central European tektite, from the Middle Miocene deposit of Jankov (southern Bohemia, Czech Republic). Our fission-track age () agrees with a recent ⁴⁰Ar/³⁹Ar age, , based on several determinations on Moldavites from different sediments, including the Jankov deposit. This result indicates that the U and Th thin film neutron dosimetry represents a reliable alternative for an absolute approach in fission-track dating.     

Interaction of meso-tetrakis (4-sulfonatophenyl) porphyrin with cationic CTAC micelles investigated by small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR)

Small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) have been used to investigate the interaction of the water-soluble meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS(4)) with cationic cethyltrimethylammonium chloride (CTAC) micelles. To evaluate if the porphyrin protonation state affects its interaction with the micelle, both SAXS and EPR measurements were performed at pH 4.0 and 9.0. The best-fit SAXS curves were obtained assuming for CTAC micelle a prolate ellipsoidal shape in the absence and upon incorporation of 2-10 mM TPPS(4). SAXS results show that the presence of porphyrin impacts on micellar hydrophobic core, leading to a micellar reassembling into smaller micelles. Lineshapes of EPR spectra of 5- and 16-doxyl stearic acids (5- and 16-DSA, respectively) bound to 100 mM CTAC micelles exhibited slight changes as a function of porphyrin concentration. Spectral simulations revealed an increase of mobility restriction for both spin probes, especially at higher porphyrin concentration, where a small reduction of environment polarity was also observed for 16-DSA. The spin labels monitored only slight differences between pH 4.0 and 9.0, in agreement with the SAXS results.     

A chemometric analysis of ab initio vibrational frequencies and infrared intensities of methyl fluoride

Factorial design and principal component analyses are applied to CH₃F infrared frequencies and intensities calculated from ab initio wave functions. In the factorial analysis, the quantitative effects of changing from a 6–31G to a 6–311G basis, of including polarization and diffuse orbitals, and of correcting for electron correlation using the second-order Møller-Plesset procedure are determined for all frequencies and intensities. The most significant main effect observed for the frequencies corresponds to the shift from Hartree-Fock to MP2 calculations, which tends to lower all frequency values by approximately 100 cm⁻¹. For the intensities, the main effects are larger for the CF stretching and the CH₃ asymmetric stretching modes. Interaction effects between two or more of the four factors are found to be of minor importance, except for the interaction between correlation and polarization. The principal component analysis indicates that wave functions with polarization and diffuse orbitals at the second-order Møller-Plesset level provide the best estimates for the harmonic frequencies, but not for the intensities. For the frequencies, the first principal component distinguishes between MP2 and Hartree-Fock calculations, while the second component separates the wave functions with polarization orbitals from those without these orbitals. For the intensities, the separation is similar but less well defined. This analysis also shows that wave function optimization to calculate accurate intensities is more difficult than an optimization for frequencies. © 1996 by John Wiley & Sons, Inc.     

Biosensor for phenol based on the direct electron transfer blocking of peroxidase immobilising on silica–titanium

Horseradish peroxidase (HRP) was immobilised on silica gel modified with titanium oxide. This material was employed to prepare modified carbon paste electrode. The direct electron transfer of the hydrogen peroxide reduction by HRP was blocked when immobilised on silica–titanium. This biosensor presented a very sensitive response for phenol (1 μmol l⁻¹) at an applied potential of 0 mV vs SCE. The best condition was achieved in phosphate buffer pH 6.8, ratio of hydrogen peroxide/phenol higher than 0.35. The biosensor showed a linear response range between 10 and 50 μmol l⁻¹ of phenol, adjusted by the equation j=−32.8+16.3 [phenol], for n=5 with a correlation coefficient of 0.9995. The response time of the biosensor was about 3 s.     

Determination of salicylate in blood serum using an amperometric biosensor based on salicylate hydroxylase immobilized in a polypyrrole-glutaraldehyde matrix

The use of an amperometric biosensor for the salicylate determination in blood serum is described. The biosensor is based on salicylate hydroxylase (EC 1.14.13.1) electropolymerized onto a glassy carbon-working electrode with polypyrrole and glutaraldehyde, to improve the biosensor lifetime. The hexacyanoferrate (II) was also incorporated to work as a redox mediator to minimize possible interferences. The salicylate is enzymatically converted to catechol, which is monitored amperometrically by its electrooxidation at+0.170 V versus SCE (saturated calomel electrode). Salicylate determination was carried out maintaining the ratio between beta-NADH and salicylate at 4:1 (30 degrees C). The amperometric response of the biosensor was linearly proportional to the salicylate concentration between 2.3x10(-6) and 1.4x10(-5) mol l(-1), in 0.1 mol l(-1) phosphate buffer (pH 7.8), containing 0.1 mol l(-1) KCl and 5.0x10(-4) mol l(-1) Na(2)H(2)EDTA, as supporting electrolyte. The recovery studies, in the presence of several interfering compounds, showed recoveries between 96.4 and 104.8%. The useful lifetime of the biosensor in the concentration range evaluated was at least 40 days, in continuous use. Blood serum samples analyzed by this biosensor showed a good correlation compared to the spectrophotometric method (Trinder) used as reference, presenting relative deviations lower than 7.0%.     

Ion recognition properties of poly[Cu(3-MeOsalpd)] films

Poly[Cu(3-MeOsalpd)] films with good physical, chemical and electrochemical stability may be potentiodynamically electrodeposited with high deposition efficiency from acetonitrile solutions of the monomer. Comparative coulometric assays with the Ni-based analogue show that the metal in the salen motif does play a role in the electronic structure of the polymer, but that the electroactive response is ligand (not metal) based. The dynamics of redox switching are ultimately limited by coupled electron/counter ion diffusion, but this process is sufficiently rapid that it influences the voltammetric response only for thick films (Γ >420?nmol?cm^?2) at high scan rates. Redox cycling in monomer-free electrolyte shows a voltammetric signature that responds, via interaction with the pseudo-crown ether receptor sites, to the presence of Li⁺, K⁺, Mg²⁺ and Ba²⁺ ions in solution. The most prominent change is associated with the first anodic peak in the i-E signature. For each of the metal ions considered, this peak potential responds logarithmically to concentration in a manner that varies with individual complexed cation and film thickness and to an extent greater than predicted by the Nernst equation. The film characteristics offer some analytical promise, including a trade-off between sensitivity and dynamic range and signal amplification, possibly due to supramolecular effects.