Modification of the methylene blue method for spectrophotometric selenium determination

The spectrophotometric method for Se(IV) determination based on its catalytic effect in the reduction of Methylene Blue (MB) by sulphide is modified. The variables that affect the decolorization of MB were taken into account: reagent concentrations, order of addition, mixing and standing times, pH, ionic strength, temperature, solution volume, wavelength, etc. The results of this study allowed a decrease of the determination limit and, by selection of the appropriate analytical conditions, choice of optimum linear range according to the selenium content in the sample. The lower limit range is 15-75 mug/1., with 3% relative standard deviation and no systematic errors. Procedures for overcoming several potential interferences were studied. The proposed method was applied to several environmental samples and the results were compared with those obtained by other standard methods.     

Intramolecular interactions in the triplet excited states of benzophenone-thymine dyads

Time-resolved and product studies on the synthesized dyads 1 and 2 have provided evidence that the benzophenone-to-thymine orientation strongly influences intramolecular photophysical and photochemical processes. The prevailing reaction mechanism has been established as a Paterno-Büchi cycloaddition to give oxetanes 3-6; however, the ability of benzophenone to achieve a formal hydrogen abstraction from the methyl group of thymidine has also been evidenced by the formation of photoproducts 7 and 8. These processes have been observed only in the case of the cisoid dyad 1. Adiabatic photochemical cycloreversion of the oxetane ring is achieved upon direct photolysis to give the starting dyad 1 in its excited triplet state. The photobiological implications of the above results are discussed with respect to benzophenone-photosensitized damage of thymidine.     

On the Concentration Dependence of the Cluster Fractal Dimension in Colloidal Aggregation

We have undertaken the task to calculate, by means of extensive numerical simulations and by different procedures, the cluster fractal dimension (d) of colloidal aggregates at different initial colloid concentrations. Our first approach consists in obtaining d from the slope of the log-log plots of the radius of gyration versus size of all the clusters formed during the aggregation time. In this way, for diffusion-limited colloidal aggregation, we have found a square root type of increase of the fractal dimension with concentration, from its zero-concentration value: d = d⁰ _f + a , with d⁰ _f = 1.80 ± 0.01, a = 0.91 ± 0.03 and = 0.51 ± 0.02, and where is the volume fraction of the colloidal particles. In our second procedure, we get the d via the particle-particle correlation function g_cluster(r) and the structure function S_cluster(q) of individual clusters. We first show that the stretched exponential law g_cluster(r) = Ar^{d –3}e^–(r/) gives an excellent fit to the cutoff of the g(r). Here, A, a and are parameters characteristic of each of the clusters. From the corresponding fits we then obtain the cluster fractal dimension. In the case of the structure function S_cluster (q), using its Fourier transform relation with g_cluster(r) and introducing the stretched exponential law, it is exhibited that at high q values it presents a length scale for which it is linear in a log-log plot versus q, and the value of the d extracted from this plot coincides with the d of the stretched exponential law. The concentration dependence of this new estimate of d, using the correlation functions for individual clusters, agrees perfectly well with that from the radius of gyration versus size. It is however shown that the structure factor S(q) of the whole system (related to the normalized scattering intensity) is not the correct function to use when trying to obtain a cluster fractal dimension in concentrated suspensions. The log-log plot of S(q) vs. q proportions a value higher than the true value. Nevertheless, it is also shown that the true value can be obtained from the initial slope of the particle-particle correlation function g(r), of the whole system. A recipe is given on how to obtain approximately this g(r) from a knowledge of the S(q), up to a certain maximum q value.     

Catalytic performance of Fe-modified AlPO4 catalyst in cyclohexene skeletal isomerization

Iron-modified AlPO₄ catalysts (0.5–5 wt.% Fe₂O₃) have been prepared by impregnation until incipient wetness with different iron(III) salts. The resulting solids, after characterization by surface area, XRD and DRIFT measurements, were checked for their surface acidity through their catalytic activity towards cyclohexene skeletal isomerization test reaction. It was found that iron(III) increases the catalytic performance of AlPO₄ and besides, the iron(III) salt used strongly influences the activity. Thus, the catalysts prepared using complexed iron(III) salts as starting materials presented higher specific activity (and hence, acidity) than those prepared using iron(III) nitrate. Moreover, a maximum in activity was found for 2 wt.% Fe₂O₃.     

Calibration of the LNMRI Secondary Standard Ionization Chamber for 131I capsules used in nuclear medicine

The radionuclide ¹³¹I has been increasingly used in nuclear medicine therapy procedures. Nowadays, the ¹³¹I source administered to the patient is manufactured in two different geometries: solution and capsules. The purpose of this study is the accurate measurement of the activity present in a ¹³¹I capsule without destroys it. The methodology to determine the capsules activity is to obtain the calibration factor of an IG12 secondary standard activity measurement system based on the IG12 well-type ionization chamber set up at Brazilian national metrology laboratory for ionizing radiation (LNMRI) of institute of radiation protection and dosimetry (IRD).The result obtained, 6.4670?±?0.0381?×?10^?18?A?Bq^?1, is quite similar to the calibration factor of the ¹³¹I solution contained in the standard ampoule geometry, 6.4515?±?0.0368?×?10^?18?A?Bq^?1. After obtaining the calibration factor it was used to measure ¹³¹I therapy capsules in order to check the performance of radionuclide calibrators of some Brazilian nuclear medicine centers.     

Measurements and correlation of vapour-liquid equilibria of 2-butanone and hydrocarbons binary systems at two different pressures

Consistent isobaric vapour-liquid equilibrium data have been measured for 2-butanone + n-hexane, 2-butanone + n-heptane, and 2-butanone + 2,2,4-trimethylpentane at two different pressures. All binary systems present a minimum boiling azeotrope at both pressures, and show that the azeotropic compositions are weakly dependent on pressure. The equilibrium data were correlated using the Wilson, NRTL, and UNIQUAC models for which the parameters are reported.     

Ionic liquid matrices for MALDI-TOF-MS analysis of intact glycoproteins

2,5-Dihydroxybenzoic acid (DHB) has been demonstrated to be a more suitable matrix than 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid, SA) to obtain reliable molecular mass values of intact glycoproteins because it prevents sugar fragmentation. Lack of spot homogeneity during the crystallization step was prevented by drying the sample-matrix mixture under vacuum conditions. Nevertheless, this sample-matrix preparation procedure requires a specific experimental setup and may be time-consuming. In this work, we investigated the effectiveness of different ionic liquid matrices (ILMs) with SA and DHB on the ionization of a set of intact glycoproteins with several degrees of glycosylation. The obtained results demonstrate that some of the tested ILMs allow detection of the studied intact glycoproteins. Furthermore, the selected optimum conditions solve the reproducibility issue of using the DHB as a solid matrix without the vacuum drying method and, surprisingly, avoid sugar fragmentation when both SA and DHB were used as ILMs.     

Theoretical insight into the spectroscopy and photochemistry of isoalloxazine, the flavin core ring

The electronic singlet-singlet and singlet-triplet electronic transitions of the isoalloxazine ring of the flavin core are studied using second-order perturbation theory within the framework of the CASPT2//CASSCF protocol. The main features of the absorption spectrum are computed at 3.09, 4.28, 4.69, 5.00, and 5.37 eV. The lowest singlet (S1) and triplet (T1) excited states are found to be both of pi character with a singlet-triplet splitting of 0.57 eV. On the basis of the analysis of the computed spin-orbit couplings and the potential energy hypersurfaces built for the relevant excited states, the intrinsic mechanism for photoinduced population of T1 is discussed. Upon light absorption, evolution of the lowest singlet excited state along the relaxation pathway leads ultimately to the population of the lowest triplet state, which is mediated by a singlet-triplet crossing with a state of npi* type. Subsequently a radiationless decay toward T1 through a conical intersection takes place. The intersystem crossing mechanism and the internal conversion processes documented here provide a plausible route to access the lowest triplet state, which has a key role in the photochemistry of the flavin core ring and is mainly responsible for the reactivity of the system.     

Birkhoff‐Frink representations as functors

In an earlier article we characterized, from the viewpoint of set theory, those closure operators for which the classical result of Birkhoff and Frink, stating the equivalence between algebraic closure spaces, subalgebra lattices and algebraic lattices, holds in a many‐sorted setting. In the present article we investigate, from the standpoint of category theory, the form these equivalences take when the adequate morphisms of the several different species of structures implicated in them are also taken into account. Specifically, our main aim is to provide a functorial rendering of the Birkhoff‐Frink representation theorems for both single‐sorted algebras and many‐sorted algebras, by defining the appropriate categories and functors, covariant and contravariant, involved in the process (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Effect of deposited bismuth on the potential of maximum entropy of Pt(111) single-crystal electrodes

The effect of bismuth adsorption on the entropy of formation of the double layer on Pt(111) electrodes has been studied with the laser-induced temperature jump method. The coulostatic response to the temperature change induced by pulsed laser illumination allows the estimation of the sign and magnitude of the thermal coefficient of the potential drop at the interphase. This is related to the entropy of formation of the double layer, and the particular potential where this thermal coefficient becomes zero can be identified with the potential of maximum entropy of double-layer formation (pme). The effect of bismuth adsorption on the pme depends on the adatom coverage. At high coverages, a marked decrease of the pme is observed. This trend follows the change of the potential of zero charge expected from work function measurements, and it is likely due to the change in the orientation of solvent molecules induced by surface dipoles originated between the adatom and the substrate. At low coverage, the pme increases with the bismuth coverage. The disruption of the water structure due to the presence of the bismuth adatoms is tentatively proposed as the most likely explanation for this behavior.