The stability of trace metals suspensions in heavy crudes as determined by neutron activation analysis

The importance of trace elements in petroleum has increased, since the role of nonhydrocarbon components has been recognized in the elucidation of the mechanisms of migration and origin of crudes. On the other hand, knowledge of trace elements in petroleum is also important in the refining and processing of crudes. In developing different instrumental analytical techniques for trace analysis of crudes, little attention has been devoted to the scatter of data due to poor sampling and to the proper nature of the matrix. In the present paper the results of the determination of 17 trace elements including Co, Zn, Fe, V, Ni, Mo, Ba, Cs, Au, Br, Se, Sc, As, Mn, La, Eu and Cu are presented. A multielemental neutron activation analysis in a purely instrumental form was performed on a homogenized sample and the results are compared to those obtained on the same sample after a seven month period. The results clearly show that the crude loses its induced homogeneous nature and that a standard heavy crude can not be stocked for comparison purposes. For the heavy asphaltene fractions, results of the analysis of the same trace elements are presented and the possibility of its use as a standard is discussed.     

Peroxycarbenium-mediated C-C bond formation: applications to the synthesis of hydroperoxides and peroxides

The Lewis acid-mediated reaction of alkene nucleophiles with peroxyacetals provides an effective route for the synthesis of homologated peroxides and hydroperoxides. In the presence of Lewis acids such as TiCl(4), SnCl(4), and trimethylsilyl triflate, peroxyacetals and peroxyketals undergo reaction with allyltrimethylsilane, silyl enol ethers, and silyl ketene acetals to afford homoallyl peroxides, 3-peroxyketones, and 3-peroxyalkanoates, respectively. Reactions of peroxyacetals are Lewis acid dependent; TiCl(4) promotes formation of ethers while SnCl(4) and trimethylsilyl triflate promote formation of peroxides. Lewis acid-promoted reactions of silylated hydroperoxyacetals furnish silylated hydroperoxides, which can be deprotected to homologated hydroperoxides. Hydroperoxyketals undergo Lewis acid-mediated allylation to furnish 1,2-dioxolanes via attack of hydroperoxide on the intermediate carbocation. Lewis acid-mediated cyclization of unsaturated peroxyacetals furnishes 1,2-dioxanes, 1,2-dioxepanes, and 1,2-dioxacanes through 6-endo/exo, 7-endo/endo, and 8-endo/endo pathways. The corresponding reactions involving 6-endo/endo and 5-endo/exo pathways were unsuccessful.     

A simple method for the activation analysis of mercury in fish by using iodinated resins

After irradiation and decay periods, samples are dissolved in 20 min in a sulfuric-nitric acid mixture containing a metavanadate salt. The aqueous phase is neutralized and passed through an iodinated resin which selectively isolates mercury from all other interfering radionuclides. The activity of¹⁹⁷Hg is determined by γ-spectrometry using a thin Na(Tl) detector. A sensitivity limit of 2.5 ppb^* is obtained and the relative standard deviation of the method is 6.7% at a level of 85 ppb.     

Determination of traces of vanadium in catalysts by neutron activation analysis

Determination of vanadium at very low levels in impregnated alumina-based catalysts by neutron activation analysis is reported. A pre-irradiation separation of vanadium, based on methyl isobutyl ketone extraction, yields high decontamination factors for Mo and Al. The technique is simple, precise and quantitative; recoveries of 50 ng of vanadium are obtained with a precision better than 10%. Results obtained for standard reference materials were, on average, within 3% of the reported values. The procedure can be applied satisfactorily to catalysts, metals, alloys, and geological samples. Detection limits of 5 ng can be easily achieved.     

Neural correlates of semantic and syntactic processes in the comprehension of case marked pronouns: Evidence from German and Dutch

Background  

It is well known that both semantic and syntactic information play a role in pronoun resolution in sentences. However, it is unclear what the relative contribution of these sources of information is for the establishment of a coreferential relationship between the pronoun and the antecedent in combination with a local structural case constraint on the pronoun (i.e. case assignment of a pronoun under preposition governing). In a prepositional phrase in German and Dutch, it is the preposition that assigns case to the pronoun. Furthermore, in these languages different overtly case-marked pronouns are used to refer to male and female persons. Thus, one can manipulate biological/syntactic gender features separately from case marking features.     

Microwave heating of ceramic composites

The microwave heating of a ceramic composite is modelled andanalysed. The composite consists of many small ceramic particlesembedded in a ceramic cement. The composite is assumed to bewell insulated, and each particle is assumed to be in imperfectthermal contact with the surrounding cement. Based on thesetwo assumptions an asymptotic theory exploiting the small Biotnumber and small non-dimensional contact conductance is developed.Our asymptotic theory yields a set of nonlinear partial differentialequations which govern the temperature in the composite. Theseare reduced to a set of coupled nonlinear ordinary differentialequations in which the surface area of each particle entersas a parameter. Recent experiments with such composites haveshown that the steady-state temperature of the composite isstrongly dependent upon the radii of the embedded particles.Our model captures this effect. In fact, our analysis showsthat the assumption of imperfect thermal contact between theparticles and the ceramic cement is essential for this trendto be established.