Time-resolved fluorescence polarization anisotropy of multimodal samples: the asphaltene case

In this work we describe an application of the time-resolved fluorescence polarization anisotropy (TRFPA) technique to the analysis of asphaltenes, complex mixtures of high-molecular weight compounds, typically present in petroleum oils. Our asphaltene samples consist of nanometer-sized polydispersed particles, whose lighter fraction showed a relatively high fluorescence quantum yield. Most of the fluorescence intensity observed from the complex sample originated from a well defined sample fraction presenting a large fluorescence yield. Consequently, the TRFPA analysis only provided the average size of more fluorescing particles, that, in our case, were the smaller ones. Larger and less fluorescing aggregates did not significantly contribute to the TRFPA signal. Hence, to overcome intrinsic limitations of the TRFPA technique in characterizing multimodal samples, we preliminarily fractionated our complex samples by means of size exclusion chromatography (SEC), thus obtaining nearly monomodal fractions of the original samples. This procedure allowed to estimate also the size of less fluorescing and larger particles. A comparison of particle size estimate by means of TRFPA and SEC methods was also used to acquire information about occurrence of aggregation phenomena, and about the kind and strength of the chemical bonds linking chromophores to each other or to their parent particle. PACS 82.53.Uv; 82.80.Bg; 47.57.E-     

Analysis of trace elements in trunk wood by thick-target PIXE using dry ashing for preconcentration

Thick-target Particle Induced X-ray Emission (TTPIXE) was used for the quantitative determination of trace-element concentrations in trunk wood. The wood samples were preconcentrated by dry ashing to improve the reliability of the sampling and the sensitivity of the analytical method. Samples of Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) were collected from a polluted area (Harjavalta) as well as from a relatively nonpolluted area (Merimasku) in southwestern Finland. The elements studied were P, S, K, Ca, Mn, Fe, Ni, Cu, Pb, Rb, Sr, Ba, Cd and Ag. TTPIXE combined with dry ashing is a sensitive and reliable analytical technique for most elements studied. The method was validified by using several certified reference materials and also by ICP-MS analysis. Due to the low ash content (0.2–0.4%) in wood a high preconcentration factor can be obtained. Differences in trace-element uptake were observed between the two tree species studied. Trunk wood from the polluted area contained higher concentrations of heavy metal ions. Received: 30 August 1996 / Revised: 11 October 1996 / Accepted: 3 November 1996     

Ab initio melting curve of copper by the phase coexistence approach

Ab initio calculations of the melting properties of copper in the pressure range 0-100 GPa are reported. The ab initio total energies and ionic forces of systems representing solid and liquid copper are calculated using the projector augmented wave implementation of density functional theory with the generalized gradient approximation for exchange-correlation energy. An initial approximation to the melting curve is obtained using an empirical reference system based on the embedded-atom model, points on the curve being determined by simulations in which solid and liquid coexist. The approximate melting curve so obtained is corrected using calculated free energy differences between the reference and ab initio system. It is shown that for system-size errors to be rendered negligible in this scheme, careful tuning of the reference system to reproduce ab initio energies is essential. The final melting curve is in satisfactory agreement with extrapolated experimental data available up to 20 GPa, and supports the validity of previous calculations of the melting curve up to 100 GPa.     

Zur elektrolytischen Bestimmung des Mangans, Nickels, Zinks und Bleies

Ohne Zusammenfassung     

Quasi-measures with image transformations as generalized variables

We introduce an image transformation as a generalization of measurable maps. Structure properties of the image transformation is given. Unique extension of image transformations from solid sets is proven. The multidimensional median and sample median are presented as image transformations, providing a construction suitable for probability theorists.     

Melting curve of MgO from first-principles simulations

First-principles calculations based on density functional theory, both with the local density approximation (LDA) and with generalized gradient corrections (GGA), have been used to simulate solid and liquid MgO in direct coexistence in the range of pressure 0 < or = p < or = 135 GPa. The calculated LDA zero pressure melting temperature is T(LDA)m = 3110 +/- 50 K, in good agreement with the experimental data. The GGA zero pressure melting temperature T(GGA)m = 2575 +/- 100 K is significantly lower than the LDA one, but the difference between the GGA and the LDA is greatly reduced at high pressure. The LDA zero pressure melting slope is dT/dp approximately 100 K/GPa, which is more than 3 times higher than the currently available experimental one from Zerr and Boehler [Nature (London) 371, 506 (1994)]. At the core mantle boundary pressure of 135 GPa MgO melts at Tm = 8140 +/- 150 K.     

Rates of convergence for the laws of large numbers for independent Banach-valued random variables

In this paper, results of Lai, Heyde, and Rohatgi concerning the convergence rates for the laws of large numbers are extended for the case of independent random variables taking values in a separable Banach space.