Analytische Chemie heute

Ohne Zusammenfassung     

Scaling in the structure of directory trees in a computer cluster

We describe the topological structure and the underlying organization principles of the directories created by users of a computer cluster. Users create trees with a scale-free degree distribution whose properties are reproduced by a growth and preferential attachment mechanism with a single parameter. The degree distribution has a nonuniversal exponent associated with different values of the parameter. However, the distribution of branch sizes has a universal exponent analytically obtained from the model.     

A general method for the synthesis of bastaranes and isobastaranes: first total synthesis of bastadins 5, 10, 12, 16, 20, and 21

A general strategy for the synthesis of twenty naturally occurring bastadins (all but bastadin 3) is presented. A key retrosynthetic disconnection of the two amide bonds, common in all target molecules, bisects the macrocyclic core into two diaryl ether fragments, an alpha,omega-diamine (western part) and an alpha,omega-dicarboxylic acid (eastern part). Efficient preparation of the synthetically challenging o-mono or dibromo-substituted diaryl ether linkages was achieved employing the diaryl iodonium salt method. Regarding the western part, variations of the aliphatic chain were more efficiently secured by the preparation of two different alpha,omega-aminonitrile moieties. Cobalt boride mediated reduction of the nitrile functionality established the required diamines and, at the same time, provided the necessary variation of the aromatic-ring bromination pattern. Regarding the eastern part, two different dicarboxyl precursors had to be prepared in order to accommodate bromination-pattern variations. Coupling and subsequent macrolactamization of different combinations of these key intermediates may lead at will to any member of this family of marine natural products. Four bastaranes (bastadins 5, 10, 12 and 16) and two isobastaranes (bastadins 20 and 21) were synthesized as a demonstration of the flexibility and efficiency of the approach presented.     

Depth of Higher Associated Graded Rings

The depth of the associated graded ring of the powers of anideal I of a local ring R is studied. It is proved that thedepth of the associated graded ring of Iⁿ is asymptoticallyconstant when n tends to infinity, and this value is characterizedin terms of Valabrega–Valla conditions of I^m for somelarge integer m 0. As a corollary, a generalization is obtainedof the 2-dimensional algebraic version of the Grauert–Riemenschneidervanishing theorem (due to Huckaba and Huneke) to ideals satisfyingthe second Valabrega–Valla condition. The positivenessof Hilbert coefficients is also studied, and Valabrega–Vallaconditions are linked to the vanishing of the cohomology groupsof the closed fiber of the blowing up of Spec(R) along the closedsub-scheme defined by I.     

Generalized Cofibration Categories and Global Actions

We develop an axiomatic homotopy theory for categories with a family of natural cylinders indexed by a set with relations, generalizing Baues I-categories. The main examples of -cofibration categories are the categories of global actions and simplicial complexes.     

Bounds for solutions of a differential inequality

This work compares the solutions of an th order differential inequality plus boundary conditions with the solution of the related differential equation with boundary conditions. The differential operator is assumed to be disconjugate. It is proved that under suitable conditions the ratio of these solutions is monotone. The solution of the inequality can be replaced by the corresponding Green's function.

     

Beyond Hebb: exclusive-OR and biological learning

A learning algorithm for multilayer neural networks based on biologically plausible mechanisms is studied. Motivated by findings in experimental neurobiology, we consider synaptic averaging in the induction of plasticity changes, which happen on a slower time scale than firing dynamics. This mechanism is shown to enable learning of the exclusive-OR (XOR) problem without the aid of error backpropagation, as well as to increase robustness of learning in the presence of noise.     

Removal of graffiti from the mortar by using Q-switched Nd:YAG laser

This paper presents part of the larger study on microstructural features of mortars and it's effects on laser cleaning process. It focuses on the influence of surface roughness, porosity and moisture content of mortars on the removal of graffiti by Nd:YAG laser. The properties of this laser are as follows: wavelength (λ) 1.06 μm, energy: 500 mJ per pulse, pulse duration: 10 ns. The investigation shows that the variation of laser fluence with the number of pulses required for the laser cleaning can be divided into two zones, namely effective zone and ineffective zone. There is a linear relationship observed between number of pulses required for laser cleaning and the laser fluence in the effective zone, while the number of pulses required for the laser cleaning is almost constant even though the laser fluence increases in the ineffective zone. Moreover, surface roughness, porosity and moisture content of mortar samples have influence on the laser cleaning process. The effect of these parameters become however negligible at the high level of laser fluence. The number of pulses required for the laser cleaning is low for smooth surface or less porous mortar. Furthermore, the wetness of the samples facilitates the cleaning process.     

Investigations of trace elements in high salinity waters by ICP-MS

A separation and enrichment procedure for the analysis of trace elements (Cd, Co, Cu, Fe, Mn, Ni, Pb, Tl, U, Zn) in seawater or brines from ore dumps by ICP-MS was established based on the complexation with sodium diethyldithiocarbamate-trihydrate (Na-DDTC) and separation on a phenyl column. The method was tested with seawater samples from the hydrothermal system of Axial Seamount, Juan de Fuca Ridge. The analytical results demonstrated the influence of hydrothermal activity on the concentration of Fe and Mn. Received: 2 December 1998 / Revised: 25 February 1999 / Accepted: 1 March 1999