Untersuchungen zum Desorptionsprozess in der UV-matrix-unterstützten Laserdesorption/-ionisation mit einer Flüssigmatrix

Der Desorptionsprozess in der UV-MALDI-MS mit der Flüssigmatrix 3-Nitrobenzylalkohol und einem frequenzvervierfachten Nd:YAG-Laser wird mit Hilfe der photoakustischen Analyse, der Hochgeschwindigkeitsfotografie der desorbierten Teilchenwolke sowie der UV-Laser-Nachionisation untersucht. Auf Grundlage der experimentellen Befunde wird ein qualitatives Desorptionsmodell entwickelt, das einen zweistufigen Prozess annimmt, bei dem im ersten Schritt eine Schicht-für-Schicht-Phasenexplosion der obersten Probenschichten nach Überhitzung bis in den Bereich der Spinodalen stattfindet, woraufhin ein Teil des verbleibenden überhitzten Restvolumens im zweiten Schritt durch einen normalen Kochvorgang abgetragen wird. Die Ergebnisse der photoakustischen Analyse deuten dabei auf einen Zusammenhang zwischen dem Einsetzen der Phasenexplosion und einer effizienten Analytionenbildung hin, was sowohl mit einem Energy-Pooling-Ionisationsmodell als auch mit einem Cluster-Ionisationsmechanismus vereinbar ist

Infrared laser post-ionization of large biomolecules from an IR-MALD(I) plume

AbstractA two-infrared laser desorption/ionization method is described. A first laser, which was either an Er:YAG laser or an optical parametric oscillator (OPO), served for ablation/vaporization of small volumes of analyte/matrix sample at fluences below the ion detection threshold for direct matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS). A second IR-laser, whose beam intersected the expanding ablation plume at a variable distance and time delay, was used to generate biomolecular ions out of the matrix-assisted laser desorption (MALD) plume. Either one of the two above lasers or an Er:YSGG laser was used for post-ionization. Glycerol was used as IR-MALDI matrix, and mass spectra of peptides, proteins, as well as nucleic acids, some of which in excess of 105 u in molecular weight, were recorded with a time-of-flight mass spectrometer. A mass spectrum of cytochrome c from a water ice matrix is also presented. The MALD plume expansion was investigated by varying the position of the post-ionization laser beam above the glycerol sample surface and its delay time relative to the desorption laser. Comparison between the OPO (pulse duration, τL = 6 ns) and the Er:YAG laser (τL ∼120 ns) as primary excitation laser demonstrates a significant effect of the laser pulse duration on the MALD process

Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutile TiO2(110)

Greuling A, Rahe P, Kaczmarski M, Kühnle A, Rohlfing M. Combined NC-AFM and DFT study of the adsorption geometry of trimesic acid on rutile TiO2(110). Journal of Physics : Condensed Matter. 2010;22(34): 345008.The adsorption behavior of trimesic acid (TMA) on rutile TiO2(110) is studied by means of non-contact atomic force microscopy (NC-AFM) and density-functional theory (DFT). Upon low-coverage adsorption at room temperature, NC-AFM imaging reveals individual molecules, centered above the surface titanium rows. Based on the NC-AFM results alone it is difficult to deduce whether the molecules are lying flat or standing upright on the surface. To elucidate the detailed adsorption geometry, we perform DFT calculations, considering a large number of different adsorption positions. Our DFT calculations suggest that single TMA molecules adsorb with the benzene ring parallel to the surface plane. In this configuration, two carboxylic groups can anchor to the surface in a bidentate fashion with the oxygen atoms binding to surface titanium atoms while the hydrogen atoms approach oxygen atoms within the bridging oxygen rows. The most favorable adsorption position is obtained in the presence of a hydroxyl defect, allowing for additional binding of the third carboxylic group

Exact Kohn-Sham exchange kernel for insulators and its long-wavelength behavior

We present an exact expression for the frequency-dependent Kohn-Sham exact-exchange (EXX) kernel for periodic insulators, which can be employed for the calculation of electronic response properties within time-dependent (TD) density-functional theory. It is shown that the EXX kernel has a long-wavelength divergence behavior of the exact full exchange-correlation kernel and thus rectifies one serious shortcoming of the adiabatic local-density approximation and generalized-gradient approximations kernels. A comparison between the TDEXX and the GW-approximation-Bethe-Salpeter-equation approach is also made.Comment: two column format 6 pages + 1 figure, to be publisehd in Physical Review

Toward Molecular Nanowires Self-Assembled on an Insulating Substrate: Heptahelicene-2-carboxylic acid on Calcite (1014)

Rahe P, Nimmrich M, Greuling A, et al. Toward Molecular Nanowires Self-Assembled on an Insulating Substrate: Heptahelicene-2-carboxylic acid on Calcite (1014). Journal of Physical Chemistry C. 2010;114(3):1547-1552.Molecular self-assembly is employed for creating unidirectional molecular nanostructures on a truly insulating substrate, namely the (10 (1) over bar4) cleavage plane of calcite. The molecule used is racemic heptahelicene-2-carboxylic acid, which forms structures, well-aligned along the [010] crystallographic direction and stable at room temperature. Precise control of both molecule-substrate and molecule-molecule interaction is required, leading to the formation of such wire-like structures of well-defined width and lengths exceeding 100 nm. This subtle balance is governed by the heptahelicene-2-carboxylic acid used in this study, allowing for both hydrogen bond formation as well pi-pi stacking

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Hardware-Accelerated Wireless Sensor Network for Distributed Structural Health Monitoring

Civil infrastructure objects are subject to safety-related issues such as increasing loads and extended service live. Costly manual inspections of these structures should therefore be supplemented by automated continuous monitoring. In this work, a hardware accelerated wireless sensor network built entirely with energy-efficient embedded components is proposed as the basis for a distributed structural health monitoring (SHM) implementation. In addition to detection and localization of structural damage, the energy-efficiency of the wireless data acquisition system is the major topic of this work. By utilizing the Random-Decrement (RD) technique, the structure’s modal parameters are acquired based on ambient excitation such as wind or traffic. The RD functions are calculated by a Field-Programmable Gate Arrays (FPGA) designed for mobile applications. To demonstrate the benefits of the proposed monitoring network, the model of a truss bridge is excited by a train set to simulate realistic operational excitations. Dominant mode shapes of the bridge model are extracted from the RD functions using frequency domain Operational Modal Analysis and compared to previously determined reference measurements. The loosening of a single bolted joint simulates damage and is found to be reflected in significant deviations of the first vertical bending mode, located at 68 Hz