Low temperature scanning force microscopy of the Si(111)-(7x7) surface

A low temperature scanning force microscope (SFM) operating in a dynamic mode in ultrahigh vacuum was used to study the Si(111)- (7x7) surface at 7.2 K. Not only the twelve adatoms but also the six rest atoms of the unit cell are clearly resolved for the first time with SFM. In addition, the first measurements of the short range chemical bonding forces above specific atomic sites are presented. The data are in good agreement with first principles computations and indicate that the nearest atoms in the tip and sample relax significantly when the tip is within a few A of the surface.     

Controlling the spectral shape of coherent soft X-rays

We demonstrate the generation of arbitrarily shaped spectra of coherent soft X-ray radiation by adaptive control of the driving laser pulse. The selection of both narrow and wide spectral ranges as well as suppression of high-harmonic emission in a selected wavelength region is achieved. Feedback-controlled spectral engineering of coherent soft X-rays paves the way to shaping of soft X-ray pulses and coherent control of electron dynamics in the sub-femtosecond regime.     

Chains of reinsurance

A one-dimensional, hierarchical system of reinsurance is considered. A member of the chain is in direct contact with only two other members: the one from which coverage is bought and the one to which coverage is sold. Exceptions are the first link (which does not sell any reinsurance coverage) and the last link (which does not buy any). The problem is to find the values of the quotas and the loadings that are optimal in some sense. Assuming exponential utility functions and a normal distribution for the claims, an explicit solution is found for two versions of the model.     

Characterization of rodlike aggregates generated from a cationic surfactant and a polymerizable counterion

The polymerization of elongated micellar structures offers a novel approach to the production of high-aspect-ratio, water-soluble amphiphilic nanorods. A cationic surfactant with a vinyl-containing counterion, cetyltrimethylammonium 4-vinylbenzoate, has been synthesized and polymerized to produce high-aspect-ratio nanoparticles which are insensitive to changes in solution conditions. Aggregates are polymerized over a range of initiator concentrations allowing for control of the product length. Small-angle neutron scattering and light scattering are used to characterize the dimensions of the polymerized aggregates, showing them to have a fixed radius of 2 nm and contour lengths varying from 96 to 340 nm. Proton NMR verifies the chemical structure and provides insight into the mobility of the aggregate components. Finally, gel permeation chromatography of the polymer extracted from the aggregates indicates that the polymerization reaction controls the aggregate dimensions.     

Stringent tests of shell model calculations in fp shell nuclei (46, 48)Ti and (50,52)Cr from measurements of g factors and B(E2) values

Measurements of magnetic moments and lifetimes of 2(+)(1) and 4(+)(1) states of (46,48)Ti and (50,52)Cr were performed with high accuracy via projectile Coulomb excitation and the technique of transient magnetic fields. The high quality of the data allows for the first time to establish stringent constraints on large scale shell model calculations. Whereas the global behavior of the data is well explained by full fp shell model calculations, distinct deviations in the g factors and B(E2) values of (46,48)Ti from theoretical predictions can be attributed to excitations of the 40Ca core. This suggestion is supported by recent Monte Carlo calculations which provide evidence that 48Ca is a better inert core.     

High-throughput characterization of sediment organic matter by pyrolysis–gas chromatography/mass spectrometry and multivariate curve resolution: A promising analytical tool in (paleo)limnology

Molecular-level chemical information about organic matter (OM) in sediments helps to establish the sources of OM and the prevalent degradation/diagenetic processes, both essential for understanding the cycling of carbon (C) and of the elements associated with OM (toxic trace metals and nutrients) in lake ecosystems. Ideally, analytical methods for characterizing OM should allow high sample throughput, consume small amounts of sample and yield relevant chemical information, which are essential for multidisciplinary, high-temporal resolution and/or large spatial scale investigations. We have developed a high-throughput analytical method based on pyrolysis–gas chromatography/mass spectrometry and automated data processing to characterize sedimentary OM in sediments. Our method consumes 200 μg of freeze-dried and ground sediment sample. Pyrolysis was performed at 450 °C, which was found to avoid degradation of specific biomarkers (e.g., lignin compounds, fresh carbohydrates/cellulose) compared to 650 °C, which is in the range of temperatures commonly applied for environmental samples. The optimization was conducted using the top ten sediment samples of an annually resolved sediment record (containing 16–18% and 1.3–1.9% of total carbon and nitrogen, respectively). Several hundred pyrolytic compound peaks were detected of which over 200 were identified, which represent different classes of organic compounds (i.e., n-alkanes, n-alkenes, 2-ketones, carboxylic acids, carbohydrates, proteins, other N compounds, (methoxy)phenols, (poly)aromatics, chlorophyll and steroids/hopanoids). Technical reproducibility measured as relative standard deviation of the identified peaks in triplicate analyses was 5.5 ± 4.3%, with 90% of the RSD values within 10% and 98% within 15%. Finally, a multivariate calibration model was calculated between the pyrolytic degradation compounds and the sediment depth (i.e., sediment age), which is a function of degradation processes and changes in OM source type. This allowed validation of the Py–GC/MS dataset against fundamental processes involved in OM cycling in aquatic ecosystems.     

Structural elucidation of metabolites of ritonavir and indinavir by liquid chromatography-mass spectrometry

The structural elucidation of metabolites of ritonavir and indinavir, HIV-protease inhibitor drugs, by liquid chromatography-electrospray ionization mass spectrometry is described. Ritonavir and indinavir were biotransformed separately by incubation with transplant quality human liver microsomes. The incubation mixture was then analyzed by HPLC coupled to ion trap (ITMS) and triple quadrupole mass analyzers. The metabolites retained most of the structural features of the parent molecules. Baseline chromatographic resolution of isobaric species by gradient elution HPLC permitted rapid structural identification of these metabolites. Both drugs were biotransformed primarily by oxidative and hydrolytic pathways to numerous metabolites that retained many of the features of the parent molecules. Triple quadrupole and ion trap mass spectrometry were applied jointly to thoroughly detect and thoroughly characterize these metabolites. Furthermore, retention-time and data-dependent scanning assured acquisition of detailed MS-MS spectra for rapid detection of metabolic pathways of ritonavir and indinavir. Comparison of the ITMS and triple quadrupole data showed qualitative and quantitative differences in the mass spectral patterns, suggesting that these instruments should be used in parallel to ensure comprehensive metabolite detection and characterization by LC-MS.