Ionenmobilitätsspektrometrie

Ion mobility spectrometers are extremely fast and sensitive sensors for trace gases, which identify these according to their motion through a neutral gas under the influence of an electric field. In this work, the basics of ion mobility are summarized, different areas of application for ion mobility spectrometers are explained and three basic types of ion mobility spectrometers are presented.     

Elucidation of the structure of poly(γ-benzyl-l-glutamate) nanofibers and gel networks in a helicogenic solvent

The synthesis, characterization, self-assembly, and gel formation of poly(γ-benzyl-l-glutamate) (PBLG) in a molecular weight range from ca. 7,000–100,000 g/mol and with narrow molecular weight distribution are described. The PBLG is synthesized by the nickel-mediated ring-opening polymerization and is characterized by size-exclusion chromatography coupled with multiple-angle laser light scattering, NMR, and Fourier transform infrared spectroscopy. The self-assembly and thermoreversible gel formation in the helicogenic solvent toluene is investigated by transmission electron microscopy, atomic force microscopy, small-angle X-ray scattering, and synchrotron powder X-ray diffraction. At concentrations significantly below the minimum gelation concentration, spherical aggregates are observed. At higher concentrations, gels are formed, which show a 3D network structure composed of nanofibers. The proposed self-assembly mechanism is based on a distorted hexagonal packing of PBLG helices parallel to the axis of the nanofiber. The gel network forms due to branching and rejoining of bundles of PBLG nanofibers. The network exhibits uniform domains with a length of 200?±?42 nm composed of densely packed PBLG helices.      

Nonparametric Gaussian inference for stable processes

Statistical Inference for Stochastic Processes - Jump processes driven by $$\alpha $$ -stable Lévy processes impose inferential difficulties as their increments are heavy-tailed and the...     

Bradbury-Nielsen vs. Field switching shutters for high resolution drift tube ion mobility spectrometers

A key component in the design of every drift tube ion mobility spectrometer (IMS) is the ion shutter which controls the injection of ions into the drift tube. Especially, compact drift tube IMS require very short injection pulses to achieve high resolution and therefore require fast ion shutters. Thus, it is important to find an ion shutter principle that can be readily scaled towards these short injection widths without causing major non-idealities in the injection process, such as drift field inhomogeneities, ion loss and ion discrimination by mobility. In this paper, we compare different ion shutter principles, foremost the Bradbury-Nielsen gate and a field switching design. It is shown through theoretical considerations and field simulations that the Bradbury-Nielsen shutter is more universally applicable and typically less complex for long injections widths but field inhomogeneities associated with its operating principle impede the scaling process. Thus, the currently less used field switching shutters will become the superior principle when very short injection widths are required, as this shutter principle allows for single digit microsecond widths.     

Photoracemization broadening of selective reflection and polarization band of glassy chiral-nematic films

Glass-forming liquid crystals consisting of a cyclohexane central core with (S)-1-phenyl-ethylamine and (4-cyanophenyl)naphthalene pendants were synthesized as hosts for racemizable (R)-dinaphtho[2,1-d:1',2'-f][1,3]dioxepin, a chiral dopant. Chiral-nematic films 14, 22, and 35 mum thick were prepared for thermal and photoinduced racemization at temperatures from 95 to 130°C, i.e. in the mesomorphic temperature range, over a period of hours to days. Spatially modulated photoracemization was accomplished with an insignificant contribution from the thermal process at temperatures around 100°C over a period of up to 3 h. With an absorbance per unit thickness of 6.2 mum^-1 at 334 nm, the photochemical process was essentially confined to the irradiated surface, thereby setting up counter-diffusion of the two enantiomers through the film, and hence the pitch gradient as visualized by atomic force microscopy. The significantly widened selective reflection band was interpreted with the Good-Karali theory extended for a gradient-pitch film. Furthermore, the bandwidth was found to increase with decreasing racemization temperature or with increasing film thickness, further validating the presence of a pitch gradient as a result of controlled photoracemization.