Stable isotope biogeochemistry of the sulfur cycle in modern marine sediments: I. Seasonal dynamics in a temperate intertidal sandy surface sediment

A biogeochemical and stable isotope geochemical study was carried out in surface sediments of an organic-matter poor temperate intertidal sandy surface sediment (German Wadden Sea of the North Sea) to investigate the activity of sulfate-reducing bacteria and the dynamics of the vertical partitioning of sedimentary sulfur, iron, and manganese species in relation to the availability of total organic carbon (TOC) and mud contents. The contents and stable isotopic compositions ((34)S/(32)S) of total reduced inorganic sulfur species (TRIS) and dissolved sulfate were measured. Maximum oxygen penetration depths were estimated from the onset of a blackening of the sediments due to FeS accumulation and ranged from 5 to 10 mm below surface (mmbsf). A zone of relatively moderate relative organic-matter enrichment was found between 5 and 20 mmbsf leading to enhanced activities of sulfate-reducing bacteria with sulfate-reduction rates (SRR) up to 350 nmol cm(-3) d(-1). Below this zone, microbial SRR dropped significantly. Depth integrated SRR seem to depend not only on temperature but also on the availability of reactive organic matter. The sulfur-isotopic composition of TRIS was depleted in (34)S by 33-40 per thousand with respect to coexisting dissolved sulfate (constant at about +21 per thousand vs. Vienna-Canyon Diablo Troilite (V-CDT)). Since sulfate reduction is not limited by dissolved sulfate (open system), depth variations of the isotopic composition of TRIS reflect changes in overall isotope effect due to superimposed microbial and abiotic reactions. Most of the solid-phase iron and manganese was bonded to (non-reactive) heavy minerals. However, a layer of reactive Fe(III) and Mn(IV) oxi(hydroxi)des was found in the uppermost sediment section due to re-oxidation of dissolved Fe(II) and Mn(II) species at the sediment-water interface. Metal cycling below the surface is at least partially coupled to intense sulfur cycling.     

Electrochemical Proton Intercalation in Vanadium Pentoxide Thin Films and its Electrochromic Behavior in the near-IR Region

This work examines the proton intercalation in vanadium pentoxide (V₂O₅) thin films and its optical properties in the near-infrared (near-IR) region. Samples were prepared via direct current magnetron sputter deposition and cyclic voltammetry was used to characterize the insertion and extraction behavior of protons in V₂O₅ in a trifluoroacetic acid containing electrolyte. With the same setup chronopotentiometry was done to intercalate a well-defined number of protons in the H_xV₂O₅ system in the range of x=0 and x=1. These films were characterized with optical reflectometry in the near-IR region (between 700 and 1700 nm wavelength) and the refractive index n and extinction coefficient k were determined using Cauchy ’s dispersion model. The results show a clear correlation between proton concentration and n and k.     

Inorganic arsenic speciation in water and seawater by anodic stripping voltammetry with a gold microelectrode

The determination of arsenic in sea and freshwater by anodic stripping voltammetry (ASV) was revisited because of problems related to unstable peaks and inconveniently strong acidic conditions used by existing methods. Contrary to previous work it was found, that As(III) can be determined by ASV using a gold microwire electrode at any pH including the neutral pH typical for natural waters. As(V) on the other hand, requires acidification to pH 1, but this is still a much milder condition than used previously. This is the basis of a new method for the chemical speciation of arsenic in natural waters. The limits of detection are 0.2 nM As(III) at pH 8 and 0.3 nM combined arsenic (III + V) at pH 1 with a 30 s deposition time. These limits are lowered by extending the deposition time. The detection step at pH 8 was stripping chronopotentiometry (SC) as this was found to give a lower detection limit than ASV. Copper is co-determined simultaneously with arsenic. The method was applied successfully to the determination of arsenic as well as copper in samples from the Irish Sea, mineral water and tap water.     

Enhancement of intensities in glow discharge mass spectrometry by using mixtures of argon and helium as plasma gases

Glow discharge mass spectrometry (GD-MS) is an excellent technique for fast multi-element analysis of pure metals. In addition to metallic impurities, non-metals also can be determined. However, the sensitivity for these elements can be limited due to their high first ionization potentials. Elements with a first ionization potential close to or higher than that of argon, which is commonly used as discharge gas in GD-MS analysis, are ionized with small efficiency only. To improve the sensitivity of GD-MS for such elements, the influence of different glow-discharge parameters on the peak intensity of carbon, chlorine, fluorine, nitrogen, phosphorus, oxygen, and sulfur in pure copper samples was investigated with an Element GD (Thermo Fisher Scientific) GD-MS. Discharge current, discharge gas flow, and discharge gas composition, the last of which turned out to have the greatest effect on the measured intensities, were varied. Argon–helium mixtures were used because of the very high potential of He to ionize other elements, especially in terms of the high energy level of its metastable states. The effect of different Ar–He compositions on the peak intensity of various impurities in pure copper was studied. With Ar–He mixtures, excellent signal enhancements were achieved in comparison with use of pure Ar as discharge gas. In this way, traceable linear calibration curves for phosphorus and sulfur down to the μg kg⁻¹ range could be established with high sensitivity and very good linearity using pressed powder samples for calibration. This was not possible when pure argon alone was used as discharge gas. This contribution is based on a presentation given at the Colloquium for Analytical Atomic Spectroscopy (CANAS ’07) held March 18–21, 2007 in Constance, Germany.     

Multivariate data analysis for enhanced interpretation of electrochemical impedance spectra of gramicidin-ion interactions in phospholipid monolayers

This article describes a multifrequency electrochemical impedance study of phospholipid monolayers on a mercury drop electrode in solutions containing electrolytes and gramicidin derivatives: gramicidin A (gA), gramicidin-BOC (g-BOC), and desformylgramicidin (g-des). The impedance spectra have been studied individually (univariate approach) and also transformed using a multivariate data reduction method (multivariate approach). It was shown that the two approaches are complementary. Thus the formation of K+-conducting channels is observed in gA only, and these channels can be distinguished from an interaction of all gramicidin derivatives with Mg2+. An unknown peptide interaction in the monolayer was observed on a slow time scale.     

Chronocoulometric and rotating disc electrode determination of the charge propagation current through poly-4-vinylpyridine films containing hexachloroiridate anions

Charge propagation currents through polymer films coated on electrodes which contain redox active sites are usually determined by potential step methods. The aim of this study was to compare the charge propagation current obtained from chronocoulometric measurements with charge transport through a poly-4-vinylpyridine/IrCl₆²⁻ film during the mediated oxidation of Fe²⁺ at a rotating disc electrode. The two methods gave identical results for the model system chosen.     

Micronization of Pharmaceutical Substances by Rapid Expansion of Supercritical Solutions (RESS): Experiments and Modeling

An increasing number of newly developed pharmaceutical substances are poorly soluble in both aqueous and organic media. Thus, the application of oral or injectable drugs is often limited by its low bioavailability. An alternative and promising method to improve the bioavailability of pharmaceutical agents is the production of nanoscale particles by the rapid expansion of supercritical solutions (RESS). Our research is aimed towards an improved understanding of the underlying physical phenomena of the relationship between the process conditions and the particle characteristics. Therefore, experimental investigations and numerical simulations were performed. RESS experiments with the pharmaceutical substances β‐sitosterol, griseofulvin, and ibuprofen led to particle sizes in the range of 240±80 nm. In addition, as one step towards intravenous application of poorly soluble drugs, β‐sitosterol was used to produce aqueous suspensions of a water‐insoluble drug with a particle size smaller than or equal to those produced by RESS into air. RESS modeling is focused on the flow through the nozzle, the supersonic free jet, the mach shock, and particle growth in the expansion unit. The comparison with experimental results shows a good agreement in the general trends but does not match exactly the measured mean particle sizes.     

Assembly and decomposition of building blocks to analyze polymer NEXAFS spectra

The poly(ethylene-terephthalate) and poly(bisphenol-A-carbonate) compounds are used to illustrate the possibilities and limitations of building-block analysis of polymer near-edge X-ray absorption spectra. Experimental spectra obtained for thin polymer films are analyzed using theoretical static exchange calculations of C_1s and O_1s excitation spectra on model molecules selected as possible building blocks. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 63: 749–765, 1997     

Polarization‐dependent Raman spectroscopy of LiBH4 single crystals and Mg(BH4)2 powders

The phonons and the crystal structure of the complex hydride LiBH₄ are studied on single crystals using micro‐Raman spectroscopy. The symmetry of the modes is determined by polarization‐dependent measurements at liquid helium temperature, allowing a better comparison and a more reliable assignment to the computed phonon wavenumbers. This has led to the revision of some former assignments made from Raman measurements on polycrystalline samples. In addition, a higher integration time allowed the detection of very weak lines, so that 35 out of 36 predicted Raman lines have been identified. We have also performed explorative Raman measurements on Mg(BH₄)₂ powders. In contrast to LiBH₄, the very poor crystallinity of this material inhibits the exploitation of the full potential of Raman spectroscopy. Only broad lines are observed, which we compare to phonon wavenumbers calculated for various possible structures using density functional theory. Copyright © 2011 John Wiley & Sons, Ltd.     

A sedimented sample of a 59 kDa dodecameric helicase yields high-resolution solid-state NMR spectra

Crystal clear: Preparing solid-state NMR samples that yield high-resolution spectra displaying high sensitivity is time-consuming and complicated. A sample of the 59?kDa protein DnaB, prepared simply by preparative centrifugation, provides spectra that are as good as the ones from carefully grown microcrystals.