Analysis of polysulfides in drinking water distribution systems using headspace solid-phase microextraction and gas chromatography–mass spectrometry

Sulfide and polysulfides are strong nucleophiles and reducing agents that participate in many environmentally significant processes such as the formation of sulfide minerals and volatile organic sulfur compounds. Their presence in drinking water distribution systems are of particular concern and need to be assessed, since these species consume disinfectants and dissolved oxygen, react with metal ions to produce insoluble metal sulfides, and cause taste and odour problems. The analysis of sulfide and polysulfides in drinking water distribution systems is challenging due to their low concentrations, thermal instability and their susceptibility to undergo oxidation and disproportionation reactions. This paper reports on the development and optimisation of a rapid, simple, and sensitive method for the determination of sulfide and polysulfides in drinking water distribution systems. The method uses methyl iodide to derivatise sulfide and polysulfides into their corresponding dimethyl(poly)sulfides, which are then extracted using solid-phase microextraction in the headspace mode and analysed by gas chromatography–mass spectrometry. Good sensitivity was achieved for the analysis of dimethyl(poly)sulfides, with detection limits ranging from 50 to 240 ng L⁻¹. The method also demonstrated good precision (repeatability: 3–7%) and good linearity over two orders of magnitude. Matrix effects from raw drinking water containing organic carbon (3.8 mg L⁻¹) and from sediment material from a drinking water distribution system were shown to have no interferences in the analysis of dimethyl(poly)sulfides. The method provides a rapid, robust, and reliable mean to analyse trace levels of sulfides and polysulfides in aqueous systems. The new method described here is more accessible and user-friendly than methods based on closed-loop stripping analysis, which have been traditionally used for the analysis of these compounds. The optimised method was used to analyse samples collected from various locations in a drinking water distribution system. Some of the samples were shown to contain inorganic polysulfides, and their presence was associated with high sediment density in the system and the absence of disinfectant residual in the bulk water.     

An Organic–Inorganic Hybrid Exhibiting Electrical Conduction and Single‐Ion Magnetism

The first three‐dimensional (3D) conductive single‐ion magnet (SIM), (TTF)₂[Co(pdms)₂] (TTF=tetrathiafulvalene and H₂pdms=1,2‐bis(methanesulfonamido)benzene), was electrochemically synthesised and investigated structurally, physically, and theoretically. The similar oxidation potentials of neutral TTF and the molecular precursor [HNEt₃]₂[M(pdms)₂] (M=Co, Zn) allow for multiple charge transfers (CTs) between the SIM donor [M(pdms)₂]^n? and the TTF^.+ acceptor, as well as an intradonor CT from the pdms ligand to Co ion upon electrocrystallisation. Usually TTF functions as a donor, whereas in our system TTF is both a donor and an accepter because of the similar oxidation potentials. Furthermore, the [M(pdms)₂]^n? donor and TTF^.+ acceptor are not segregated but strongly interact with each other, contrary to reported layered donor–acceptor electrical conductors. The strong intermolecular and intramolecular interactions, combined with CT, allow for relatively high electrical conductivity even down to very low temperatures. Furthermore, SIM behaviour with slow magnetic relaxation and opening of hysteresis loops was observed. (TTF)₂[Co(pdms)₂] ( 2‐Co ) is an excellent building block for preparing new conductive SIMs.     

O−O Bond Formation and Liberation of Dioxygen Mediated by N5‐Coordinate Non‐Heme Iron(IV) Complexes

Formation of the O?O bond is considered the critical step in oxidative water cleavage to produce dioxygen. High‐valent metal complexes with terminal oxo (oxido) ligands are commonly regarded as instrumental for oxygen evolution, but direct experimental evidence is lacking. Herein, we describe the formation of the O?O bond in solution, from non‐heme, N₅‐coordinate oxoiron(IV) species. Oxygen evolution from oxoiron(IV) is instantaneous once meta‐chloroperbenzoic acid is administered in excess. Oxygen‐isotope labeling reveals two sources of dioxygen, pointing to mechanistic branching between HAT (hydrogen atom transfer)‐initiated free‐radical pathways of the peroxides, which are typical of catalase‐like reactivity, and iron‐borne O?O coupling, which is unprecedented for non‐heme/peroxide systems. Interpretation in terms of [Fe^IV(O)] and [Fe^V(O)] being the resting and active principles of the O?O coupling, respectively, concurs with fundamental mechanistic ideas of (electro‐) chemical O?O coupling in water oxidation catalysis (WOC), indicating that central mechanistic motifs of WOC can be mimicked in a catalase/peroxidase setting.     

In Silico Design of Cylindrophanes: The Role of Functional Groups in a Fluoride Selective Host

Molecular recognition is the key driver in the formation of supramolecular complexes, enabling the selective encapsulation of specific guests. Here, we explore the delicate balance between different energetic terms in the formation of an efficient host for fluoride anions based on a cylindrophane structure, which can be achieved by the incorporation of ligand sites into a cyanuric acid based cyclophane framework, resulting a close proximity between the ammonium hydrogens and the anion. This study describes the character and contribution of different energetic and repulsive terms that favor the efficient inclusion of fluoride. Our findings are useful for further rational design and synthesis of efficient and highly selective fluoride hosts, which have been generally less well described than complexing agents for other halides.     

Approaches to Model Nanotube-Reinforced Polymers using Micromechanics and Finite Elements

In this work, the applicability of new analytical methods for modeling polymer nanocomposites is examined. The influence of an interphase between nanotube and matrix could be captured by this modeling strategy. However, it is shown that there are some difficulties in calculation of composites including extremely weak fibers with stiff coatings. Furthermore, finite element calculations are shown for comparison. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

l‐Seryl‐l‐phenyl­alanine

The peptide bond in the crystal structure of the title compound, C₈H₁₆N₂O₄, deviates substantially from planarity in the same manner as in other l ‐Ser‐l ‐Xaa dipeptides, where Xaa is a hydro­phobic residue.     

Palladium‐Catalyzed Decarboxylative γ‐Arylation for the Synthesis of Tetrasubstituted Chiral Allenes

An enantiospecific palladium‐catalyzed decarboxylative coupling of acyclic β,γ‐alkynoic acids with various aryl iodides to chiral tetrasubstituted allenes is described. The coupling reaction comprises a decarboxylative γ‐palladation of α,α‐disubstituted carboxylic acids to provide the tetrasubstituted allenes with complete point‐to‐axial chirality transfer in excellent yields.     

Comparison of analytical and numerical methods for homogenization of nanotube-reinforced polymers

Carbon nanotubes are increasingly getting impact as reinforcing material for polymer based nanocomposites. Hence, new modeling strategies are necessary to calculate the behavior of these materials. In the last years some attempts have been made using and developing classical micromechanical models. On the other hand numerical homogenization methods are available to tackle this problem. Examples for both types of modeling strategies are presented with focus on the nanotube geometry. The nanotubes are modeled as hollow tubes as well as as isotropic and transversely isotropic cylinders. As expected the results of numerical and analytical methods are identical for isotropic cylinder inclusions. Small deviations occur for transversely isotropic cylinders in transverse direction. In the case of hollow tube inclusions, the analytical models lead to lower stiffness values in transverse direction and for shear. The largest deviations occur for longitudinal shear with magnitudes smaller than 10%. In contrast the effort to get numerical results is enormous, so that the analytical models are still useful. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)