Comparison of Stir Bar Sorptive Extraction and Solid Phase Extraction as Enrichment Techniques in Combination with Column Liquid Chromatography for the Determination of Polycyclic Aromatic Hydrocarbons in Water Samples

The commonly used solid-phase extraction (SPE) was compared with the recently developed stir bar sorptive extraction (SBSE) to determine EPA-PAHs from precipitation water samples. SPE was performed using C18 PAH-filled cartridges; magnetic stirring rods 10 mm long and coated with 0.5 mm (24 µL) polydimethylsiloxane were used for SBSE. Determination was performed by high performance liquid chromatography and fluorescence detection. The investigations were performed at a concentration level of 30 ng/L, adjusted to actual PAH concentration levels of precipitation water. The recoveries ranged between 54% (SBSE) and 20% (SPE) respectively and about 100%, while the standard deviation ( n = 5) varied in the range of 4.7-13.5% (SBSE) and 3.6-16.9% (SPE) except for the more volatile compounds acenaphthene and fluorene. The detection limits calculated were between 0.4 and 5.0 ng/L. Both techniques were applied to study the PAH contents of precipitation water of several polluted sites in the city of Halle (Saxony-Anhalt, Germany). The advantages and disadvantages of the methods used are discussed.     

On the subsidiary condition in quantum electrodynamics

It is proven that no charged state obtained by applying a local operator to the vacuum can satisfy the Gupta-Bleuler subsidiary condition. The meaning of this result is illustrated in the simple model whereby the vector potential is coupled to an external classical current. Alternative ways of handling the difficulty with the subsidiary condition are listed.     

Structural investigation of bismuth borate glasses and crystalline phases

Glasses of the system: xBi₂O₃-(100−x)B₂O₃ (x = 20 to 66 mol%) were prepared and characterized by density, DSC, UV-visible absorption and ¹¹B MAS-NMR spectroscopy. Glass molar volume increases while the glass transition temperature decreases with Bi₂O₃ concentration. Densities of some bismuth borate glasses are found to be greater or very close to those of single crystal phases with equal composition. B¹¹ MAS-NMR studies determined that the fraction of tetrahedrally coordinated borons (N₄) is maximum at 42 mol% of Bi₂O₃ and that there is a local maxima in N₄ at Bi₂O₃ concentration of 50 mol%. Glasses containing Bi₂O₃ concentration of 33 mol% and higher show an unusual, intense absorption band just below the optical band gap. Two crystalline phases: Bi₃B₅O₁₂ and Bi₄B₂O₉ were prepared by devitrification of glasses and characterized by X-ray diffraction, FTIR and ¹¹B MAS-NMR studies. Both crystalline phases contained significantly lower N₄ than glasses with equal composition.     

Stress, strain, and NMR

Experimental and ab initio results that demonstrate the effect of stress on the nuclear magnetic resonance spectra of materials are shown. The design of a cell that generates uniaxial compressive stress is presented, and results on gallium phosphide and lead nitrate single crystals that illustrate the observable results of the stress are shown. Tensors that relate stress and strain to changes in the chemical shielding tensors and the electric field gradient tensors are defined formally. The elements of these tensors are then computed by a density functional theory approach that makes use of planewaves and pseudopotentials. The experimental results are interpreted with the aid of the calculations. Extensions to spinning samples and to the interpretation of optical phenomena in materials are discussed.     

Equation of state of gluon plasma from a fundamental modular region

Despite considerable practical success in dealing with the gluon plasma, finite-temperature perturbation theory suffers at the fundamental level from infrared divergences discovered by Linde. However, if gauge or Gribov copies are properly eliminated from the physical state space, infrared modes are strongly suppressed. We describe the gluon plasma in zeroth order as a gas of free quasiparticles with a temperature-independent dispersion relation of Gribov type, E(k)=k(2)sqrt[+M4 / k(2)], that results from the reduction of the physical state space. The effective mass M2 / k controls infrared divergences and allows finite calculable corrections. The equation of state of this gas is calculated and compared with numerical lattice data.     

Through-bond connectivity in solids by continuous-wave spin lock

A simple two-dimensional correlation experiment that enables determination of through-bond connectivity in the solid state is described. The experiment is performed under fast magic angle spinning (MAS) conditions. After the initial pi/2 pulse, the magnetization develops freely under the MAS Hamiltonian. The t1-period is followed by a strong spin locking pulse used as mixing period. The dipolar coupling is averaged out by magic angle spinning, and the chemical shifts and r.f.-offsets are scaled by the applied spin locking field. Hence, for strong locking conditions, the isotropic J-coupling is the dominant interaction. The mixing Hamiltonian is thus identical to the well-known TOCSY-Hamiltonian, resulting in a net through-bond magnetization transfer. The mixing-time dependence of the exchange rates is investigated. Applications to crystalline P4S7 and MgP4O11 are shown.     

Zur multivariaten Auswertung von Ringversuchen

Summary The advantages and drawbacks of some graphical methods as interpreting aids for cooperative tests are demonstrated using results of slag sample analyses (5 laboratories, 7 components). In cases of low dimensions (components) the method of regular polygons has been found most informative. For higher dimensions modern data analysis techniques, such as principal components analysis (pca), correspondence analysis and discriminant analysis are to be preferred. From pca of correlation matrices one can also derive best choices of feature subsets to construct good polygons. The sensitivity of partial correlation coefficients due to deviating laboratory manners can be assessed by leaving-one-out comparisons.     

Every gauge orbit passes inside the Gribov horizon

TheL ² topology is introduced on the space of gauge connectionsA and a natural topology is introduced on the group of local gauge transformationsGT. It is shown that the mappingGT×AA defined byAA ^g=g^*Ag+g^*dg is continuous and that each gauge orbit is closed. The Hilbert norm of the gauge connection achieves its absolute minimum on each gauge orbit, at which point the orbit intersects the region bounded by the Gribov horizon.CNR, GNFMResearch supported in part by the National Science Foundation under grant no. PHY 87-15995     

Mono‐ and Bis(imidazolidinium ethynyl) Cations and Reduction of the Latter To Give an Extended Bis‐1,4‐([3]Cumulene)‐p‐carboquinoid System

An extended π‐system containing two [3]cumulene fragments separated by a p‐carboquinoid and stabilized by two capping N‐heterocyclic carbenes (NHCs) has been prepared. Mono‐ and bis(imidazolidinium ethynyl) cations have also been synthesized from the reaction of an NHC with phenylethynyl bromide or 1,4‐bis(bromoethynyl)benzene. Cyclic voltammetry coupled with synthetic and structural studies showed that the dication is readily reduced to a neutral, singlet bis‐1,4‐([3]cumulene)‐p‐carboquinoid as a result of the π‐accepting properties of the capping NHCs.     

Observable effects of mechanical stress induced by sample spinning in solid state nuclear magnetic resonance

The stress-induced change in chemical shielding induced by sample spinning is measured and interpreted theoretically. By considering the rotating sample as an elastic body in the plane-strain approximation, the internal stress field as a function of sample size, rotation frequency, and elastic constants is determined. This stress field and the dependence of chemical shielding on strain, as determined by first-principles calculations, are combined to predict the shielding dependence on rotation frequency under isothermal conditions in single crystal gallium phosphide. The prediction is in good qualitative agreement with the experiment. Little to no effect is detected in powder samples of both gallium phosphide and copper iodide, and it is argued that this follows from the stress distribution in granular material, as opposed to bulk crystals. Finally, the temperature and pressure dependence of the chemical shielding is estimated from these considerations and found consistently to underestimate the experimental values, indicating the importance of finite-temperature anharmonic effects even in very simple solids.