Proficiency testing in analytical chemistry,microbiology and laboratory medicine: discussions on current practice and future directions

Accreditation and Quality Assurance - A summary of the working group discussions on proficiency testing (PT) and external quality assessment (EQA) held at the Eurachem Workshop, Portorož,...     

Spin-dependent thermoelectric effect in Co2Fe0.4Mn0.6Si thin film with perpendicular magnetic anisotropy

We report the anomalous Nernst effect in trilayers containing a thin film of the half-metallic ferromagnetic Heusler alloy Co₂Fe_0.4Mn_0.6Si with perpendicular magnetic anisotropy. The structure is MgO/CFMS/Pd and we have studied the variation of anomalous Hall and Nernst effects as a function of CFMS and Pd thickness. The anomalous Nernst coefficient reaches 0.5 μV/K at room temperature and we have observed a strong dependence of the anomalous Nernst coefficient on the thickness of both layers. Our results indicate that inducing perpendicular magnetic anisotropy in a strongly spin-polarising Heusler alloy such as CFMS is very promising for new thermoelectric devices based on exploiting the anomalous Nernst effect.     

氨基酸DLH及其稀土镧配合物的高压红外和拉曼光谱研究

测定了DL-2-氨基-4-磺酸基-丁酸 [DLH, DL-Homocysteic acid, (NH+₃)-CH(COOH)-(CH₂)₂-SO-₃] 及其稀土La配合物[La(DLH)₂Cl₃·H₂O=LaL₂]在不同压力下的红外和拉曼光谱。DLH 在50 kbar左右压力以下存在两个压力诱导相转变区,它们分别在17和37 kbar左右,两者均为二级相转变,认为分子间氢键的存在是出现两个压力诱导相转变区的原因。在红外光谱中,SO-₃的对称伸缩振动的压力灵敏度(dν/dp)表现出与其他振动模式不同的变化趋势,它们在低压相区的平均压力灵敏度为0.30 cm-1·(kbar)-1、中压相区为0.32 cm-1·(kbar)-1、高压相区为0.41 cm-1·(kbar)-1,低压相区与高压相区的比值为0.72, 而其他振动模式刚好相反,低压相区与高压相区的比值为4.8。稀土La配合物LaL₂的生成,改变了分子间的氢键,在50 kbar左右压力以下只观察到1个压力诱导相转变区(27 kbar附近)。在红外光谱中,配合物LaL₂中SO-₃的反对称伸缩振动的压力灵敏度(dν/dp)也表现出与其他振动模式不同的变化趋势,它们在低压相区的平均压力灵敏度与高压相区的平均压力灵敏度的比值为0.43, 而其他振动模式的比值为2.5。     

Post-buckled propagation model for compressive fatigue of impact damaged laminates

An analytical model for prediction of compressive fatigue threshold strains in composite plates with barely visible impact damage (BVID) is presented. The model represents the complex damage morphology as a single circular delamination at a critical level and calculates the strain at which thin-film buckling of the circular delaminated region occurs. The threshold strain is defined as the strain at which the strain energy release rate for the fracture of post-buckled delaminated plies along the delamination is equal to the critical Mode I value (G_1C) for the resin. The model predicts the critical through-thickness level for delamination, the stability of delamination growth and also the sensitivity to experimental error in geometric measurements of the damage area. Results obtained using the model show an agreement of fatigue strain to within 4% of experimental values for four sets of data reported in the literature.     

Regiospecificity of the peptidyl tRNA ester within the ribosomal P site

The ribosomal peptidyl transferase center is expected to be regiospecific with regard to its tRNA substrates, yet the ester linkages between the tRNA and the amino acid or peptide are susceptible to isomerization between the O2' and O3' hydroxyls of the terminal A76 ribose sugar. To establish which isomer of the P site tRNA ester is utilized by the ribosome, we prepared two nonisomerizable transition state inhibitors with either an A76 O2' or O3' linkage. Strong preferential binding to the O3' regioisomer indicates that the peptidyl transferase proceeds through a transition state with an O3'-linked peptide in the P-site.     

Controlling regiochemistry in Negishi carboaluminations. Fine tuning the ligand on zirconium

The species on the zirconocene catalyst is changed from two Cp's to the Brintzinger ligand and catalytic amounts of MAO are used to usually effect a >99% regiocontrol of Negishi carboaluminations of 1-alkynes in toluene.     

Photodissociation of 1-bromo-2-butene, 4-bromo-1-butene, and cyclopropylmethyl bromide at 234 nm studied using velocity map imaging

We present photofragment imaging experiments to characterize potential photolytic precursors of three C4H7 radical isomers: 1-methylallyl, cyclopropylmethyl, and 3-buten-1-yl radicals. The experiments use 2+1 resonance enhanced multiphoton ionization (REMPI) with velocity map imaging to state-selectively detect the Br(2P(3/2)) and Br(2P(1/2)) atoms as a function of their recoil velocity imparted upon photodissociation of 1-bromo-2-butene, cyclopropylmethyl bromide, and 4-bromo-1-butene at 234 nm as well as the angular distributions of the photofragments. Energy and momentum conservation allows the internal energy distribution of the nascent momentum-matched radicals to be derived. The radicals are detected with single photon photoionization at 157 nm. In the case of the 1-methylallyl radical the photoionization cross section is expected to be independent of internal energy in the range of 7-30 kcal/mol. Thus, comparison of the product recoil kinetic energy distribution derived from the measurement of the 1-methylallyl velocity distribution, detecting the radicals with 157 nm photoionization, with a linear combination of the Br atom recoil kinetic energy distributions allows us to derive reliable REMPI line strength ratios for the detection of Br atoms and to test the assumption that the photoionization cross section does not strongly depend on the internal energy of the radical. This line strength ratio is then used to determine the branching to the Br(2P(3/2)) and Br(2P(1/2)) product channels for the other two photolytic systems and to determine the internal energy distribution of their momentum-matched radicals. (We also revisit earlier work on the photodissociation of cyclobutyl bromide which detected the Br atoms and momentum-matched cyclobutyl radicals.) This allows us to test whether the 157 nm photoionization of these radicals is insensitive to internal energy for the distribution of total internal (vibrational+rotational) energy produced. We find that 157 nm photoionization of cyclopropylmethyl radicals is relatively insensitive to internal energy, while 3-buten-1-yl radicals show a photoionization cross section that is markedly dependent on internal energy with the lowest internal energy radicals not efficiently detected by photoionization at 157 nm. We present electronic structure calculations of the radicals and their cations to understand the experimental results.     

Muonium studies of p-type semi-conducting diamond under conditions of UV-visible illumination

This work reports on the promptly forming fraction and the spin relaxation rate of the isotropic muonium (Mu_T) component in p-type semi-conducting diamond, measured under the condition of illumination. The data are the first such investigations for diamond. A broad band illumination with wavelengths ranging from 0.5 μm to 3 μm was obtained from a Xenon lamp. The energy of the photons was sufficient to excite electrons from the valence band to the 0.28 ppm boron impurity band (0.37 eV). The Transverse Field Muon Spin Rotation (TF-μSR) measurements were conducted as a function of temperature, ranging from 5 K to 300 K. An illumination effect at temperatures below 100 K is observed. It is not yet clear from these data whether the effect is due to Mu_t scattering off delocalized holes, which are removed by illumination or whether there is prompt trapping of Mu_t at boron impurities (passivation) which is affected by illumination. This revised version was published online in August 2006 with corrections to the Cover Date.     

Iron(III) coordination chemistry of alterobactin A: a siderophore from the marine bacterium Alteromonas luteoviolacea

Alterobactin A is a siderophore produced by the oceanic bacterium Alteromonas luteoviolacea. The thermodynamic stability constant of the ferric alterobactin A (Alt-A) complex was estimated from electrochemical measurements on the basis of a previously reported linear relationship between the reduction potentials and the pH-independent stability constants for known iron(III) complexes. The reduction potential of the ferric alterobactin A complex determined by square wave voltammetry is -0.972 V vs SCE and reversible, corresponding to a thermodynamic stability constant of 10(51+/-2). Potentiometric titration of Fe(III)-Alt-A shows the release of six protons on complexation of Fe(III) to Alt-A. The 1H NMR resonances of the Ga(III)-Alt-A complex show that the C-4, C-5, and C-6 catecholate protons and the C(alpha) and C(beta) protons of both beta-hydroxyaspartate moieties are shifted downfield relative to the free ligand, which along with the potentiometric titration data is consistent with a complex in which Fe(III) is coordinated by both catecholate oxygen atoms and both oxygen atoms of each beta-hydroxyaspartate. The UV-vis spectrum of Fe(III)-Alt-A is invariant over the pH range 4-9, indicating the coordination does not change over a wide pH range. In addition, in the absence of a coordinated metal ion, the serine ester of Alt-A hydrolyzes forming Alt-B.     

Simulations of concentrated suspensions of rigid fibers: relationship between short-time diffusivities and the long-time rotational diffusion

Brownian dynamics simulations of the behavior of suspensions of fibers demonstrate that the scaling of the rotational diffusivity with respect to the number density (nL3) is a sensitive function of the thickness and the parameter L2D(R0)/D(T0), where D(R0) is the rotational diffusivity at infinite dilution, D(T0) is the average center-of-mass diffusivity at infinite dilution, and L is the fiber length. Existing theories for the long-time rotational diffusivities of rigid fibers in the semidilute and concentrated regimes fail to accurately account for the relationship with the dilute values of the rotational and translational diffusivities of the various physical models used to simulate the fibers. The concentration regime studied in this work ranges from a number density of nL3 approximately 0-150, which is below the transition from an isotropic to nematic state. The effect of the fiber thickness was studied by performing simulations of rods with aspect ratios (fiber length over diameter) of 25, 50, and 500, as well as performing projections for infinitely thin fibers. The excluded volume of the rods was enforced through the use of short-range potentials. For a rod with an aspect ratio of 50 with a parameter of L2D(R0)/D(T0)=9, which corresponds to a slender-body model of the individual fibers, the rotational diffusivity (D(R)) scales as D(R)/D(R0) approximately (nL3)(-1.9) in the concentration regime of 70 < or = nL3 < or = 150. Similarly with a parameter of L2D(R0)/D(T0)=4, corresponding to a rigid-dumbbell model, the rotational diffusivity scales as D(R)/D(R0) approximately (nL3)(-1.1) over the same range of concentrations. For rods with aspect ratios of 25, it is observed that a difference in the scaling is seen for L2D(R0)/D(T0) approximately < 8, with higher values of this ratio exhibiting essentially the same scaling. Additional values of the ratio L2D(R0)/D(T0) were investigated to determine the overall behavior of the suspension dynamics with respect to this parameter. These findings resolve discrepancies between simulation results for rotational diffusivities reported by previous investigators and provide new insights for the development of an accurate theory for the diffusivity of rigid rods suspended in solution.