Purity determination as needed for the realisation of primary standards for elemental determination: status of international comparability

Within the National Metrology Institutes (NMIs) and designated laboratories, an interlaboratory comparison, CCQM-P107, was conducted to verify the degree of international comparability concerning the results of purity analysis. The mass fractions of Ag, Bi, Cd, Cr, Ni, Tl at the lower mg/kg-level in a high purity zinc material were determined, but the real measurand in metrological sense was the sum of the six mass fractions. Homogeneity was investigated by glow discharge mass spectrometry, reference values were obtained using isotope dilution mass spectrometry. Six NMIs participated, contributing eight independent data sets. The agreement amongst the results of the participants, their median and the agreement with the reference values were usually excellent and in almost all cases below the target uncertainty of 30% relative. In this manner, the accuracy of results and the comparability between the participants was demonstrated to be established.     

A dielectric study of poly(vinyl acetate) using a pulse-probe technique

There is considerable literature available that describes our understanding of the viscoelastic properties of polymers subjected to mechanical stresses or deformations. What we refer to here as a pulse-probe technique is one method that is commonly used to study the time-dependent behavior of materials in histories, e.g., temperature-jump or step-deformations, that exhibit fading memory responses. In the linear case the behavior is well understood in the context of Boltzmann superposition ideas. However, there is only limited work available that investigates the dielectric response of materials within this same context. In the present study, we present an investigation of the dielectric behavior of poly(vinyl acetate) (PVAc) using a two-step pulse-probe technique. Time domain dielectric experiments were performed in the vicinity of the glass transition temperature. After establishing the linear response function in single-step experiments, two types of pulse-probe experiments were performed. In one, the time duration t ₁ of the first step in the probe was varied. In the second case, the magnitude of the field E ₁ applied to the sample for the first step was varied. We not only demonstrate the existence of the memory effect in the dielectric response, but also find that the responses are consistent with the linear Boltzmann superposition principle. Evidence of deviations from linear superposition at the highest electric fields is also presented.     

Direct conversion of β-hydroxyketones to cyclic disiloxanes

β-Hydroxyketones can be directly converted to cyclic disiloxanes using diphenylchlorosilane in the presence of imidazole and an amine base. The reaction is proposed to proceed via a nucleophilic activation mechanism through a cyclic chairlike transition state affording hydrosilylated products with high diastereoselectivity.     

Stochastic least-action principle for the incompressible Navier-Stokes equation

We formulate a stochastic least-action principle for solutions of the incompressible Navier-Stokes equation, which formally reduces to Hamilton’s principle for the incompressible Euler solutions in the case of zero viscosity. We use this principle to give a new derivation of a stochastic Kelvin Theorem for the Navier-Stokes equation, recently established by Constantin and Iyer, which shows that this stochastic conservation law arises from particle-relabelling symmetry of the action. We discuss issues of irreversibility, energy dissipation, and the inviscid limit of Navier-Stokes solutions in the framework of the stochastic variational principle. In particular, we discuss the connection of the stochastic Kelvin Theorem with our previous “martingale hypothesis” for fluid circulations in turbulent solutions of the incompressible Euler equations.     

Comparison of membrane assisted solvent extraction,stir bar sorptive extraction,and solid phase microextraction in analysis of tetramine in food

Three environmentally friendly extraction techniques, membrane assisted solvent extraction (MASE), stir bar sorptive extraction (SBSE), and headspace solid phase microextraction (HS‐SPME), were compared for the direct analysis of the highly toxic rodenticide tetramine in food. The optimized MASE method was applied to seven foods fortified with tetramine and compared to previously reported SBSE and HS‐SPME results. Parameters such as the standard addition linearity (MASE (0.964–0.999), SBSE (0.966–0.999), HS‐SPME (0.955–0.999)), recovery (MASE (12–86%), SBSE (36–130%), HS‐SPME (50–200%)), reproducibility (MASE (3.0–30%), SBSE (4.4–9.6%), HS‐SPME (1–12%)), and LOD (MASE (1.6–6.4 ng/g), SBSE (0.2–2.1 ng/g), HS‐SPME (0.9–4.3 ng/g)) were compared.     

Determination of thermodynamic pKa values of benzimidazole and benzimidazole derivatives by capillary electrophoresis

Thermodynamic pK_a values for benzimidazole and several substituted benzimidazoles were determined by CE. Electrophoretic mobilities of benzimidazoles were determined by CE at different pH levels and ionic strengths. The dependence of electrophoretic mobilities on pH was used to obtain pK_a values at different ionic strengths. Extrapolations to zero ionic strength were used to determine the thermodynamic pK_a values. Using this method the thermodynamic pK_a values of 15 benzimidazoles were determined and found to range from 4.48 to 7.38. Results from the CE measurements were compared with spectrophotometric measurements which were evaluated at wavelengths where the highest absorbance difference for varying pH was recorded. The two analytical techniques were in good agreement.     

Bioinspired vesicle restraint and mobilization using a biopolymer scaffold

Biology employs vesicles to package molecules (e.g., neurotransmitters) for their targeted delivery in response to specific spatiotemporal stimuli. Biology is also capable of employing localized stimuli to exert an additional control on vesicle trafficking; intact vesicles can be restrained (or mobilized) by association with (or release from) a cytoskeletal scaffold. We mimic these capabilities by tethering vesicles to a biopolymer scaffold that can undergo (i) stimuli-responsive network formation (for vesicle restraint) and (ii) enzyme-catalyzed network cleavage (for vesicle mobilization). Specifically, we use the aminopolysaccharide chitosan as our scaffold and graft a small number of hydrophobic moieties onto its backbone. These grafted hydrophobes can insert into the bilayer to tether vesicles to the scaffold. Under acidic conditions, the vesicles are not restrained by the hydrophobically modified chitosan (hm-chitosan) because this scaffold is soluble. Increasing the pH to neutral or basic conditions allows chitosan to form interpolymer associations that yield a strong, insoluble restraining network. Enzymatic hydrolysis of this scaffold by chitosanase cleaves the network and mobilizes intact vesicles. Potentially, this approach will provide a controllable means to store and liberate vesicle-based reagents/therapeutics for microfluidic/medical applications.     

High-capacity binding of proteins by poly(acrylic acid) brushes and their derivatives

Polymeric coatings with high protein-binding capacities are important for increasing the output of affinity-based protein purification and decreasing the detection limits of antibody microarrays. This report describes the use of thick poly(acrylic acid) (PAA) brushes to immobilize as much as 80 monolayers of protein. The brushes were prepared using a recently developed procedure that allows polymerization of 100-nm-thick poly(tert-butyl acrylate) films from a surface in just 5 min along with hydrolysis of these films to PAA in 15 min. Covalent binding of bovine serum albumin (BSA) to PAA brushes that were activated using standard coupling agents, however, resulted in immobilization of less than two monolayers of BSA because of competitive hydrolysis of the esters in the activated film. In contrast, derivatization of PAA with nitrilotriacetate (NTA)-Cu2+ complexes yielded films capable of binding many monolayers of protein via metal-ion affinity interactions. For example, derivatization of 55-nm-thick PAA films with NTA-Cu2+ allowed immobilization of about 15 monolayers (5.8 microg/cm2 or 58 nm) of BSA. The binding capacity was even higher for myoglobin (7.7 microg/cm2) and anti-IgG (9.6 microg/cm2). Remarkably, electrostatic adsorption of lysozyme in 55-nm-thick, underivatized PAA resulted in as much as 80 monolayers (16.2 microg/cm2 or 162 nm) of adsorbed protein. Polymer synthesis, derivatization, and swelling, as well as BSA immobilization kinetics and thermodynamics were characterized using reflectance FT-IR spectroscopy, ellipsometry, and protein assays.