On-line delta(18)O measurement of organic and inorganic substances

A method for the automated sample conversion and on-line oxygen isotope ratio (delta(18)O) determination for organic and inorganic substances is presented. The samples are pyrolytically decomposed at 1400 degrees C in the presence of nickelized graphite. With the system presented organic as well as inorganic samples such as nitrates, sulphates and phosphates of 50-100 &mgr;g O can be analyzed for their delta(18)O values with a standard deviation usually better than 0.5 per thousand. Additionally, carbon isotope ratios of organic substances and nitrogen isotope ratios of inorganic nitrogenous compounds are available in the same sample run. Data for international and some inter-laboratory reference materials are presented to show the accuracy and reliability of the method. The effect of some additives on the CO yield was checked for substances which do not pyrolyze completely. Copyright 1999 John Wiley & Sons, Ltd.     

Cluster size analysis of two-dimensional order in colloidal gold nanoparticle arrays

A protocol for cluster size distribution analysis was developed in order to parametrize local two-dimensional (2D) order in a quantitative manner, using mean cluster sizes and fractional hcp cluster formation (fhcp). Cluster size analysis was performed on 2D arrays of Au nanoparticles encapsulated in resorcinarene tetrathiol, which were organized into close-packed films at aqueous interfaces. The degree of monolayer formation and 2D order within the self-assembled nanoparticle arrays was observed to be strongly dependent on the amount and type of electrolyte (chloride and/or citrate) adsorbed on the nanoparticle surface, prior to encapsulation and extraction to the solvent interface. Increasing the concentration of adsorbed electrolyte could promote monoparticulate film formation but had a variable effect on local 2D order.     

A clinical system for accurate assessment of tissue iron concentration

Summary This paper describes a new computer-enhanced dual-channel SQUID susceptometer designed to achieve accuratein vivo estimates of tissue storage iron concentrations. We discuss the practical design considerations, instrumental noise performance and anticipated clinical resolution of the new system. Work supported by NIH grants HL24198 and AM25105.     

An Infrared Study of the Amine-Catalyzed Reaction of Methoxymethylsilanes with Silica

A thin film infrared technique was used to investigate the reaction of methoxysilanes and amines with the silica surface. The low-frequency region contains bands due to Si-O-Si modes that are used to distinguish between hydrogen-bonded and chemisorbed species. It is shown that the competitive adsorption of amines and CH(3)OSi(CH(3))(3) differs from the results obtained using (CH(3)O)(2)Si(CH(3))(2) or (CH(3)O)(3)SiCH(3). The monomethoxysilane does not displace preadsorbed triethylamine whereas the triethylamine is displaced from the surface by both (CH(3)O)(2)Si(CH(3))(2) and (CH(3)O)(3)SiCH(3). In the reverse sequence, the triethylamine displaces all three methoxysilanes on the surface. When 1:1 mixtures of methoxysilanes and triethylamine (or propylamine) are co-added to silica, the amine preferentially adsorbs and is only displaced by subsequent chemisorption of the silane. The implication of these results for using a two-step amine-catalyzed reaction of methoxysilanes on silica is discussed. Copyright 2000 Academic Press.     

Hydrothermal synthesis, X-ray structure and complex magnetic behaviour of Ba4(C2O4)Cl2[[Fe(C2O4)(OH)]4

Hydrothermal reaction of iron(III) chloride, barium chloride and sodium oxalate in a narrow stoichiometry range produces the title compound Ba4(C2O4)Cl2[[Fe(C2O4)(OH)]4] (1). This new iron(II) oxalate crystallises in the tetragonal space group P42/mnm: a = 13.811(3), c = 7.026(2) A. The structure consists of parallel chains of mu2-hydroxy-bridged iron(II) ions. These are connected by bridging oxalates to form an anionic framework with large channels that contain the remaining barium, chloride and oxalate counter ions. Magnetisation studies on an oriented single crystal of 1 revealed a magnetic phase transition at 32 K and a strong easy-plane anisotropy at all temperatures. Above Tc the compound behaves as an S = 2XY antiferromagnetic chain, showing a broad maximum in the susceptibility at about 70 K. We determined the intrachain coupling J and the interchain coupling J' to be -7 cm(-1) and +0.4 cm(-1), respectively. The low-temperature phase is an ordered antiferromagnetic state. Zero- and longitudinal-field muon spin-rotation/relaxation studies support this interpretation; below Tc oscillations in the muon spin-autocorrelation function are observed giving unambiguous evidence for a non-zero sublattice magnetisation and proof of a long-range magnetically ordered state.