Preservation and storage techniques for low-level aqueous mercury speciation

Although researchers today generally employ appropriate techniques for the storage and preservation of aqueous samples for ambient-level mercury (ppb) speciation, these methods continue to be poorly documented. Numerous experiments were thus conducted to investigate the effects of acidification and bottle type on holding time for various mercury species [elemental mercury (Hg(0)), ionic mercury (Hg(II)), dimethyl mercury (DMHg), monomethyl mercury (MMHg), and dissolved-to-particulate ratio] as well as total mercury (THg). We documented that THg is stable for at least 300 days when stored at 0.4-0.5% acidity in either Teflon or glass bottles. In cases where THg is adsorbed to bottle walls, the addition of BrCl at least 24 h before analysis allowed all Hg to be quantitatively recovered. Polyethylene bottles allowed diffusion of Hg(0) through the bottle walls to or from the sample, depending on the Hg concentration of the sample and storage atmosphere. MMHg in freshwater samples can be stored refrigerated and unacidified for days to weeks with no observed degradation of MMHg. For long-term storage (at least 250 days), samples should be acidified with 0.4% HCl (v/v) and kept in the dark to avoid photodegradation (approximate t(1/2)=6 months). For saltwater samples, preservation with 0.2% (v/v) H(2)SO(4) is preferred to avoid exceeding the optimal chloride concentration if the distillation procedure is used for MMHg determination. For volatile species (Hg(0) and DMHg), samples should be collected in completely full glass bottles with Teflon-lined caps, as these species are lost rapidly (t(1/2)=10-20 h) from Teflon and polyethylene bottles. Because acids can enhance the rapid oxidation of volatile species, these samples should be stored refrigerated and unacidified and processed within 1-2 days if they cannot be purged and trapped in the field. Hg(II) and the dissolved-to-particulate ratio are more stable and can be stored for a period of days to weeks without preservation.     

Cloning of human Stat5B. Reconstitution of interleukin-2-induced Stat5A and Stat5B DNA binding activity in COS-7 cells

We have isolated a second human Stat5 cDNA, Stat5B, and demonstrated that the genes encoding both Stat5A and Stat5B are located at chromosome 17q11.2. Both genes were constitutively transcribed in peripheral blood lymphocytes. By using specific antisera, we demonstrated that both Stat5A and Stat5B are activated by interleukin-2 (IL-2) in peripheral blood lymphocytes, natural killer-like YT leukemia cells, and human T cell lymphotropic virus type I-transformed MT-2 T cells. In COS-7 cells, which constitutively express the Janus family tyrosine kinase Jak1, reconstitution of IL-2-induced Stat5A and Stat5B DNA binding activities was dependent on the coexpression of Jak3 along with the IL-2 receptor beta chain and the common cytokine receptor gamma-chain. This IL-2-induced Stat5 activation was dependent on the presence of either of two tyrosines (Tyr-392 or Tyr-510) in the IL-2 receptor beta chain, indicating that either of these two tyrosines can serve as a docking site. Moreover, we demonstrated that human Stat5 activation is also dependent on Tyr-694 in Stat5A and Tyr-699 in Stat5B, indicating that these tyrosines are required for dimerization. The COS-7 reconstitution system described herein provides a valuable assay for further elucidation of the IL-2-activated JAK-STAT pathway.     

Single-chip 4×500-MBd CMOS transceiver

A CMOS chip containing four 500-MBd serializer/deserializer pairs has been designed to relieve interconnect congestion in an ATM switch system. The 9.7×9.7 mm² chip fabricated in a 0.8-μm technology is packaged on a ceramic ball grid array and dissipates 3.5 W. It replaces a 72-wire parallel interface with an eight-line serial interface transparent to the user and supports transmission at 1.6 Gb/s per direction in full-duplex mode. Virtually error-free operation in a system environment over electrical serial links having up to 9 dB loss at 500 MHz has been realized using signal predistortion for the serial bit stream and PLL clock recovery for each of the four receivers. Interface timing and serial-link driver strength are programmable     

Calibration and modeling of a dual-axis inclinometer

Methods are described to calibrate and model a dual-axis inclinometer, using a single-axis sine bar, by rotating the inclinometer axially on the sine bar and using a cross-coupled polynomial expansion model. A simple combination of angle blocks and fixturing is used to generate the axial rotations. An example of how the method was used to calibrate the model CST92 panel corner setting tools for the Robert C. Byrd Green Bank Telescope (GBT) is described.     

The kinetics of CO oxidation by adsorbed oxygen on well‐defined gold particles on TiO2(110)

Very tiny Au particles on TiO₂ show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO₂(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO₂(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO₂, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O₂ desorption temperatures (741 K) from ultrathin gold particles on TiO₂(110) than from thicker particles (545 K). This implies that O_a bonds much more strongly to ultrathin islands of Au. Thus from Brønsted relations, ultrathin gold particles should be able to dissociatively adsorb O₂ more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO₂) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (CO_g + O_a → CO_2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Brønsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).     

Mechanical properties of ceria nanorods and nanochains; the effect of dislocations, grain-boundaries and oriented attachment

We predict that the presence of extended defects can reduce the mechanical strength of a ceria nanorod by 70%. Conversely, the pristine material can deform near its theoretical strength limit. Specifically, atomistic models of ceria nanorods have been generated with full microstructure, including: growth direction, morphology, surface roughening (steps, edges, corners), point defects, dislocations and grain-boundaries. The models were then used to calculate the mechanical strength as a function of microstructure. Our simulations reveal that the compressive yield strengths of ceria nanorods, ca. 10 nm in diameter and without extended defects, are 46 and 36 GPa for rods oriented along [211] and [110] respectively, which represents almost 10% of the bulk elastic modulus and are associated with yield strains of about 0.09. Tensile yield strengths were calculated to be about 50% lower with associated yield strains of about 0.06. For both nanorods, plastic deformation was found to proceed via slip in the {001} plane with direction <110>--a primary slip system for crystals with the fluorite structure. Dislocation evolution for the nanorod oriented along [110] was nucleated via a cerium vacancy present at the surface. A nanorod oriented along [321] and comprising twin-grain boundaries with {111} interfacial planes was calculated to have a yield strength of about 10 GPa (compression and tension) with the grain boundary providing the vehicle for plastic deformation, which slipped in the plane of the grain boundary, with an associated <110> slip direction. We also predict, using a combination of atomistic simulation and DFT, that rutile-structured ceria is feasible when the crystal is placed under tension. The mechanical properties of nanochains, comprising individual ceria nanoparticles with oriented attachment and generated using simulated self-assembly, were found to be similar to those of the nanorod with grain-boundary. Images of the atom positions during tension and compression are shown, together with animations, revealing the mechanisms underpinning plastic deformation. For the nanochain, our simulations help further our understanding of how a crystallising ice front can be used to 'sculpt' ceria nanoparticles into nanorods via oriented attachment.     

Numerical and experimental study on multiple-spout fluidized beds

In this paper we study the effect of multiple spouts on the bed dynamics in a pseudo-2D triple-spout fluidized bed, employing the discrete particle model (DPM) and non-intrusive measurement techniques such as particle image velocimetry (PIV) and positron emission particle tracking (PEPT). A flow regime map was constructed, revealing new regimes that were not reported so far. The multiple-interacting-spouts regime (C) has been studied in detail for a double- and triple-spout fluidized bed, where the corresponding fluidization regime for a single-spout fluidized bed has been studied as a reference case. The experimental results obtained with PIV and PEPT agree very well for all the three cases, showing the good performance of these techniques. The DPM simulation results slightly deviate from the experiments which is attributed to particle–wall effects that are more dominant in pseudo-2D beds than in 3D systems. The investigated multiple-interacting-spouts regime is a fully new flow regime that does not appear in single-spout fluidized beds. Two flow patterns have been observed, viz. particle circulation in between the spouts near the bottom of the bed, and an apparent single-spout fluidization motion at a higher location upwards in the bed. These findings show that the presence of multiple spouts in a spout fluidized bed highly affect the flow behaviour, which cannot be distinguished by solely investigating single-spout fluidized beds.     

Growth of vertically aligned bamboo-like carbon nanotubes from ammonia/methane precursors using a platinum catalyst

Vertically aligned multi-walled carbon nanotubes have been synthesized by microwave plasma enhanced chemical vapor deposition using ammonia/methane source gas mixtures and a platinum thin film catalyst on thermally oxidized silicon substrates. The SiO₂ layer served as a diffusion barrier to reduce PtSi formation and aide the formation of well dispersed catalytic nanoparticles. Nevertheless, some degree of substrate–catalyst interaction occurred even with this diffusion barrier present. Carbon nanotube growth using a relatively thick catalyst film was either inhibited or prevented altogether when increasing the duration of plasma pretreatment or by moving the substrate closer to the center of the plasma. This was primarily attributed to increased substrate–catalyst interactions resulting in catalyst poisoning. Decreasing the plasma pretreatment time or moving the substrate away from the center of the plasma improved the growth, increasing the length of the nanotubes. Varying the catalyst film thickness enabled control of the diameter and density of the carbon nanotubes. Transmission electron microscopy revealed a bamboo-like structure with the orientation of internal compartments consistent with a base-growth mode.     

Correlation between Grain Size and Thermal Expansion for Aluminum Titanate Materials

Al₂TiO₅ materials were sintered under different conditions to produce different grain sizes. The resultant microcracked materials exhibited a range of bulk thermal expansions which showed a strong correlation with average grain size. An Al₂TiO₅ average grain size of 3 to 4 μm was the minimum at which the size and population of microcracks effectively reduced the apparent thermal expansion of the polycrystalline material. Further increases in grain size resulted in a rapid drop in the bulk thermal expansion, followed by diminishing decreases with further increases in grain size. Small amounts of phase stabilizers (<2.1 wt% MgO or Fe₂O₃) or limited mullite additions in mullite—aluminum titanate composites had no significant effect on this correlation.