Molecular flexibility effects upon liquid dynamics

Simulation results for the diffusive behavior of polymer chain/penetrant systems are analyzed. The attractive range and flexibility of simple chain molecules were varied in order to gauge the effect on dynamics. In all cases, the dimensionless diffusion coefficient, D*, is found to be a smooth, single-valued function of the packing fraction, eta. The functions D*(eta) are found to be power laws with exponents that are sensitive to both chain stiffness and particle type. For a specific system type, the D*'s for both penetrant and chain-center-of-mass extrapolate to zero at the same packing fraction, eta0. This limiting packing fraction is interpreted to be the location of the glass transition, and (eta0-eta), the distance to the glass transition.     

Determination of113mCd in natural water

A method has been developed for the determination of^113mCd from fallout in large-volume environmental water samples. In the analytical procedure a known amount of cadmium carrier is added to the sample and the cadmium is preconcentrated as insoluble hydroxide by coprecipitation with ferric hydroxide. The separated cadmium is purified from other interfering elements by an anion—exchange procedure. Finally, cadmium is coprecipitated with copper sulfide, and the radioactivity of^113mCd is measured in a lowbackground, gas-flow proportional beta counter. The cadmium recovery is established by determining the amount of Cd in the precipitate using an energy dispersive x-ray fluorescence spectrometer. For a 24hr counting period a lower limit of detection around 50μBq^113mCd/1 of sample has been attained. Samples of Lake Michigan water were analyzed: the concentration of^113mCd in 1980 was 177±5 μBq/l.     

Circles minimize most knot energies

We define a new class of knot energies (known as renormalization energies) and prove that a broad class of these energies are uniquely minimized by the round circle. Most of O'Hara's knot energies belong to this class. This proves two conjectures of O'Hara and of Freedman, He, and Wang. We also find energies not minimized by a round circle. The proof is based on a theorem of Lük? on average chord lengths of closed curves.     

Speciation of dimethylarsinyl-riboside derivatives (arsenosugars) in marine reference materials by HPLC-ICP-MS

Anion and cation exchange HPLC-ICP-MS was used to separate and detect mixtures of four dimethylarsinyl-riboside derivatives (arsenosugars), in the presence of eight other arsenic species naturally occurring in the marine environment. The separations achieved showed that two arsenosugars 11 and 13 (cf. Table 2) were present in shellfish certified reference materials (CRMs) and in a lobster hepatopancreas CRM. The concentration of the two arsenosugars in the shellfish samples amounts to 18% of the total arsenic as compared to arsenobetaine at 9–13% and dimethylarsinate at 4–9% of the total arsenic. Additionally, a chromatographic peak with the same retention time as that of 2-dimethylarsinylacetic acid was detected in the shellfish samples. Further support of the identity of this peak was gained after spiking the sample extracts with the standard substance which resulted in a single, but larger peak. The indication that this novel arsenical is present in shellfish, and the recently reported finding of arsenocholine in seafood supports a proposed marine biosynthetic pathway of arsenic that includes both of these compounds as the immediate precursors of arsenobetaine, the end-product of the marine arsenic metabolism.     

Size and aerial density distributions of Ge nanocrystals in a SiO2 layer produced by molecular beam epitaxy and rapid thermal processing

We discuss the distribution of size and aerial density of Ge nanocrystals in a metal-oxide-semiconductor (MOS) memory structure fabricated by molecular beam epitaxy combined with rapid thermal processing; the size and aerial density of Ge nanocrystals are controlled by varying the thickness of the deposited Ge layer and the processing time. Variation of tunnel oxide thickness is demonstrated with the extension of the processing time. The effect of processing time and tunnel oxide thickness on the electrical properties of the MOS structures is investigated by high frequency capacitance–voltage measurements. PACS  61.46.+w; 81.07.-b; 81.07.Bc; 81.07.Ta     

The GaAs (001)−c(4×4) and (2×4) reconstructions: A comparative photoemission study

Surface sensitive photoemission measurements of As(3d) core levels give new experimental evidence that the MBE-grown GaAs (0 0 1)?c(4×4) surface is richer in As than the 2×4 surface and that it is produced by an As overlayer chemisorbed onto the As atoms terminating the polar GaAs(0 0 1) surface. A study of As thermal desorption shows that the c(4×4) structure persists over a broad range of surface stoichiometry.     

HRGC separation of PCB congeners

High resolution gas chromatography has greatly changed the field of polychlorinated biphenyl (PCB) analysis. From a rough estimation of the total PCB concentration on packed column GC, the development of commercially available fused silica capillary columns in the late 1970s and early 1980s has advanced the analysis of PCBs to a point where they can be accurately measured as separated congeners. The state-of-the-art of PCB analysis in the 1990s is routine isomer-specific analysis with multi-column techniques using long (<50 m) narrow (> 0.25 mm) columns. A broad spectrum of commercially available stationary phases have been completely characterized with regard to their PCB elution profiles.     

The fractional chromatic number of mycielski's graphs

James Propp The most familiar construction of graphs whose clique number is much smaller than their chromatic number is due to Mycielski, who constructed a sequence G_n of triangle-free graphs with X(G_n) = n. In this article, we calculate the fractional chromatic number of G_n and show that this sequence of numbers satisfies the unexpected recurrence a_n+1 = a_n + (1/a_n). © 1995 John Wiley & Sons, Inc.