Application of selected reaction monitoring tandem mass spectrometry to the quantitative determination of an arsenic-containing nucleoside in a crude biological extract

HPLC electrospray selected reaction monitoring tandem mass spectrometry was used for the determination of a key arsenic metabolite, 5'-dimethylarsinoyladenosine, at trace levels (ca. 20 microg As l(-1)) in a crude extract of kidney from the clam Tridacna derasa.     

Thio arsenosugars identified as natural constituents of mussels by liquid chromatography-mass spectrometry

Two novel thio arsenosugars have been identified by liquid chromatography-mass spectrometry as significant arsenic constituents in samples of mussels.     

Non-arrhenius behavior of the island density in metal heteroepitaxy: Co on Cu(001)

We present a combined theoretical and experimental study of island nucleation and growth in the deposition of Co on Cu(001)-a prototype for understanding heteroepitaxial growth involving intermixing. Experimentally, ion scattering is employed. Using density-functional theory, we obtain energy barriers for the various elementary processes and incorporate these into a kinetic Monte Carlo program to simulate the heteroepitaxial growth. Both the simulations and the experiments show a unique N-shape dependence of the island density on temperature that stems from the interplay and competition of the different processes involved.     

Terahertz conductivity and possible BLOCH gain in semiconductor superlattices

We have investigated terahertz emission due to dynamical electron transport in wide-miniband GaAs/Al(0.3)Ga0.7As superlattices. By noting that the time-domain THz emission spectroscopy inherently measures the step-response of the electron system to the bias electric field, the obtained THz spectra were compared with the high-frequency conductivities predicted for miniband transport. Excellent agreement between theory and experiment strongly supports that the THz gain due to Bloch oscillating electrons persists at least up to 1.7 THz. It was also found that Zener tunneling into the second miniband sets the high-frequency limit to the THz gain for the samples studied here.     

Theory of the Greenspan viscometer

We present a detailed acoustic model of the Greenspan acoustic viscometer, a practical instrument for accurately measuring the viscosity eta of gases. As conceived by Greenspan, the viscometer is a Helmholtz resonator composed of two chambers coupled by a duct of radius rd. In the lowest order, eta=pi f rho(rd/Q)2, where f and Q are the frequency and quality factor of the isolated Greenspan mode, and rho is the gas density. In this level of approximation, the viscosity can be determined by measuring the duct radius and frequency response of the resonator. In the full acoustic model of the resonator, the duct is represented by a T-equivalent circuit, the chambers as lumped impedances, and the effects of the diverging fields at the duct ends by lumped end impedances with inertial and resistive components. The model accounts for contributions to 1/Q from thermal dissipation (primarily localized in the chambers) and from a capillary used for filling and evacuating the resonator. A robust, prototype instrument is being used for measuring the viscosity of reactive gases used in semiconductor processing. For well-characterized surrogate gases, the prototype viscometer generated values of eta that were within +/-0.8% of published reference values throughout the pressure range 0.2-3.2 MPa. Remarkably, we achieved this level of agreement by only slight adjustment of the numerically calculated inertial and resistive end effect parameters to improve the agreement with helium reference values. No other parameters were adjusted.     

Advanced flicker spectroscopy of fluid membranes

The bending elasticity of a fluid membrane is characterized by its modulus and spontaneous curvature. We present a new method, advanced flicker spectroscopy of giant nonspherical vesicles, which makes it possible to simultaneously measure both parameters for the first time. Our analysis is based on the generation of a large set of reference data from Monte Carlo simulations of randomly triangulated surfaces. As an example of the potential of the procedure, we monitor thermal trajectories of vesicle shapes and discuss the elastic response of zwitterionic membranes to transmembrane pH gradients. Our technique makes it possible to easily characterize membrane curvature as a function of environmental conditions.     

Anechoic chamber qualification: traverse method,inverse square law analysis method,and nature of test signal

Qualification of anechoic chambers is intended to demonstrate that the chamber supports the intended free-field environment within some permissible tolerance bounds. Key qualification issues include the method used to obtain traverse data, the analysis method for the data, and the use of pure tone or broadband noise as the chamber excitation signal. This paper evaluates the relative merits of continuous versus discrete traverses, of fixed versus optimal reference analysis of the traverse data, and of the use of pure tone versus broadband signals. The current practice of using widely space discrete sampling along a traverse is shown to inadequately sample the complexity of the sound field extant with pure tone traverses, but is suitable for broadband traverses. Continuous traverses, with spatial resolution on the order of 15% of the wavelength at the frequency of interest, are shown to be necessary to fully resolve the spatial complexity of pure tone qualifications. The use of an optimal reference method for computing the deviations from inverse square law is shown to significantly improve the apparent performance of the chamber for pure tone qualifications. Finally, the use of broadband noise as the test signal, as compared to pure tone traverses over the same span, is demonstrated to be a marginal indicator of chamber performance.     

Shortcut to a Fermi-degenerate gas of molecules via cooperative association

We theoretically examine the creation of a Fermi-degenerate gas of molecules by considering a photoassociation or Feshbach resonance applied to a degenerate Bose-Fermi mixture of atoms. This problem raises interest because, unlike bosons, fermions in general do not behave cooperatively, so that the collective conversion of a degenerate gas atoms into a macroscopic number of diatomic molecules is not to be expected. Nevertheless, we find that the coupled Fermi system displays collective Rabi-like oscillations and a rapid adiabatic passage between atoms and molecules, thereby mimicking Bose-Einstein statistics. Cooperative association of a degenerate mixture of Bose and Fermi gases could therefore serve as a shortcut to a degenerate gas of Fermi molecules.