4,559 research outputs found
New analytical techniques for mycotoxins in complex organic matrices
Air samples are collected for analysis from the Ames Solid Waste Recovery System. The high level of airborne fungi within the processing area is of concern due to the possible presence of toxic mycotoxins, and carcinogenic fungal metabolites. An analytical method has been developed to determine the concentration of aflatoxins B1, B2, G1, and G2 in the air of the plant which produces Refuse Derived Fuel (RDF);After extraction with methanol, some components in the matrix are precipitated by dissolving the sample in 30% acetonitrile/chloroform. An aliquot of this solution is injected onto a Styragel column where the sample components undergo simultaneous size exclusion and reverse phase partitioning. Additional studies have provided a more thorough understanding of solvent related non-exclusion effects on size exclusion gels. The Styragel column appears to have a useable lifetime of more than six months. After elution from Styragel, the sample is diverted to a second column containing Florisil which has been modified with oxalic acid and deactivated with water;Aflatoxins are eluted with 5% water/acetone. After removal of this solvent, the sample is dissolved in 150 (mu)L of a spotting solvent and the entire sample applied to a thin layer chromatography (TLC) plate using a unique sample applicator developed here. The aflatoxins on the TLC plate are analyzed by laser fluorescence;A detection limit of 10 pg is possible for aflatoxin standards using a nitrogen laser as the excitation source. Sample concentrations are determined by comparing with an internal standard, a specially synthesized aflatoxin derivative. In two separate RDF samples, aflatoxin B1 was found at levels of 6.5 and 17.0 ppb. The analytical method has also proven useful in the analysis of contaminated corn and peanut meal samples;In a separate study, the spore pigment in Aspergillus flavus wasisolated. The mass spectrum indicates a molecular weight in excessof 700. Only aliphatic hydrocarbons have been identified in the massspectrum of products from a permanganate oxidation. The isolatedpigment is obviously a mixture and no further work was done to;elucidate structures;(\u271)This research was supported by U.S. Department of Energy, Contract No. W-7405-Eng-82
Generation and remote detection of THz sound using semiconductor superlattices
The authors introduce a novel approach to study the propagation of high
frequency acoustic phonons in which the generation and detection involves two
spatially separated superlattices apart. Propagating modes
of frequencies up to escape from the superlattice where they
are generated and reach the second superlattice where they are detected. The
measured frequency spectrum reveals finite size effects, which can be accounted
for by a continuum elastic model.Comment: Submitted to Applied Physics Letter
UV slope of z3 bright () Lyman-break galaxies in the COSMOS field
We analyse a unique sample of 517 bright () LBGs at redshift
z3 in order to characterise the distribution of their UV slopes
and infer their dust extinction under standard assumptions. We exploited
multi-band observations over 750 arcmin of the COSMOS field that were
acquired with three different ground-based facilities: the Large Binocular
Camera (LBC) on the Large Binocular Telescope (LBT), the Suprime-Cam on the
SUBARU telescope, and the VIRCAM on the VISTA telescope (ULTRAVISTA DR2). Our
multi-band photometric catalogue is based on a new method that is designed to
maximise the signal-to-noise ratio in the estimate of accurate galaxy colours
from images with different point spread functions (PSF). We adopted an improved
selection criterion based on deep Y-band data to isolate a sample of galaxies
at to minimise selection biases. We measured the UV slopes ()
of the objects in our sample and then recovered the intrinsic probability
density function of values (PDF()), taking into account the
effect of observational uncertainties through detailed simulations. The
galaxies in our sample are characterised by mildly red UV slopes with
throughout the enitre luminosity range that is probed by
our data (). The resulting dust-corrected
star formation rate density (SFRD) is , corresponding to a contribution of about 25% to the
total SFRD at z3 under standard assumptions. Ultra-bright LBGs at match the known trends, with UV slopes being redder at decreasing redshifts,
and brighter galaxies being more highly dust extinct and more frequently
star-forming than fainter galaxies. [abridged]Comment: Matched to journal version. 11 pages, 13 figures, Astronomy &
Astrophysics in pres
Effect of Fibonacci Modulation On Superconductivity
We have studied finite-sized single band models with short range pairing
interactions between electrons in presence of diagonal Fibonacci modulation in
one dimension. Two models, namely the attractive Hubbard model and the
Penson-Kolb model, have been investigated at half-filling at zero temperature
by solving the Bogoliubov-de Gennes equations in real space within a mean field
approximation. The competition between ``disorder'' and the pairing interaction
leads to a suppression of superconductivity (of usual pairs with zero
centre-of-mass momenta) in the strong-coupling limit while an enhancement of
the pairing correlation is observed in the weak-coupling regime for both the
models. However, the dissimilarity of the pairing mechanisms in these two
models brings about notable difference in the results. The extent to which the
bond ordered wave and the -paired (of pairs with centre-of-mass momenta =
) phases of the Penson-Kolb model are affected by the disorder has also
been studied in the present calculation. Some finite size effects are also
identified.Comment: 14 pages, 13 figure
Dynamical phenomena in Fibonacci Semiconductor Superlattices
We present a detailed study of the dynamics of electronic wavepackets in
Fibonacci semiconductor superlattices, both in flat band conditions and subject
to homogeneous electric fields perpendicular to the layers. Coherent
propagation of electrons is described by means of a scalar Hamiltonian using
the effective-mass approximation. We have found that an initial Gaussian
wavepacket is filtered selectively when passing through the superlattice. This
means that only those components of the wavepacket whose wavenumber belong to
allowed subminibands of the fractal-like energy spectrum can propagate over the
entire superlattice. The Fourier pattern of the transmitted part of the
wavepacket presents clear evidences of fractality reproducing those of the
underlying energy spectrum. This phenomenon persists even in the presence of
unintentional disorder due to growth imperfections. Finally, we have
demonstrated that periodic coherent-field induced oscillations (Bloch
oscillations), which we are able to observe in our simulations of periodic
superlattices, are replaced in Fibonacci superlattices by more complex
oscillations displaying quasiperiodic signatures, thus sheding more light onto
the very peculiar nature of the electronic states in these systems.Comment: 7 pagex, RevTex, 5 Postscript figures. Physical Review B (in press
Raman scattering by optical phonons in In1−y−zAlyGazAs lattice matched to InP
We report on Raman scattering by longitudinal optical phonons in In1−y−zAlyGazAs (1−y−z=0.53) lattice matched to InP. The quaternary alloys were grown on (001) InP by molecular beam epitaxy. The phonon spectra exhibit three‐mode behavior. The frequencies of AlAs‐ and GaAs‐like modes vary linearly with the concentration of Al (or Ga) while the position of the InAs‐like phonon remains nearly constant. The data show no evidence of alloy clustering.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71267/2/APPLAB-53-17-1652-1.pd
Phonon Universal Transmission Fluctuations and Localization in Semiconductor Superlattices with a Controlled Degree of Order
We study both analytically and numerically phonon transmission fluctuations
and localization in partially ordered superlattices with correlations among
neighboring layers. In order to generate a sequence of layers with a varying
degree of order we employ a model proposed by Hendricks and Teller as well as
partially ordered versions of deterministic aperiodic superlattices. By
changing a parameter measuring the correlation among adjacent layers, the
Hendricks- Teller superlattice exhibits a transition from periodic ordering,
with alterna- ting layers, to the phase separated opposite limit; including
many intermediate arrangements and the completely random case. In the partially
ordered versions of deterministic superlattices, there is short-range order
(among any conse- cutive layers) and long range disorder, as in the N-state
Markov chains. The average and fluctuations in the transmission, the
backscattering rate, and the localization length in these multilayered systems
are calculated based on the superlattice structure factors we derive
analytically. The standard deviation of the transmission versus the average
transmission lies on a {\it universal\/} curve irrespective of the specific
type of disorder of the SL. We illustrate these general results by applying
them to several GaAs-AlAs superlattices for the proposed experimental
observation of phonon universal transmission fluctuations.Comment: 16-pages, Revte
Field‐dependent linewidths and photoluminescence energies in GaAs‐AlGaAs multiquantum well modulators
Photoluminescence linewidths and transition energies have been measured in GaAs‐AlGaAs multiple quantum wells with large (≥160 Å) barrier widths as a function of applied transverse electric field. The experimental data agree well with values calculated by using a recently developed variational technique. It is apparent that heterointerface roughness is the dominant line broadening mechanism. The emission intensity decreases rapidly with field, principally due to carrier tunneling at high fields. At 80 kV/cm a shift of 20 meV in the emission energy is observed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70001/2/APPLAB-48-19-1246-1.pd
Vertical Transmission of Herpes Simplex Virus Acquired After Primary Outbreak in Second Trimester of a Dichorionic Twin Gestation
Background: The incidence of genital herpes simplex virus (HSV) has increased in recent years, particularly among women of reproductive age. This places more neonates at risk for severe morbidity and mortality. Treatment recommendations for primary disease in pregnancy are lacking, particularly for those who acquire. HSV remote from term
Methane, ammonia, and their irradiation products at the surface of an intermediate-size KBO? A portrait of Plutino (90482) Orcus
Orcus is an intermediate-size 1000km-scale Kuiper Belt Object in 3:2
mean-motion resonance with Neptune, in an orbit very similar to that of Pluto.
We present visible and near-infrared photometry and spectroscopy obtained with
the Keck 10m-telescope and the Gemini 8m-telescope . We confirm the unambiguous
detection of crystalline water ice as well as absorption in the 2.2\mu m
region. Both in the visible and near-infrared Orcus' spectral properties appear
to be homogeneous over time (and probably rotation) at the resolution
available. From Hapke radiative transfer models involving intimate mixtures of
various ices we find for the first time that ammonium (NH+4) and traces of
ethane (C2 H6), which are most probably solar irradiation products of ammonia
and methane, and a mixture of methane and ammonia (diluted or not) are the best
candidates to improve the description of the data with respect to a simple
water ice mixture (Haumea type surface). The possible more subtle structure of
the 2.2\mu m band(s) should be investigated thoroughly in the future for Orcus
and other intermediate size Plutinos to better understand the methane and
ammonia chemistry at work, if any. We investigated the thermal history of Orcus
with a new 3D thermal evolution model. Simulations over 4.5 x109 yrs with an
input 10% porosity, bulk composition of 23% amorphous water ice and 77% dust,
and cold accretion show that even with the action of long-lived radiogenic
elements only, Orcus should have a melted core and most probably suffered a
cryovolcanic event in its history which brought large amounts of crystalline
ice to the surface. The presence of ammonia in the interior would strengthen
the melting process. The crystalline water ice possibly brought to the surface
by a past cryovolcanic event sbe detectable after several billion years despite
the irradiation eects, as demonstrated by recent laboratory experiments.Comment: 15 pages, 9 figure
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