4,039 research outputs found
Fat transforms ascorbic acid from inhibiting to promoting acid-catalysed N-nitrosation
<b>Background</b>: The major potential site of acid nitrosation is the proximal stomach, an anatomical site prone to
a rising incidence of metaplasia and adenocarcinoma. Nitrite, a pre-carcinogen present in saliva, can be
converted to nitrosating species and N-nitroso compounds by acidification at low gastric pH in the presence
of thiocyanate.
<b>Aims</b>: To assess the effect of lipid and ascorbic acid on the nitrosative chemistry under conditions simulating
the human proximal stomach.
<b>Methods</b>: The nitrosative chemistry was modelled in vitro by measuring the nitrosation of four secondary
amines under conditions simulating the proximal stomach. The N-nitrosamines formed were measured by gas
chromatography–ion-trap tandem mass spectrometry, while nitric oxide and oxygen levels were measured
amperometrically.
<b>Results</b>: In absence of lipid, nitrosative stress was inhibited by ascorbic acid through conversion of nitrosating
species to nitric oxide. Addition of ascorbic acid reduced the amount of N-nitrosodimethylamine formed by
fivefold, N-nitrosomorpholine by .1000-fold, and totally prevented the formation of N-nitrosodiethylamine
and N-nitrosopiperidine. In contrast, when 10% lipid was present, ascorbic acid increased the amount of Nnitrosodimethylamine,
N-nitrosodiethylamine and N-nitrosopiperidine formed by approximately 8-, 60- and
140-fold, respectively, compared with absence of ascorbic acid.
<b>Conclusion</b>: The presence of lipid converts ascorbic acid from inhibiting to promoting acid nitrosation. This
may be explained by nitric oxide, formed by ascorbic acid in the aqueous phase, being able to regenerate
nitrosating species by reacting with oxygen in the lipid phase
Trapping cold atoms near carbon nanotubes: thermal spin flips and Casimir-Polder potential
We investigate the possibility to trap ultracold atoms near the outside of a
metallic carbon nanotube (CN) which we imagine to use as a miniaturized
current-carrying wire. We calculate atomic spin flip lifetimes and compare the
strength of the Casimir-Polder potential with the magnetic trapping potential.
Our analysis indicates that the Casimir-Polder force is the dominant loss
mechanism and we compute the minimum distance to the carbon nanotube at which
an atom can be trapped.Comment: 8 pages, 3 figure
Size Effects in Carbon Nanotubes
The inter-shell spacing of multi-walled carbon nanotubes was determined by
analyzing the high resolution transmission electron microscopy images of these
nanotubes. For the nanotubes that were studied, the inter-shell spacing
is found to range from 0.34 to 0.39 nm, increasing with
decreasing tube diameter. A model based on the results from real space image
analysis is used to explain the variation in inter-shell spacings obtained from
reciprocal space periodicity analysis. The increase in inter-shell spacing with
decreased nanotube diameter is attributed to the high curvature, resulting in
an increased repulsive force, associated with the decreased diameter of the
nanotube shells.Comment: 4 pages. RevTeX. 4 figure
A Monolithic Time Stretcher for Precision Time Recording
Identifying light mesons which contain only up/down quarks (pions) from those
containing a strange quark (kaons) over the typical meter length scales of a
particle physics detector requires instrumentation capable of measuring flight
times with a resolution on the order of 20ps. In the last few years a large
number of inexpensive, multi-channel Time-to-Digital Converter (TDC) chips have
become available. These devices typically have timing resolution performance in
the hundreds of ps regime. A technique is presented that is a monolithic
version of ``time stretcher'' solution adopted for the Belle Time-Of-Flight
system to address this gap between resolution need and intrinsic multi-hit TDC
performance.Comment: 9 pages, 15 figures, minor corrections made, to appear as JINST_008
Raman imaging and electronic properties of graphene
Graphite is a well-studied material with known electronic and optical
properties. Graphene, on the other hand, which is just one layer of carbon
atoms arranged in a hexagonal lattice, has been studied theoretically for quite
some time but has only recently become accessible for experiments. Here we
demonstrate how single- and multi-layer graphene can be unambiguously
identified using Raman scattering. Furthermore, we use a scanning Raman set-up
to image few-layer graphene flakes of various heights. In transport experiments
we measure weak localization and conductance fluctuations in a graphene flake
of about 7 monolayer thickness. We obtain a phase-coherence length of about 2
m at a temperature of 2 K. Furthermore we investigate the conductivity
through single-layer graphene flakes and the tuning of electron and hole
densities via a back gate
Characteristics of the terrestrial field-aligned current system
We present the first ever comprehensive statistical study of the
spatiotemporal characteristics of field-aligned currents in the terrestrial
magnetosphere-ionosphere system using multi point measurements. We determine
how the FAC density, variability and scale size are coupled. The three ST 5
satellites were in a pearls-on-a-string formation making measurements of the
magnetic field with variable inter-spacecraft separations ranging from a few
seconds to about 10 min. More than 4700 sets of satellite passes are
analyzed using a robust correlation analysis aimed at determining the
variability of the FAC system as a function of scale size and satellite
spacing. We find significant differences between the FAC characteristics on
the dayside and on the nightside in terms of dynamics of the current
systems. On the dayside the FAC characteristics are found to be independent
of IMF <I>B</I><sub>z</sub> and geomagnetic activity while the nightside indicates increased
variability during disturbed conditions. The boundary separating highly and
poorly correlated FACs can be fitted by a linear line for satellite
separations shorter than 60 s (dayside) and 160 s (nightside). We interpret
this as the dayside and nightside magnetospheric reconfiguration times
respectively. For times exceeding this the FAC characteristics are suggested
to be controlled by the solar wind (dayside) and plasma sheet (nightside)
dynamics. Finally, the characteristics of FAC system with scale sizes larger
than ~200 km (at ionospheric altitude) appear to be stable and
repeatable on time scales of the order of a minute (i.e. comparable to the
low-altitude orbiting satellite's traverse time across the auroral belt). In
this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on
average the large-scale currents with scale sizes of the Region1 and Region2
are quasi-persistently significant in the transport of energy and momentum
between the magnetosphere and the ionosphere
Superlattice properties of carbon nanotubes in a transverse electric field
Electron motion in a (n,1) carbon nanotube is shown to correspond to a de
Broglie wave propagating along a helical line on the nanotube wall. This
helical motion leads to periodicity of the electron potential energy in the
presence of an electric field normal to the nanotube axis. The period of this
potential is proportional to the nanotube radius and is greater than the
interatomic distance in the nanotube. As a result, the behavior of an electron
in a (n,1) nanotube subject to a transverse electric field is similar to that
in a semiconductor superlattice. In particular, Bragg scattering of electrons
from the long-range periodic potential results in the opening of gaps in the
energy spectrum of the nanotube. Modification of the bandstructure is shown to
be significant for experimentally attainable electric fields, which raises the
possibility of applying this effect to novel nanoelectronic devices.Comment: 7 pages, 3 figure
Possible Superconductivity at 37 K in Graphite-Sulfur Composite
Sulfur intercalated graphite composites with diamagnetic transitions at 6.7 K
and 37 K are prepared. The magnetization hysteresis loops (MHL), Xray
diffraction patterns, and resistance were measured. From the MHL, a slight
superconducting like penetration process is observed at 15 K in low field
region. The XRD shows no big difference from the mixture of graphite and sulfur
indicating that the volume of the superconducting phase (if any) is very small.
The temperature dependence of resistance shows a typical semiconducting
behavior with a saturation in low temperature region. This saturation is either
induced by the de-localization of conducting electrons or by possible
superconductivity in this system.Comment: CHIN. PHYS.LETT v18 1648 (2001
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