3,493 research outputs found
Spontaneous-Desorption Ionizer for a TOF-MS
A time-of-flight mass spectrometer (TOF-MS) like the one mentioned in the immediately preceding article has been retrofitted with an ionizer based on a surface spontaneous-desorption process. This ionizer includes an electron multiplier in the form of a microchannel plate (MCP). Relative to an ionizer based on a hot-filament electron source, this ionizer offers advantages of less power consumption and greater mechanical ruggedness. The current density and stability characteristics of the electron emission of this ionizer are similar to those of a filament-based ionizer. In tests of various versions of this ionizer in the TOF-MS, electron currents up to 100 nA were registered. Currents of microamperes or more - great enough to satisfy requirements in most TOFMS applications - could be obtained by use of MCPs different from those used in the tests, albeit at the cost of greater bulk. One drawback of this ionizer is that the gain of the MCP decreases as a function of the charge extracted thus far; the total charge that can be extracted over the operational lifetime is about 1 coulomb. An MCP in the ion-detector portion of the TOF-MS is subject to the same limitation
Direct tissue analysis of phospholipids in rat brain using MALDI-TOFMS and MALDI-ion mobility-TOFMS
After water, lipids are the most common biomolecules found in the brain (12%). A brief perusal of the physiology, anatomy, and pathophysiology of the brain illustrates the importance of lipids. Recent advances in mass spectrometry have allowed the direct probing of tissues. However, most studies have focused on proteins. In the present work, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) and MALDI-ion mobility (IM)-TOFMS were employed for direct analysis of phospholipids in rat brain tissue. Molecular ions (MH+) corresponding to phosphatidylcholines, phosphatidylethanolamines, and sphingomyelin, were recorded. When studying pharmacology, we learn that many therapeutic compounds are stored in the body’s adipose tissue. MALDI-TOFMS and MALDI- IM-TOFMS were thus used to analyze rat brain tissue with chlorisondamine added directly onto the tissue slice. With both techniques, noncovalent complexes between the tissue phospholipids and chlorisondamine were detected. In addition, MALDI-IM-TOFMS of noncovalent complexes between phospholipids and chlorisondamine displayed a mobility between that of an isobaric lipid and peptide
Hybrid Ion-Detector/Data-Acquisition System for a TOF-MS
A modified ion-detector/data-acquisition system has been devised to increase the dynamic range of a time-of-flight mass spectrometer (TOF-MS) that, previously, included a microchannel-plate detector and a data-acquisition system based on counting pulses and time-tagging them by use of a time-to-digital converter (TDC). The dynamic range of the TOF-MS was limited by saturation of the microchannel plate detector, which can handle no more than a few million counts per second. The modified system includes (1) a combined microchannel plate/discrete ion multiplier and (2) a hybrid data-acquisition system that simultaneously performs analog current or voltage measurements and multianode single-ion-pulse-counting time-of-flight measurements to extend the dynamic range of a TDC into the regime in which a mass peak comprises multiple ions arriving simultaneously at the detector. The multianode data are used to determine, in real time, whether the detector is saturated. When saturation is detected, the data-acquisition system selectively enables circuitry that simultaneously determines the ion-peak intensity by measuring the time profile of the analog current or voltage detector-output signal
Drinfeld Twists and Symmetric Bethe Vectors of Supersymmetric Fermion Models
We construct the Drinfeld twists (factorizing -matrices) of the
-invariant fermion model. Completely symmetric representation of the
pseudo-particle creation operators of the model are obtained in the basis
provided by the -matrix (the -basis). We resolve the hierarchy of the
nested Bethe vectors in the -basis for the supersymmetric model.Comment: Latex File, 24 pages, no figure, some misprints are correcte
Shot noise-mitigated secondary electron imaging with ion count-aided microscopy
Modern science is dependent on imaging on the nanoscale, often achieved
through processes that detect secondary electrons created by a highly focused
incident charged particle beam. Scanning electron microscopy is employed in
applications such as critical-dimension metrology and inspection for
semiconductor devices, materials characterization in geology, and examination
of biological samples. With its applicability to non-conducting materials (not
requiring sample coating before imaging), helium ion microscopy (HIM) is
especially useful in the high-resolution imaging of biological samples such as
animal organs, tumor cells, and viruses. However, multiple types of measurement
noise limit the ultimate trade-off between image quality and the incident
particle dose, which can preclude useful imaging of dose-sensitive samples.
Existing methods to improve image quality do not fundamentally mitigate the
noise sources. Furthermore, barriers to assigning a physically meaningful scale
make these modalities qualitative. Here we introduce ion count-aided microscopy
(ICAM), which is a quantitative imaging technique that uses statistically
principled estimation of the secondary electron yield. With a readily
implemented change in data collection, ICAM nearly eliminates the influence of
source shot noise -- the random variation in the number of incident ions in a
fixed time duration. In HIM, we demonstrate 3x dose reduction; based on a good
match between these empirical results and theoretical performance predictions,
the dose reduction factor is larger when the secondary electron yield is
higher. ICAM thus facilitates imaging of fragile samples and may make imaging
with heavier particles more attractive
Determinant Representations of Correlation Functions for the Supersymmetric t-J Model
Working in the -basis provided by the factorizing -matrix, the scalar
products of Bethe states for the supersymmetric t-J model are represented by
determinants. By means of these results, we obtain determinant representations
of correlation functions for the model.Comment: Latex File, 41 pages, no figure; V2: minor typos corrected, V3: This
version will appear in Commun. Math. Phy
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SMART-1 Impact Ground-based campaign
Based on predictions of impact magnitude and cloud ejecta dynamics, we organized a SMART-1 ground-based observation campaign to perform coordinated measurements of the impact. Results from the coordinated multi-site campaign will be discussed
Investigating the topology of interacting networks - Theory and application to coupled climate subnetworks
Network theory provides various tools for investigating the structural or
functional topology of many complex systems found in nature, technology and
society. Nevertheless, it has recently been realised that a considerable number
of systems of interest should be treated, more appropriately, as interacting
networks or networks of networks. Here we introduce a novel graph-theoretical
framework for studying the interaction structure between subnetworks embedded
within a complex network of networks. This framework allows us to quantify the
structural role of single vertices or whole subnetworks with respect to the
interaction of a pair of subnetworks on local, mesoscopic and global
topological scales.
Climate networks have recently been shown to be a powerful tool for the
analysis of climatological data. Applying the general framework for studying
interacting networks, we introduce coupled climate subnetworks to represent and
investigate the topology of statistical relationships between the fields of
distinct climatological variables. Using coupled climate subnetworks to
investigate the terrestrial atmosphere's three-dimensional geopotential height
field uncovers known as well as interesting novel features of the atmosphere's
vertical stratification and general circulation. Specifically, the new measure
"cross-betweenness" identifies regions which are particularly important for
mediating vertical wind field interactions. The promising results obtained by
following the coupled climate subnetwork approach present a first step towards
an improved understanding of the Earth system and its complex interacting
components from a network perspective
MAGIC Upper Limits for two Milagro-detected, Bright Fermi Sources in the Region of SNR G65.1+0.6
We report on the observation of the region around supernova remnant G65.1+0.6
with the stand-alone MAGIC-I telescope. This region hosts the two bright GeV
gamma-ray sources 1FGL J1954.3+2836 and 1FGL J1958.6+2845. They are identified
as GeV pulsars and both have a possible counterpart detected at about 35 TeV by
the Milagro observatory. MAGIC collected 25.5 hours of good quality data, and
found no significant emission in the range around 1 TeV. We therefore report
differential flux upper limits, assuming the emission to be point-like (<0.1
deg) or within a radius of 0.3 deg. In the point-like scenario, the flux limits
around 1 TeV are at the level of 3 % and 2 % of the Crab Nebula flux, for the
two sources respectively. This implies that the Milagro emission is either
extended over a much larger area than our point spread function, or it must be
peaked at energies beyond 1 TeV, resulting in a photon index harder than 2.2 in
the TeV band.Comment: 8 pages, 3 figures, 1 tabl
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