663 research outputs found
Continuous-flow IRMS technique for determining the 17O excess of CO2 using complete oxygen isotope exchange with cerium oxide
This paper presents an analytical system for analysis of all single
substituted isotopologues (<sup>12</sup>C<sup>16</sup>O<sup>17</sup>O,
<sup>12</sup>C<sup>16</sup>O<sup>18</sup>O, <sup>13</sup>C<sup>16</sup>O<sup>16</sup>O) in nanomolar quantities
of CO<sub>2</sub> extracted from stratospheric air samples. CO<sub>2</sub> is
separated from bulk air by gas chromatography and CO<sub>2</sub> isotope ratio
measurements (ion masses 45 / 44 and 46 / 44) are performed using isotope ratio
mass spectrometry (IRMS). The <sup>17</sup>O excess (Ī<sup>17</sup>O) is
derived from isotope measurements on two different CO<sub>2</sub> aliquots:
unmodified CO<sub>2</sub> and CO<sub>2</sub> after complete oxygen isotope exchange with
cerium oxide (CeO<sub>2</sub>) at 700 Ā°C. Thus, a single measurement of
Ī<sup>17</sup>O requires two injections of 1 mL of air with a CO<sub>2</sub>
mole fraction of 390 Ī¼mol mol<sup>ā1</sup> at 293 K and 1 bar pressure
(corresponding to 16 nmol CO<sub>2</sub> each). The required sample size
(including flushing) is 2.7 mL of air. A single analysis (one pair of
injections) takes 15 minutes. The analytical system is fully automated for
unattended measurements over several days. The standard deviation of the
<sup>17</sup>O excess analysis is 1.7‰. Multiple
measurements on an air sample reduce the measurement uncertainty, as
expected for the statistical standard error. Thus, the uncertainty for a
group of 10 measurements is 0.58‰ for Δ
<sup>17</sup>O in 2.5 h of analysis. 100 repeat analyses of one air sample
decrease the standard error to 0.20‰. The instrument
performance was demonstrated by measuring CO<sub>2</sub> on stratospheric air
samples obtained during the EU project RECONCILE with the high-altitude
aircraft Geophysica. The precision for RECONCILE data is 0.03‰ (1σ) for Ī“<sup>13</sup>C, 0.07‰ (1σ) for Ī“<sup>18</sup>O and 0.55‰ (1σ) for δ<sup>17</sup>O for a sample of 10
measurements. This is sufficient to examine stratospheric enrichments, which
at altitude 33 km go up to 12‰ for δ<sup>17</sup>O
and up to 8‰ for Ī“<sup>18</sup>O with respect to
tropospheric CO<sub>2</sub> : δ<sup>17</sup>O ~
21‰ Vienna Standard Mean Ocean Water (VSMOW), Ī“<sup>18</sup>O ~
41‰ VSMOW (LƤmmerzahl et al., 2002). The samples
measured with our analytical technique agree with available data for
stratospheric CO<sub>2</sub>
Single polymer adsorption in shear: flattening versus hydrodynamic lift and corrugation effects
The adsorption of a single polymer to a flat surface in shear is investigated
using Brownian hydrodynamics simulations and scaling arguments. Competing
effects are disentangled: in the absence of hydrodynamic interactions, shear
drag flattens the chain and thus enhances adsorption. Hydrodynamic lift on the
other hand gives rise to long-ranged repulsion from the surface which preempts
the surface-adsorbed state via a discontinuous desorption transition, in
agreement with theoretical arguments. Chain flattening is dominated by
hydrodynamic lift, so overall, shear flow weakens the adsorption of flexible
polymers. Surface friction due to small-wavelength surface potential
corrugations is argued to weaken the surface attraction as well.Comment: 6 pages, 4 figure
Why is the DNA Denaturation Transition First Order?
We study a model for the denaturation transition of DNA in which the
molecules are considered as composed of a sequence of alternating bound
segments and denaturated loops. We take into account the excluded-volume
interactions between denaturated loops and the rest of the chain by exploiting
recent results on scaling properties of polymer networks of arbitrary topology.
The phase transition is found to be first order in d=2 dimensions and above, in
agreement with experiments and at variance with previous theoretical results,
in which only excluded-volume interactions within denaturated loops were taken
into account. Our results agree with recent numerical simulations.Comment: Revised version. To appear in Phys. Rev. Let
Quench Induced Vortices in the Symmetry Broken Phase of Liquid He
Motivated by the study of cosmological phase transitions, our understanding
of the formation of topological defects during spontaneous symmetry-breaking
and the associated non-equilibrium field theory has recently changed.
Experiments have been performed in superfluid He to test the new ideas
involved. In particular, it has been observed that a vortex density is seen
immediately after pressure quenches from just below the transition.
We discuss possible interpretations of these vortices, conclude they are
consistent with our ideas of vortex formation and propose a modification of the
original experiments.Comment: 29 pages, RevTeX with one EPS figur
Path-integral analysis of fluctuation theorems for general Langevin processes
We examine classical, transient fluctuation theorems within the unifying
framework of Langevin dynamics. We explicitly distinguish between the effects
of non-conservative forces that violate detailed balance, and non-autonomous
dynamics arising from the variation of an external parameter. When both these
sources of nonequilibrium behavior are present, there naturally arise two
distinct fluctuation theorems.Comment: 24 pages, one figur
Entangled Polymer Rings in 2D and Confinement
The statistical mechanics of polymer loops entangled in the two-dimensional
array of randomly distributed obstacles of infinite length is discussed. The
area of the loop projected to the plane perpendicular to the obstacles is used
as a collective variable in order to re-express a (mean field) effective theory
for the polymer conformation. It is explicitly shown that the loop undergoes a
collapse transition to a randomly branched polymer with .Comment: 17 pages of Latex, 1 ps figure now available upon request, accepted
for J.Phys.A:Math.Ge
Statistical Mechanics of Membrane Protein Conformation: A Homopolymer Model
The conformation and the phase diagram of a membrane protein are investigated
via grand canonical ensemble approach using a homopolymer model. We discuss the
nature and pathway of -helix integration into the membrane that results
depending upon membrane permeability and polymer adsorptivity. For a membrane
with the permeability larger than a critical value, the integration becomes the
second order transition that occurs at the same temperature as that of the
adsorption transition. For a nonadsorbing membrane, the integration is of the
first order due to the aggregation of -helices.Comment: RevTeX with 5 postscript figure
Melting as a String-Mediated Phase Transition
We present a theory of the melting of elemental solids as a
dislocation-mediated phase transition. We model dislocations near melt as
non-interacting closed strings on a lattice. In this framework we derive simple
expressions for the melting temperature and latent heat of fusion that depend
on the dislocation density at melt. We use experimental data for more than half
the elements in the Periodic Table to determine the dislocation density from
both relations. Melting temperatures yield a dislocation density of (0.61\pm
0.20) b^{-2}, in good agreement with the density obtained from latent heats,
(0.66\pm 0.11) b^{-2}, where b is the length of the smallest
perfect-dislocation Burgers vector. Melting corresponds to the situation where,
on average, half of the atoms are within a dislocation core.Comment: 18 pages, LaTeX, 3 eps figures, to appear in Phys. Rev.
Onsager-Machlup theory and work fluctuation theorem for a harmonically driven Brownian particle
We extend Tooru-Cohen analysis for nonequilirium steady state(NSS) of a
Brownian particle to nonequilibrium oscillatory state (NOS) of Brownian
particle by considering time dependent external drive protocol. We consider an
unbounded charged Brownian particle in the presence of an oscillating electric
field and prove work fluctuation theorem, which is valid for any initial
distribution and at all times. For harmonically bounded and constantly dragged
Brownian particle considered by Tooru and Cohen, work fluctuation theorem is
valid for any initial condition(also NSS), but only in large time limit. We use
Onsager-Machlup Lagrangian with a constraint to obtain frequency dependent work
distribution function, and describe entropy production rate and properties of
dissipation functions for the present system using Onsager-Machlup functional.Comment: 6 pages, 1 figur
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