2,512 research outputs found
Calibrant Delivery for Mass Spectrometry
This article describes a means of sampling ions that are created at a location remote from the primary ion source used for mass spectral analysis. Such a source can be used for delivery of calibrant ions on demand. Calibrant ions are sprayed into an atmospheric pressure chamber, at a position substantially removed from the sampling inlet. A gas flow sweeps the calibrants towards the sampling inlet, and a new means for toggling the second ion beam into the instrument can be achieved with the use of a repelling field established by an electrode in front of the sampling inlet. The physical separation of two or more sources of ions eliminates detrimental interactions due to gas flows or fields. When using a nanoflow electrospray tip as the primary ion source, the potential applied to the tip completely repels calibrant ions and there is no compromise in terms of electrospray performance. When calibrant ions are desired, the potential applied to the nanoflow electrospray tip is lowered for a short period of time to allow calibrant ions to be sampled into the instrument, thus providing a means for external calibration that avoids the typical complications and compromises associated with dual spray sources. It is also possible to simultaneously sample ions from multiple ion beams if necessary for internal mass calibration purposes. This method of transporting additional ion beams to a sampling inlet can also be used with different types of atmospheric pressure sources such as AP MALDI, as well as sources configured to deliver ions of different polarity
Collision-induced dissociation of bradykinin ions in the interface region of an ESI-MS
AbstractBy applying different electric field strengths to the orifice–skimmer region of an electrospray ionization mass spectrometer, the rate of dissociation can be varied based on the amount of internal energy acquired by an ion through collisions with the curtain gas molecules. Both the Arrhenius equation and Rice-Ramsperger-Kassel (RRK) theory can be used to predict the rate of dissociation of internally excited molecules. A previously determined model for collision-induced dissociation is tested by comparison of predicted and experimentally observed orifice–skimmer potential differences for dissociation of ions. The rate of collision-induced dissociation of bradykinin ions is determined by monitoring the fragments produced in a mass spectrometer. The semi-quantitative model is found to yield effective predictions when accurate Arrhenius and RRK parameters are utilized
Mesoscopic Fermi gas in a harmonic trap
We study the thermodynamical properties of a mesoscopic Fermi gas in view of
recent possibilities to trap ultracold atoms in a harmonic potential. We focus
on the effects of shell closure for finite small atom numbers. The dependence
of the chemical potential, the specific heat and the density distribution on
particle number and temperature is obtained. Isotropic and anisotropic traps
are compared. Possibilities of experimental observations are discussed.Comment: 8 pages, 9 eps-figures included, Revtex, submitted to Phys. Rev. A,
minor changes to figures and captions, corrected typo
Momentum flux density, kinetic energy density and their fluctuations for one-dimensional confined gases of non-interacting fermions
We present a Green's function method for the evaluation of the particle
density profile and of the higher moments of the one-body density matrix in a
mesoscopic system of N Fermi particles moving independently in a linear
potential. The usefulness of the method is illustrated by applications to a
Fermi gas confined in a harmonic potential well, for which we evaluate the
momentum flux and kinetic energy densities as well as their quantal mean-square
fluctuations. We also study some properties of the kinetic energy functional
E_{kin}[n(x)] in the same system. Whereas a local approximation to the kinetic
energy density yields a multi-valued function, an exact single-valued
relationship between the density derivative of E_{kin}[n(x)] and the particle
density n(x) is demonstrated and evaluated for various values of the number of
particles in the system.Comment: 10 pages, 5 figure
Quasi-continuous atom laser in the presence of gravity
We analyse the extraction of a coherent atomic beam from a trapped
Bose-Einstein condensate using a rf transition to a non-trapping state at T=0
K. Our quantum treatment fully takes gravity into account but neglects all
interactions in the free falling beam. We obtain an analytical expression of
the output rate and of the wave function of the extracted beam, i.e. the output
mode of the ``atom laser''. Our model reproduces satisfactorily experimental
data without any adjustable parameter.Comment: 4 pages, 2 figure
Scattering of short laser pulses from trapped fermions
We investigate the scattering of intense short laser pulses off trapped cold
fermionic atoms. We discuss the sensitivity of the scattered light to the
quantum statistics of the atoms. The temperature dependence of the scattered
light spectrum is calculated. Comparisons are made with a system of classical
atoms who obey Maxwell-Boltzmann statistics. We find the total scattering
increases as the fermions become cooler but eventually tails off at very low
temperatures (far below the Fermi temperature). At these low temperatures the
fermionic degeneracy plays an important role in the scattering as it inhibits
spontaneous emission into occupied energy levels below the Fermi surface. We
demonstrate temperature dependent qualitative changes in the differential and
total spectrum can be utilized to probe quantum degeneracy of trapped Fermi gas
when the total number of atoms are sufficiently large . At smaller
number of atoms, incoherent scattering dominates and it displays weak
temperature dependence.Comment: updated figures and revised content, submitted to Phys.Rev.
Health system actors' perspectives of prescribing practices in public health facilities in Eswatini: A Qualitative Study
Rational medicines use (RMU) is the prescribing/dispensing of good quality medicines to meet individual patient's clinical needs. Policy-makers, managers and frontline providers play critical roles in safeguarding medicine usage thus ensuring their rational use. This study investigated perspectives of key health system actors on prescribing practices and factors influencing these in Eswatini. Public sector healthcare service delivery is through health facilities (public sector, not-for-profit faith-based, industrial) and community-based care. Methods A qualitative, exploratory study using semi-structured in-depth interviews with seven policymakers and managers, and 32 facility-based actors was conducted. Drawing on Social Practice Theory, material (health system context), competence (provider) and cultural (patient and provider) factors influencing prescribing practices were explored. Results Participants were aged between 21-57years, had been practicing for 1-30 years, and were a mix of doctors, nurses, pharmacists and pharmacy-technicians. Factors contributing to irrational medicines use included: Poor use of treatment guidelines, lack of RMU policies, poorly-functioning pharmaceutical and therapeutics committees, stock-outs of medicines, lack of pharmacy personnel in primary healthcare facilities, and restrictions of medicines by level of care. Provider-related factors included: Knowledge, experience and practice ethic, symptomatic prescribing, high patient numbers
Sympathetic cooling of an atomic Bose-Fermi gas mixture
Sympathetic cooling of an atomic Fermi gas by a Bose gas is studied by
solution of the coupled quantum Boltzmann equations for the confined gas
mixture. Results for equilibrium temperatures and relaxation dynamics are
presented, and some simple models developed. Our study illustrate that a
combination of sympathetic and forced evaporative cooling enables the Fermi gas
to be cooled to the degenerate regime where quantum statistics, and mean field
effects are important. The influence of mean field effects on the equilibrium
spatial distributions is discussed qualitatively.Comment: 8 pages, 9 figures, accepted for publication in Phys.Rev.Let
Effect of quantum group invariance on trapped Fermi gases
We study the properties of a thermodynamic system having the symmetry of a
quantum group and interacting with a harmonic potential. We calculate the
dependence of the chemical potential, heat capacity and spatial distribution of
the gas on the quantum group parameter and the number of spatial dimensions
. In addition, we consider a fourth-order interaction in the quantum group
fields , and calculate the ground state energy up to first order.Comment: LaTeX file, 20 pages, four figures, uses epsf.sty, packaged as a
single tar.gz uuencoded fil
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