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61.
Jonathan Levine Michael R. Savina Thomas Stephan Nicolas Dauphas Andrew M. Davis Kim B. Knight Michael J. Pellin 《International journal of mass spectrometry》2009,288(1-3):36-43
Resonance ionization mass spectrometry offers extremely high sensitivity and elemental selectivity in microanalysis, but the isotopic precision attainable by this technique has been limited. Measured isotope ratios are sensitive to small fluctuations in the pointing, pulse timing, and wavelength of the resonance lasers. We show that, by minimizing these fluctuations using feedback controls and by power-broadening the optical transitions, we are able to measure chromium isotope ratios with statistics-limited precision better than 1%. Small additional improvements in reproducibility come from careful shaping of the electric field in the region where atoms are photoionized and from minimizing pulse-to-pulse variations in the time-of-flight mass spectrometer through which the photoions travel. The increased reproducibility of isotopic measurements on standard materials has enabled us to detect anomalous chromium isotopic abundances in presolar SiC grains extracted from primitive meteorites. 相似文献
62.
Broadening-independent demodulation for wavelength modulation spectroscopy based on even-order harmonics 总被引:1,自引:0,他引:1
The line broadening of gas absorption spectrum is a significant but unknown factor in traditional demodulation of wavelength modulation spectroscopy, which is a kind of tunable diode laser absorption spectroscopy in weak absorption cases. In this paper, another signal demodulation method based on some simple algebraic operations with the amplitudes of the zero-, second-, and fourth-order harmonics was proposed. With this method the effect of spectrum broadening could be selfeliminated. That makes the amplitude of the demodulated signal is irrelevant to the broadening, and enables the direct and fast measurement without complex simulations and curve fittings in traditional method. 相似文献
63.
We modify a uniform model of single bubble sonoluminescence, in which heat diffusion, water vapor diffusion and chemical reactions
are included to describe the bubble dynamics, and the processes of electron-atom bremsstrahlung, electron-ion bremsstrahlung
and recombination radiation, radiative attachment of electrons to atoms and molecules, line emissions of OH radicals and Na
atoms are taken into account to calculate the light emission. With this model, we compute the light pulse width, the photon
number per flash, the continuum and line spectra and the gas species as the products of chemical reactions, and try to compare
with all the experimental data available. We obtain good agreement with the observations of Ar and Xe bubbles in many cases,
but fail to match the experimental data of the photon number per flash. We also find that for He bubble the computed photon
number is always too small to interpret the observations.
Supported by the National Natural Science Foundation of China (Grant Nos. 10674081 and 10434070) 相似文献
64.
Analytical approaches have been developed for one-dimensional monochromatic delta-Eddington radiative transfer equation through a vertically inhomogeneous medium. They are based on the solution of the Riccati equation that arises from the decoupling of the two-stream radiances, and seek to approximate the exponent functions in the solution as opposed to finding the solution as a whole. Depending on the case, Green–Liouville approximation or other techniques presented in this paper are utilized for finding these exponents. Though developed for atmospheric radiative transfer problems applicable to the global climate change modelling, and for non-invasive medical applications on tissue–light interactions, the techniques considered here are quiet general in nature. Hence, they can also be useful in other boundary value problems of the diffusion type that involve linear second order ordinary differential equations with variable coefficients. 相似文献
65.
Perdeuterated benzofluoranthenes have slightly shorter retention times than their equivalent unlabeled forms in a DB-5 capillary column. Under column overload conditions, perdeuterated benzofluoranthenes in moderate multifold excesses, which might be encountered in their use as internal standards and carriers for quantitative analysis, are seen to exhibit both normal and reverse solvent effects on their close eluting congeners. In some cases the effects may be used to advantage by knowledgeable analysts, but for the ignorant and unwary the effects can lead to serious errors in identification and quantification. 相似文献
66.
Frederik Detobel Veronika Fekete Wim De Malsche Selm De Bruyne Han Gardeniers Gert Desmet 《Analytical and bioanalytical chemistry》2009,394(2):399-411
The present paper provides a detailed analysis of the analyte-wall adsorption effects in nanochannels, including a random
walk study of the analyte-wall collision frequency, and uses these insights to estimate wall desorption times from chromatographic
experiments in nanochannels. Using coumarin dye analytes and using a methanol/water mixture buffered at pH 3 in 120-nm deep
channels, the surface desorption times on naked fused-silica glass were found to be maximally of the order of 60 to 150 μs,
while they were found to be on the order of 100 to 500 μs on a hydrophobically coated wall. These nonzero adsorption and desorption
times lead to an additional band broadening when conducting chromatographic separations. Shear-driven flows, requiring a noncoated
moving wall and a stationary coated wall, intrinsically turn out to be more prone to this effect than pressure-driven or electro-driven
flows for example. The present study also shows that, interestingly, the number of analyte-wall collisions increases with
the inverse of the channel depth and not with its second power, as would be expected from the Einstein–Smoluchowski relationship
for molecular diffusion. 相似文献
67.
Houde Han Zhongyi Huang 《计算数学(英文版)》2008,(5):728-739
In this paper, we propose a tailored-finite-point method for the numerical simulation of the Helmholtz equation with high wave numbers in heterogeneous medium. Our finite point method has been tailored to some particular properties of the problem, which allows us to obtain approximate solutions with the same behaviors as that of the exact solution very naturally. Especially, when the coefficients are piecewise constant, we can get the exact solution with only one point in each subdomain. Our finite-point method has uniformly convergent rate with respect to wave number k in L^2-norm. 相似文献
68.
Boleslaw Grabowski 《Pramana》1987,29(5):455-460
The impact broadening by charged electron gas is calculated. A proper account of the Debye screening has been taken as also
the curvilinearity of the perturber’s trajectories. The results have been given in the form of graphs for a ready comparison
with earlier results. 相似文献
69.
The complex nature of filling factor ν = 0 of monolayer graphene is studied in magnetotransport experiments. As a function of perpendicular magnetic field a metal-insulator transition is observed, which is attributed to disorder-induced Landau level broadening in the canted antiferromagnetic phase. In the metallic regime a separation of the zeroth Landau level appears and signs of the quantum spin Hall effect are seen near ν = 0. In addition to local transport, nonlocal transport experiments show results being consistent with helical edge transport. 相似文献
70.
In this paper, the dynamic response of gelatin-based soft material under impact loading is investigated. The dynamic tests are principally performed by the classical SHPB (Split Hopkinson Pressure Bars) technique. However, due to the very low mechanical impedance of the specimen compared with the Hopkinson bars, the feeble impact forces are measured by highly sensitive piezoelectric polyvinylidene fluoride (PVDF) pressure sensors instead of SHPB measurement system. The PVDF pressure sensors are placed on the interfaces between the specimen and the bars. During the impact test, the non-equilibrium stress state and inhomogeneous strain fields are developed in the specimen; a digital image correlation (DIC) technique is proposed to identify the inhomogeneous displacement fields using high speed photography. A non-parametric approach based on the DIC technique is developed to deduce the transient stress fields in the longitudinal and transverse directions from the displacement fields measured by DIC. The validation of the calculated stress fields is performed by comparing them with the stress measurements from the PVDF pressure sensor at the bottom end of the specimen. Furthermore, stress-strain response is carried out using this approach throughout the specimen. It is clearly shown that the average highest strain rate varies with position in the specimen. This lead to multiple stress-strain relations determined at different strain rates by only one impact test. The significant strain rate sensitivity is observed at the tested rate range from 81/s to 269/s. Under compression loading, the axial stress state is developed as a simple compression only in the central part of the specimen due to the friction at the interfaces between the specimen and the bars. According to the calculated results based on movement of “long waves”, the region of the simple compression stress state in the central part of the specimen is localized. It is observed that the axial stress is much more important than the transverse stress in the central part and this confirms the assumption of uni-axial compression stress state in the specimen. 相似文献