1,721,132 research outputs found
The inhomogeneous ionizing background following reionization
No full textWe study the spatial fluctuations in the hydrogen-ionizing background in the epoch following reionization (z ∼ 5-6). The rapid decrease with redshift in the photon mean free path (mfp), combined with the clustering of increasingly rare ionizing sources, can result in a very inhomogenous ionizing background during this epoch. We systematically investigate the probability density functions (PDFs) and power spectra of ionizing flux, by varying several parameters such as the mfp, minimum halo mass capable of hosting stars and halo duty cycle. In order to be versatile, we make use of analytic, seminumeric and numeric approaches. Our models show that the ionizing background indeed has sizable fluctuations during this epoch, with the PDFs being a factor of few wide at half of the maximum likelihood. The clustering of sources dominates the width of the PDFs, so analytic models must take large-scale clustering into account. The distributions also show marked asymmetries, with a high-value tail set by clustering on small scales, and a shorter low-value tail which is set by the mfp. The power spectrum of the ionizing background is much more sensitive to source properties than the PDF and can be well understood analytically with a framework similar to the halo model (usually used to describe dark matter clustering). Nevertheless, we find that Lyα forest spectra are extremely insensitive to the details of the ultraviolet background, despite marked differences in the PDFs and power spectra of our various ionizing backgrounds. Assuming a uniform ionizing background only underestimates the value of the mean ionization rate, 〈Γ12〉, inferred from the Lyα forest by a few per cent. Instead, analysis of the Lyα forest is dominated by the uncertainties in the density field. Thus, our results justify the common assumption of a uniform ionizing background in Lyα forest analysis even during this epoch. © 2009 RAS
Lyα damping wing constraints on inhomogeneous reionization
No full textOne well-known way to constrain the hydrogen neutral fraction, of the high-redshift intergalactic medium (IGM) is through the shape of the red damping wing of the Lyα absorption line. We examine this method's effectiveness in light of recent models showing that the IGM neutral fraction is highly inhomogeneous on large scales during reionization. Using both analytic models and 'seminumeric' simulations, we show that the 'picket-fence' absorption typical in reionization models introduces both scatter and a systematic bias to the measurement of. In particular, we show that simple fits to the damping wing tend to overestimate the true neutral fraction in a partially ionized universe, with a fractional error of ∼30 per cent near the middle of reionization. This bias is generic to any inhomogeneous model. However, the bias is reduced and can even underestimate if the observational sample only probes a subset of the entire halo population, such as quasars with large H ii regions. We also find that the damping wing absorption profile is generally steeper than one would naively expect in a homogeneously ionized universe. The profile steepens and the sightline-to-sightline scatter increases as reionization progresses. Of course, the bias and scatter also depend on and so can, at least in principle, be used to constrain it. Damping wing constraints must therefore be interpreted by comparison to theoretical models of inhomogeneous reionization. © 2008 RAS
Multivariate optimization of capillary electrophoresis methods: a critical review
No full textIn this article a review on the recent applications of multivariate techniques for optimization of electromigration methods, is presented. Papers published in the period from August 2007 to February 2013, have been taken into consideration. Upon a brief description of each of the involved CE operative modes, the characteristics of the chemometric strategies (type of design, factors and responses) applied to face a number of analytical challenges, are presented. Finally, a critical discussion, giving some practical advices and pointing out the most common issues involved in multivariate set-up of CE methods, is provided
The ionizing background at the end of reionization
No full textOne of the most sought-after signatures of reionization is a rapid increase in the ionizing background (usually measured through the Lyα optical depth towards distant quasars). Conventional wisdom associates this with the 'overlap' phase when ionized bubbles merge, allowing each source to affect a much larger volume. We argue that this picture fails to describe the transition to the post-overlap Universe, where Lyman-limit systems (LLSs) absorb ionizing photons over moderate length-scales (≲20-100 Mpc). Using an analytic model, we compute the probability distribution of the amplitude of the ionizing background throughout reionization, including both discrete ionized bubbles and LLSs (parametrized by an attenuation length, which we impose rather than attempt to model self-consistently). We show that the overlap does not by itself cause a rapid increase in the ionizing background or a rapid decrease in the mean Lyα transmission towards distant quasars. More detailed seminumeric models support these conclusions. We argue that the rapid changes should instead be interpreted as evolution in the attenuation length itself, which may or may not be directly related to overlap. © 2009 RAS
Efficient Simulations of Early Structure Formation and Reionization
No full textDetailed theoretical studies of the high-redshift universe, and especially reionization, are generally forced to rely on time-consuming N-body codes and/or radiative transfer algorithms. We present a method to construct seminumerical "simulations," which can efficiently generate realizations of halo distributions and ionization maps at high redshifts. Our procedure combines an excursion-set approach with first-order Lagrangian perturbation theory and operates directly on the linear density and velocity fields. As such, the achievable dynamic range with our algorithm surpasses the current practical limit of N-body codes by orders of magnitude. This is particularly significant in studies of reionization, where the dynamic range is the principal limiting factor, because ionized regions reach scales of tens of comoving Mpc. We test our halo-finding and ionization-mapping algorithms separately against N-body simulations with radiative transfer and obtain excellent agreement. We compute the size distributions of ionized and neutral regions in our maps. We find even larger ionized bubbles than do purely analytic models at the same volume-weighted mean hydrogen neutral fraction, x̄H I, especially early in reionization. We also generate maps and power spectra of 21 cm brightness temperature fluctuations, which for the first time include corrections due to gas bulk velocities. We find that velocities widen the tails of the temperature distributions and increase small-scale power, although these effects quickly diminish as reionization progresses. We also include some preliminary results from a simulation run with the largest dynamic range to date: a 250 Mpc box that resolves halos with masses M ≥ 2.2 × 10 8 M⊙. We show that accurately modeling the late stages of reionization, x̄H I, ≲ 0.5, requires such large scales. The speed and dynamic range provided by our seminumerical approach will be extremely useful in the modeling of early structure formation and reionization. © 2007. The American Astronomical Society. All rights reserved
High-performance liquid chromatographic assayof erythromycin from biological matrix using electrochemical or ultraviolet detection
No full textTwo chromatographic methods were developed for the determination of erythromycin A (EA) residues in animal tissues (muscle, liver, kidney and fat of cattle, pigs and poultry) and cow's milk. In addition to a more traditional method using electrochemical detection, we developed an original alternative method based on UV detection at 236 nm, by pretreating to create a chromophore in the molecule. An internal standard was used with both methods to check the variability of the analytical system. Analysis times and performance were compared. The recovery of EA from various matrices was greater than 95%. For both methods the quantification limit for EA was 0.25 μg ml-1 for plasma, 0.025 μg g-1 for milk and 0.125 μg g-1 for the other biological matrices. The methods can be used to check for EA residues in these matrices; in fact, the statutory maximum residue limits (MRLs) of EA are 0.4 μg g-1 in muscle, kidney, liver and fat of beef cattle, sheep, pigs and poultry, and 0.04 μg g-1 in cow's and sheep's milk
Determination of some quinolones in tablets, human plasma and urine by differential-pulse polarography
No full textA differential pulse polarographic method was developed for the determination of norfloxacin, cinoxacin and pipemidic. oxolinic and piromidic acids in tablets and biological fluids. Well defined peaks, useful for an accurate and precise assay, were observed in the appropriate supporting electrolyte (Britton-Robinson and phosphate buffers), depending on both the kind of preparation (tablet, plasma or urine) and the quinolone investigated. The analysis of quinolones in biological fluids requires a prior clean-up procedure (treatment with acetonitrile and 2 M potassium hydroxide for plasma and solid-liquid extraction for urine) while common excipients were found not to interfere in the tablet assay. In each of the above situations (tablet, plasma or urine), good precision of the method evaluated as the CV, was found. © 1994
Pullulan as a permanent hydrophilic coating in capillary electrophoresis: applications to penicillin G acylase as chiral selector
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