Determination of some trace elements in food and soil samples by atomic absorption spectrometry after coprecipitation with holmium hydroxide

The determination of trace elements in food and soil samples by atomic absorption spectrometry was investigated. A coprecipitation procedure with holmium hydroxide was used for separation-preconcentration of trace elements. Trace amounts of copper(II), manganese(II), cobalt(II), nickel(ll), chromium(lll), iron(Ill), cadmium(ll), and lead(ll) ions were coprecipitated with holmium hydroxide in 2.0 M NaOH medium. The optimum conditions for the coprecipitation process were investigated for several commonly tested experimental parameters, such as amount of coprecipitant, effect of standing time, centrifugation rate and time, and sample volume. The precision, based on replicate analysis, was lower than 10% for the analytes. In order to verify the accuracy of the method, the certified reference materials BCR 141 R calcareous loam soil and CRM 025-050 soil were analyzed. The procedure was successfully applied for separation and preconcentration of the investigated ions in various food and soil samples. An amount of the solid samples was decomposed with 15 mL concentrated hydrochloric acid-concentrated nitric acid (3 + 1). The preconcentration procedure was then applied to the final solutions. The concentration of trace elements in samples was determined by atomic absorption spectrometry.     

The collocation method for the solution of boundary integral equations

This article is devoted to investigating the approximate solutions to a class of boundary integral equations over a closed, bounded and smooth surface found via the collocation method. This article provides sufficient conditions for the convergence of this method in the space of continuous functions.     

Dynamic magnetic behavior of the mixed spin （2, 5/2） Ising system with antiferromagnetic/antiferromagnetic interactions on a bilayer square lattice

Using the mean-field theory and Glauber-type stochastic dynamics, we study the dynamic magnetic properties of the mixed spin （2, 5/2） Ising system for the antiferromagnetic/antiferromagnetic （AFM/AFM） interactions on the bilayer square lattice under a time varying （sinusoidal） magnetic field. The time dependence of average magnetizations and the thermal variation of the dynamic magnetizations are examined to calculate the dynamic phase diagrams. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and the effects of interlayer coupling interaction on the critical behavior of the system are investigated. We also investigate the influence of the frequency and find that the system displays richer dynamic critical behavior for higher values of frequency than that of the lower values of it. We perform a comparison with the ferromagnetic/ferromagnetic （FM/FM） and AFM/FM interactions in order to see the effects of AFM/AFM interaction and observe that the system displays richer and more interesting dynamic critical behaviors for the AFM/AFM interaction than those for the FM/FM and AFM/FM interactions.     

Asymptotic behavior of second sound thermoelasticity with internal time-varying delay

In this paper, we consider a thermoelastic system of second sound with internal time-varying delay. Under suitable assumption on the weight of the delay, we prove, using the energy method, that the damping effect through heat conduction given by Cattaneo’s law is still strong enough to uniformly stabilize the system even in the presence of time delay.     

Well posedness and asymptotic behavior of a coupled system of nonlinear viscoelastic equations

In this paper, we consider a nonlinear system of two coupled viscoelastic equations which describes the interaction between two different fields arising in viscoelasticity. We prove the well-posedness and, for a wider class of relaxation functions, establish a generalized stability result for this system.     

Dynamic magnetic behavior of the mixed-spin bilayer system in an oscillating field within the mean-field theory

The dynamic magnetic behavior of the mixed Ising bilayer system (σ=2$\sigma =2$ and S=5/2$S=5/2$), with a crystal-field interaction in an oscillating field are studied, within the mean-field approach, by using the Glauber-type stochastic dynamics for both ferromagnetic/ferromagnetic and antiferromagnetic/ferromagnetic interactions. The time variations of average magnetizations and the temperature dependence of the dynamic magnetizations are investigated. The dynamic phase diagrams are presented in the reduced temperature and magnetic field amplitude plane and they exhibit several ordered phases, coexistence phase regions and critical points as well as a re-entrant behavior depending on interaction parameters.     

Dynamic magnetic properties in the kinetic mixed spin-2 and spin-5/2 Ising model under a time-dependent magnetic field

We extend the recent paper [W. Jiang, V-C. Lo, B-D. Bai, J. Yang, Physica A 389 (2010) 2227-2233] to present a study, within a mean-field approach, the dynamic magnetic properties of the mixed spin-2 and spin-5/2 Ising ferrimagnetic system, which corresponds the molecular-based magnetic materials AFe^IIFe^III(C₂O₄)₃ [ A=N(n-C_nH_2n+1)₄, n=3-5], by using the Glauber-type stochastic dynamics. This mixed Ising ferrimagnetic system is used on a layered honeycomb lattice in which Fe^II (S=5/2) and Fe^III (σ=2) occupy sites. First, we investigate the time variations of average order parameters to find the phases in the system and then the thermal behavior of the dynamic order parameters to obtain the dynamic phase transition (DPT) points as well as to characterize the nature (first-or second-order) phase transitions. We also present the dynamic phase diagrams and study the dynamic magnetic hysteresis loop behaviors of the kinetic mixed spin-2 and spin-5/2 Ising ferrimagnetic system. The results are compared with some experimental and theoretical works and a good overall agreement is found.     

A Computer Program to Help Resolution of Complex and Poorly Resolved Cu2+ and VO2+ Ions Doped Single Crystals Electron Paramagnetic Resonance Spectra

Complex and poorly resolved Cu²⁺ and VO²⁺ doped single-crystal electron paramagnetic resonance (EPR) spectra are some of the serious problems that exist in this area. In order to help the resolution of this sort of spectra, and for easily resolvable spectra as well, a versatile computer program known as EPR RESolution, or EPRES, is presented. All detectable line positions in the single-crystal spectra taken in three mutually perpendicular planes are given as input. The program plots these line positions. The user then manually determines the lines by selecting the true data points on the plot and fitting them to a well-known variation function. If selection is not suitable, the process is canceled and renewed. By this process, as many resolvable lines as in the spectra can be resolved and determined. The user then groups the resolved lines according to the paramagnetic center to which they belong. This includes the attribution of correct nuclear spin I and M _I to correct lines. After this step, hyperfine and g tensor elements can be found, constructed, and diagonalized.

[Supplemental materials are available for this article. Go to the publisher's online edition of Spectroscopy Letters for the following free supplemental resource: a copy of the EPRES computer program.]     

Sequential Injection/Mid-Infrared Spectroscopic Analysis of an Acetone‐Butanol‐Ethanol Fermentation: Analyte Cross‐Correlation Effects

Mid‐infrared spectroscopy together with sequential injection analysis (SIA) and partial least squares (PLS) regression analysis was used to monitor acetone‐butanol‐ethanol (ABE) fermentations under different fermentation conditions. Five analytes were simultaneously predicted (acetone, acetate, butyrate, n‐butanol, and glucose). In order to compare the overall model prediction ability, a relative average of the root mean square error of prediction (RMSEP) across all five analytes was employed. To form a PLS model devoid of any cross‐correlations between analytes, a synthetic calibration data set was created by the SIA system. As a test of their robustness, PLS models from synthetic samples and those from real fermentation samples were compared and used to predict samples from the opposite data set and from independent “acid‐crash” fermentations. The PLS model developed from the synthetic samples proved to be far more robust and accurate and used fewer factors than PLS models from the real fermentations, which were found to contain analyte cross‐correlations. The use of synthetic data enabled more accurate selection of factors and showed the importance of investigating spectral regression coefficients plots to aid and confirm appropriate factor selection. In addition, an alternative method of factor selection was proposed, using a “similarity measure” between the regression coefficient plots of factors for certain analytes and their standard spectra. Predictions using this method of factor selection over the common “minimum from an error vs. factor” plot proved to be more accurate and used far fewer factors.