Computation of longitudinal motions of a viscoelastic bar

A Gelerkin type method, based on trigonometric functions and Crank-Nicolson discretizations of the time variable, is applied to compute solutions of the initial boundary value problem associated with the equation $$\rho _0 u_{tt} = a_1 u_{xx} + a_2 u_x^2 u_{xx} + a_3 u_{xtx} + f,x \in [0,1],t > 0.$$ Error estimates are derived and a numerical example is presented.     

Determination of carbonyl compounds in the atmosphere of charcoal plants by HPLC and UV detection

A chromatographic quantification method with two different mobile phases (elution conditions 1 and 2) was developed to determine carbonyl compounds (CCs) in air samples collected from charcoal production workplaces, using C18 cartridges coated with 2,4-dinitrophenylhydrazine (DNPHi). Several 2,4-dinitrophenylhydrazones (DNPHo) were separated and quantified using an HPLC system and UV detection. In 16 min, elution condition 1 successfully separated and quantified the DNPHo of 14 CC including acetaldehyde, acrolein, formaldehyde, and furfural, and estimated the sum of C4 isomers, butanal-isobutanal-butanone. This elution condition was able to resolve the pairs acrolein/furfural and propanone/propanal, which have been cited in the literature as difficult mixtures to be separated. The elution condition 2 allowed separation and quantification, in less than 30 min, of 13 out of the 17 CC listed above. This elution condition was also able to separate propanone from propanal and butanone from the other components of the C4 mixture. When the two mobile phases were used together, they allowed confirmation of the presence of the DNPHo in the real samples. Thus, both elution conditions have been shown to be appropriate to determine CC, in personal and stationary samples, collected in charcoal production plants.     

The Conley index for discontinuous vector fields

In this paper, we define the Conley index for a region of discontinuity D of a piecewise C ^k discontinuous vector field Z on an n-dimensional compact Riemannian smooth orientable manifold and prove it to be a homotopy invariant. This invariance is obtained by regularization of the discontinuous vector field. We use an adapted form of Lyapunov graph continuation to produce, in a few examples, a regularization of the discontinuous vector field with the property that the dynamics in a regularized neighborhood of D has the same Conley index as .      

Methodology to Quantify and Screen the Demineralization of Teeth by Immersing Them in Acidic Drinks (Orange Juice,Coca-Cola™, and Grape Juice): Evaluation by ICP OES

Oral health problems may occur as a result of the ingestion of acid drinks. The objective of this in vitro study was to quantify and screen the concentration of potassium (K), phosphorus (P), calcium (Ca), magnesium (Mg), manganese (Mn), zinc (Zn), iron (Fe), copper (Cu), barium (Ba), lead (Pb), arsenic (As), cadmium (Cd), aluminum (Al), cobalt (Co), chromium (Cr), molybdenum (Mo), sodium (Na), nickel (Ni), selenium (Se), and vanadium (V) released from bovine incisors during an erosive challenge at different times of exposure when immersed in Coca-Cola™, orange juice, and grape juice. A total of 240 samples of bovine incisor teeth were used for the erosive challenge and allocated in groups. Digestion of drinks was performed using microwave-assisted digestion. The content in acidic drinks was monitored before and after the erosive challenge at exposure times of 1, 5, and 60 min using inductively coupled plasma optical emission spectrometry (ICP OES). The drinks’ pH varied slightly during the erosive challenge but remained below the critical value of pH 5 to cause tooth demineralization. The concentrations of elements released from the bovine incisors during the in vitro erosive challenge depend on exposure times when immersed in acidic beverages. For some elements such as Ca, Mn, Zn, Fe, Cu, Ba, Pb, As, and Cd, quantified in acidic drinks, grape juice had greater erosive potential than Coca-Cola™ and orange juice. Quantification and monitoring of chemical elements in bovine teeth can be performed considering a longer erosive time and other types of acidic drinks. Further analysis using human teeth is still not available and must be conducted. The demineralization of teeth not only occurs in acidic beverages; physical and chemical factors play other roles and should be investigated.     

Total sulfur determination in gasoline,kerosene and diesel fuel using inductively coupled plasma optical emission spectrometry after direct sample introduction as detergent emulsions

Herein, we present the development of a procedure for the determination of total sulfur in petroleum-derived products (gasoline, kerosene and diesel fuel) employing inductively coupled plasma optical emission spectrometry (ICP OES). For this procedure, samples were prepared as emulsions that were made using concentrated nitric acid, Triton X-100, sample, and ultra pure water in proportions of 5/10/7/78% (v/v), respectively. Sample volumes were weighed because of the density differences, and oxygen was added to the sheat gas entrance of the ICP OES in order to decrease carbon deposition in the torch and to minimize background effects. A Doehlert design was applied as an experimental matrix to investigate the flow ratios of argon (sheat and plasma gas) and oxygen in relation to the signal-to-background ratio. A comparative study among the slopes of the analytical curves built in aqueous media, surfactant/HNO₃, and by spike addition for several sample emulsions indicates that a unique solution of surfactant in acidic media can be employed to perform the external calibration for analysis of the emulsions. The developed procedure allows for the determination of the total sulfur content in petroleum derivatives with a limit of detection (LOD) and limit of quantification (LOQ) of 0.72 and 2.4 μg g^− 1, respectively. Precision values, expressed as the relative standard deviations (% RSD, n = 10) for 12 and 400 μg g^− 1, were 2.2% and 1.3%, respectively. The proposed procedure was applied toward the determination of total sulfur in samples of gasoline, kerosene, and diesel fuel commercialized in the city of Niterói/RJ, Brazil. The accuracy of the proposed method was evaluated by the determination of the total sulfur in three different standard reference materials (SRM): NIST 2723a (sulfur in diesel fuel oil), NIST 1616b (sulfur in kerosene), and NIST 2298 (sulfur in gasoline). The data indicate that the methodology can be successfully applied to these types of samples. Spiking tests, conducted with the analyzed samples, indicate that recoveries are between 97 and 103%.     

Adenine Determination in the Presence of Copper in Diluted Alkaline Electrolyte by Adsorptive Stripping Voltammetry at the Mercury Film Electrode

Highly sensitive, simple and inexpensive techniques of adenine determination are particularly interesting in relation to the present development of ATP and DNA sensors. A nanomolar concentration of adenine can be determined in the presence of copper. For an accumulation time of 30 minutes, the detection limit found was 0.22 ppb (1.63×10^?9 M). The method is based on controlled adsorptive accumulation of adenine‐copper at thin‐film mercury electrode followed by linear scan voltammetric measurement of the surface species. By applying a condition time of 60 s at ?0.9 V, the same thin‐film can be used over several measurements. Optimum experimental conditions were found to be the use of a 5.0×10^?3 M NaOH solution, an accumulation potential over the ?0.20 to ?0.40 V range, and a scan rate of 100 mV s^?1. The response of adenine‐copper is linear over the concentration range 20–100 ppb. The more convenient ways to measuring adenine in the presence of metals and other nitrogenated bases were also investigated. The adenosine triphosphate (ATP) or deoxyribonucleic acid (DNA) are first treated with acid (e.g., 0.1 M perchloric acid), and the acid‐released adenine (without separation from others products of the degradation) is directly determined by adsorptive stripping voltammetry.     

Fe‐DPA as Catalyst for Oxidation of Organic Contaminants: Evidence of Homogeneous Fenton Process

Fenton‐type reactions using transition metal complexes in the decomposition of hydrogen peroxide to hydroxyl radical generation have received special attention due to their advantages over classical homogeneous processes involving soluble iron salts. Thus, the goal of this study was to investigate the use of an iron complex with the ligand pyridine‐2,6‐dicarboxylic acid (dipicolinic acid or DPA) as a homogeneous catalyst for the oxidation of the quinoline. The synthesized iron complex showed a molar ratio of 1:2 metal/ligand and was efficient in the quinoline oxidation at pH values near neutrality. The tests were monitored by mass spectrometry that allows identification of the different intermediates and showed complete oxidation of the quinolone compound. Moreover, in order to shed some light on the formation of hydroxyl radicals and the overall reaction mechanism, some theoretical calculations at the DFT level were performed. The results of this study demonstrated that the iron‐DPA complex is a good catalyst for the oxidation of quinoline by a Fenton‐like mechanism. All theoretical data show good agreement with experimental results.     

Radical CH Alkylation of BODIPY Dyes Using Potassium Trifluoroborates or Boronic Acids

A one‐step synthetic procedure for the radical C?H alkylation of BODIPY dyes has been developed. This new reaction generates alkyl radicals through the oxidation of boronic acids or potassium trifluoroborates and allows the synthesis of mono‐, di‐, tri‐, and tetraalkylated fluorophores in a good to excellent yield for a broad range of organoboron compounds. Using this protocol, multiple bulky alkyl groups can be introduced onto the BODIPY core thus creating solid‐state emissive BODIPY dyes.     

Quantification of nitrogen compounds in diesel fuel samples by comprehensive two‐dimensional gas chromatography coupled with quadrupole mass spectrometry

Although several methods for the analysis of nitrogen compounds in diesel fuel have been described in the literature, the demand for rapid, sensitive, and robust analyses has increased in recent years. In this study, a comprehensive two‐dimensional gas chromatographic method was developed for the identification and quantification of nitrogen compounds in diesel fuel samples. The quantification was performed using the standard addition method and the analysis was conducted using comprehensive two‐dimensional gas chromatography coupled with fast quadrupole mass spectrometry. This study is the first to report quantification of nitrogen compounds in diesel fuel samples using the standard addition method without fractionation. This type of analysis was previously performed using many laborious separation steps, which can lead to errors and losses. The proposed method shows good linearity for target nitrogen compounds evaluated (m‐toluidine, 4‐ethylaniline, indole, 7‐methylindole, 7‐ethylindole, carbazole, isoquinoline, 4‐methylquinoline, benzo[h]quinolone, and acridine) over a range from 0.05 to 2.0 mg/L, and limits of detection and quantification of <0.06 and 0.16 mg/L, respectively, for all nitrogen compounds studied.