Determination of oxytetracycline, tetracycline, and chlortetracycline in milk by liquid chromatography with postcolumn derivatization and fluorescence detection

A multiresidue method for isolation and liquid chromatographic determination of oxytetracycline (OTC), tetracycline (TC), and chlortetracycline (CTC) in milk is presented. The sensitivity of the method is adequate to meet the needs of regulatory agencies. The European Community established 100 micrograms/kg as the maximum residue limit (MRL) in milk for TC, CTC, and OTC. Recoveries exceeded 80% for all tetracyclines at all levels, with good precision. Correlation coefficients of standards curves for individual tetracyclines isolated from fortified samples ranged from 0.991 for CTC to 0.998 for OTC. Other antibiotics that might interfere with analysis did not interfere with elution times of OTC, TC, and CTC. The procedure is rapid, precise, and quantitative and requires minimal preparation and minimal use of organic solvents. It can be applied to routine surveillance programs. We can prepare 10 samples for analysis in about 1.45 h.     

Fractal relaxed Dirichlet problems

Given aself similar fractal K ? ? ⁿ of Hausdorff dimension α>n?2, andc ₁>0, we give an easy and explicit construction, using the self similarity properties ofK, of a sequence of closed sets? _h such that for every bounded open setΩ?? ⁿ and for everyf ∈ L²(Ω) the solutions to $$\left\{ \begin{gathered} - \Delta u_h = f in \Omega \backslash \varepsilon _h \hfill \\ u_h = 0 on \partial (\Omega \backslash \varepsilon _h ) \hfill \\ \end{gathered} \right.$$ converge to the solution of the relaxed Dirichlet boundary value problem $$\left\{ \begin{gathered} - \Delta u + uc_1 \mathcal{H}_{\left| K \right.}^\alpha = f in \Omega \hfill \\ u = 0 on \partial \Omega \hfill \\ \end{gathered} \right.$$ (H _∣ ^α denotes the restriction of the α-dimensional Hausdorff measure toK). The condition α>n?2 is strict.     

Anaerobic reduction of a sulfonated azo dye, Congo Red, by sulfate-reducing bacteria

The capacity for anaerobic decolorization of a sulfonated azo dye, Congo Red, by a strain of a sulfate-reducing bacterium was evaluated. After optimizing the growth rate of the bacteria on a simple carbon source and terminal electron acceptor pair, lactate and sulfate, respectively, the effect of the dye concentration on their growth rate was analyzed. The decolorization rate was affected by the dye concentration in the growth medium. The azo-bond cleavage mechanism of reductive decolorization with the formation of benzidine was consistent with the results, as this metabolite was identified by high-performance liquid chromatography. Several fractions of the culture medium, including lysed cell extracts, were examined for the capacity to reduce the azo dye. This reduction capacity was found in the culture medium in which the cells had previously grown. The results showed that the mechanism of reductive decolorization of this sulfonated azo dye was extracellular and nonenzymatic, consistent with the production of sulfide anion by the microorganisms while growing on lactate and sulfate. The sulfide anions were the cause of the reduction leading to the disappearance of color in the medium. To increase the rate of decolorization, the presence of ferrous ion was also necessary together with the lactate and sulfate substrates.     

Shaping Solid-State Supramolecular Cavities: Chemically Induced Accordionlike Movement of gamma-Zirconium Phosphate Containing Polyethylenoxide Pillars

The hydrophilic oxygen atoms of polyethylenoxide chains inserted as pillars in gamma-zirconium phosphate form hydrogen bonds with the acid groups of the host. As a result the pillars are almost perpendicular to the gamma layers. Upon changing the pH level of the supernatant solution the hydrogen bonds are broken and the pillars become almost perpendicular to the layers (shown schematically). Thus there is a reversible enlargement-shortening of the interlayer space.     

Superior Antibacterial Activity of Integral Lemon Pectin Extracted via Hydrodynamic Cavitation

Pectin extracted via hydrodynamic cavitation in water only from waste lemon peel and further isolated via freeze drying displays significant antibacterial activity against Staphylococcus aureus, a Gram positive pathogen which easily contaminates food. The antibacterial effect of the new IntegroPectin is largely superior to that of commercial citrus pectin, opening the way to advanced applications of a new bioproduct now obtainable in large amounts and at low cost from citrus juice industry's waste.     

Mixed-Ligand Complex Formation Equilibria of Vanadium(III) with 2,2′-Bipyridine and the Amino Acids Glycine, Proline, α-Alanine and β-Alanine Studied in 3.0 mol?dm?3 KCl at 25 °C

We studied speciation of the mixed-ligand complex formation equilibria of vanadium(III) with both 2,2??-bipyridine (Bipy) and the amino acids glycine (HGly), proline (HPro), ??-alanine (H??Ala), and ??-alanine (H??Ala) by means of electromotive forces measurements emf(H) using 3.0?mol?dm^?3 KCl as the ionic medium at 25 °C. The experimental data were analyzed by means of the computational least-squares program LETAGROP, taking into account the hydrolysis of the vanadium(III) cation, the respective stability constants of the binary complexes, and the acid/base reactions of the ligands which were kept fixed during the analysis. In all four amino acid systems studied we observed the complexes [V₂O(Bipy)(B)]³⁺, [V₂O(Bipy)₂(B)₂]²⁺, [V(OH)(Bipy)(B)₂] and [V(OH)₂(Bipy)(B)], where B represents the deprotonated form of the amino acids studied in this work. The respective stability constants were determined and the species distribution diagrams as a function of pH are briefly discussed.     

Effects of alkyl groups in the rate determining step of the Baeyer-Villiger reaction of phenyl alkyl ketones: a quantum chemistry study

In this work, we have studied the substituent effect of several alkyl groups in the rate-determining step of the catalyzed Baeyer-Villiger (BV) reaction of phenyl alkyl ketones with performic (PFA) and trifluoroperacetic (TFPAA) acids, using quantum chemistry methods. Our results reveal that the substituent effect is more pronounced in the migration step barriers than in the corresponding addition step; that could change the rate-determining step (RDS) of the reaction, as observed in the oxidation of phenyl tert-butyl ketone with both peracids. In addition, the effect of the acid/peracid pairs used is also analyzed. We have demonstrated that the addition step is less susceptible to the acid/peracid nature since the acid strength and the nucleophilicity of the peracid have opposite effects. The effect of the acid/peracid pair is much more pronounced in the migration step because it only depends on the leaving ability of the acid, which in turn depends on its strength. These observations are relevant for understanding the effects of the substrate, the peracid, and the catalyst on the switching of the RDS in the BV reaction.     

Mixed-Ligand Complex Formation Equilibria of Copper(II) with 6-Methylpicolinic Acid and Some Amino Acids

In this work, the ternary complex formation among copper(II), 6-methylpicolinic acid (H6Mepic) as primary ligand, and the amino acids aspartic acid (H₂Asp), glutamic acid (H₂Glu) and histidine (HHis) as secondary ligands, were studied in aqueous solution at 25 °C using 1.0 mol·dm^?3 KNO₃ as the ionic medium. Analysis of the potentiometric data using the least squares computational program LETAGROP indicates formation of the species [Cu(6Mepic)]⁺, Cu(6Mepic)(OH), [Cu(6Mepic)(OH)₂]^?, Cu(6Mepic)₂ and [Cu(6Mepic)₃]^? in the binary Cu(II)–H6Mepic system. In the ternary Cu(II)–H6Mepic–H₂Asp system the complexes [Cu(6Mepic)(H₂Asp)]⁺, Cu(6Mepic)(HAsp), [Cu(6Mepic)(Asp)]^? and [Cu(6Mepic)(Asp)(OH)]^2? were observed. In the case of the Cu(II)–H6Mepic–H₂Glu system the complexes Cu(6Mepic)(HGlu), [Cu(6Mepic)(Glu)]^?, [Cu(6Mepic)(Glu)(OH)]^2? and [Cu(6Mepic)(glu)(OH)₂]^3? were detected. Finally, in the Cu(II)–H6Mepic–HHis system the complexes [Cu(6Mepic)(HHis)]⁺, Cu(6Mepic)(His) and [Cu(6Mepic)(His)(OH)]^? were observed. The species distribution diagrams as a function of pH are briefly discussed.