Determination of iron at ng/ml level by solid phase spectrophotometry after preconcentration on cation exchange filters

A cationic exchanger paper is used to retain analytes in solution and, after drying, to analyze directly by measuring the UV-Vis absorbance of the paper. The method was applied to determination of iron using its known 1,10-phenanthroline complex. Using 100 ml of sample the applicable concentration range was between 1.0 and 10.0 ng/ml with a detection limit of 0.2 ng/ml and a RSD around 2%. The method was applied to determination of iron total in snow, human serum and wine.     

Synthesis and photoluminescent properties of 1,1'-binaphthyl-based chiral phenylenevinylene dendrimers

New chiral, soluble binaphthyl derivatives that incorporate stilbenoid dendrons at the 6,6'-positions have been prepared. The synthesis of the new enantiopure dendrimers was performed in a convergent manner by Horner-Wadsworth-Emmons (HWE) reaction of the appropriately functionalized 1,1'-binaphthyl derivative (R)-1 and the appropriate dendrons (R)(2n)G(n)-CHO. Different electroactive units were incorporated in the peripheral positions of the dendrons in order to tune both the optical and electrochemical behavior of these systems. Fluorescence measurements on the chiral dendrimers reveal a strong emission with maxima between 409 and 508 nm depending upon the substitution pattern. Finally, the redox properties of the dendrimers were determined by cyclic voltammetry, showing the influence of the functional groups at the peripheral positions of the dendrimer on the redox behavior of these systems.     

Simultaneous determination of arsenic, antimony and selenium by gas-phase diode array molecular absorption spectrometry, after preconcentration in a cryogenic trap

This paper describes a method for the simultaneous determination of As(III), Sb(III) and Se(IV) by combining hydride generation and gas phase molecular absorption spectrometry. A system for continuous hydride generation has been designed and developed, based on the use of a double process of gas-liquid separation, and optimal compromise operation conditions for the three compounds have been found. After generation, the hydrides are collected in a liquid nitrogen cryogenic trap, and then evaporated and driven to the flow cell of a diode array spectrophotometer, in which the transient signals over the 190–250 nm wavelength interval are measured. Under the recommended conditions (sample flow: 35 ml min⁻¹, 0.5 M HCl; reductor flow: 4 ml min⁻¹ of 4% NaBH₄, solution) linear response ranges above 50 μg 1⁻¹ for As(III), 30 μg 1⁻¹ for Sb(III) and 200 μg 1⁻¹ for Se(IV) are obtained with detection limits of 22 μg 1⁻¹, 15 μg 1⁻¹ and 65 μg 1⁻¹, respectively. Multiwavelength linear regression equations were used for the simultaneous determination of the three elements in different synthetic samples, with good precision and accuracy and to study simultaneously the interference from different chemical species for the three compounds. Results were similar to those obtained by other techniques using hydride generation.     

On-line preconcentration of inorganic mercury and methylmercury in sea-water by sorbent-extraction and total mercury determination by cold vapour atomic absorption spectrometry

A comparative study of three mercury chelate forming reagents [diethyldithiocarbamate, pyrrolidin-1-yldithioformate and diphenylthiocarbazone (dithizone)] has been carried out for the preconcentration of ultratrace amounts of inorganic mercury and methylmercury in silica C(18) minicolumns as the solid sorbent. Sample flow injection in-line sorbent extraction was coupled with continuous cold vapour atomic absorption spectrometry (CVAAS) for detection. Results showed the superiority of the carbamate type reagents over the dithizone for the on-line formation and preconcentration of the corresponding mercury chelates. Using diethyldithiocarbamate (DDC) as reagent, aqueous sample volumes of 100 ml can be preconcentrated with 100% efficiency for both inorganic mercury and methylmercury. Quantitative release of the retained DDC chelates was obtained for volumes of eluent (ethanol) of 50 microl. Following the proposed procedure, detection limits of 16 ng/l. of mercury were achieved for sample volumes of 25 ml. The relative standard deviation was +/- 3.4% at 0.5 microg/l. Hg(II) levels. The method has been successfully applied to the determination of low levels of mercury in sea-water.     

Direct electrochemical synthesis and characterization of copper(II), iron(II) and iron(III) complexes with 1- and 2-substituted pyridines

Summary The electrochemical oxidation of anodic metals (iron and copper) in MeCN solutions of 2-pyridinone (HOPy), 1-hydroxy-2-pyridinethione (HPT), 2-pyridinemethanethiol-1-oxide (HPMTO) or its dimer 2,2-dithiodimethyldipyridine-1,1-dioxide (PMTO)₂, gave the simple complexes Fe(OPy)₂ · H₂O, Cu(OPy)₂ · 3H₂O, Fe(PT)₂ · 3H₂O, Fe(PT)₃, Cu(PT)₂, Fe(PMTO)₂·3H₂O and Cu(PMTO)₂·H₂O, respectively. When 1,10-phenanthroline (phen) was added to the electrolytic phase, only two mixed complexes were obtained: Cu(OPy)₂phen·3H₂O and Fe(PT)₂phen·2H₂O. The possible molecular structures of the complexes were studied on the basis of their i.r. spectra and magnetic properties.     

Type-D vacuum solutions with cosmological constant

All type-D vacuum (nonnull orbit and null orbit) solutions with are exhibited in canonical coordinates. The nonnull orbit metrics with contain four families of solutions: the static Levi-Cività metrics, the nondivergingD's, the divergingD's, and the diverging and twisting solutions. The null orbit metrics subdivide into two subclasses of solutions: the divergenceless null orbitD's, and the diverging and twisting null orbit solution.     

A common method for the determination of several calcium channel blockers using an HPLC system with ultraviolet detection

We report a common HPLC method for the single or simultaneous determination of four calcium channel blockers (CCB), namely diltiazem (DTZ), verapamil (VER), nifedipine (NIF) and nitrendipine (NIT) and their active metabolites demetildiltiazem and deacetildiltiazem (MA and M1), norverapamil (NOR), and dehydronifedipine (DHN). DHN was first synthesised in our laboratory and different pH values of the mobil phase were subsequently prepared and tested for chromatographic separation. The detection system and the environmental light conditions were optimised. The best separations of all analytes were obtained using a C₁₈ column and a mobile phase of methanol, 0.04 M ammonium acetate, acetonitrile and triethylamine (2:2:1:0.04 v/v). Quantitation was performed using imipramine (IMI) as the internal standard. For DTZ and its metabolites (M1 and MA), the wavelength chosen was 237 nm; for VER and its metabolite NOR, it was 210 nm; and, finally for NIF and its metabolite DHN and NIT it was 216 nm. When a simultaneous analysis was carried out the wavelength was of 230 nm. The optimum pH were 7.90 and 7.10 when the separation of NIT and DTZ or VER and NIF were carried out, respectively, and 7.90 when a simultaneous separation was carried out. The detection limit of the assay was less than 8 ng ml⁻¹ for all compounds, with coefficients of variation less than 7% (for inter- and intra-day) over the concentration range of 1–1000 ng ml⁻¹. The retention times were less than 11 min. When NIF or NIT were studied, it was necessary to use a sodium vapour lamp in order to avoid the photodegradation which takes place under daylight conditions.     

Pentacoordinate nickel(II) complexes double bridged by phosphate ester or phosphinate ligands: spectroscopic, structural, kinetic, and magnetic studies

The bis(phosphatediester)-bridged complexes [[Ni([12]aneN(3))(mu-O(2)P(OR)(2))](2)](PF(6))(2) [[12]aneN(3)=Me(3)[12]aneN(3), 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene; R=Me (1), Bu (2), Ph (3), Ph-4-NO(2) (4); [12]aneN(3)=Me(4)[12]aneN(3), 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene; R=Me (5), Bu (6), Ph (7), Ph-4-NO(2) (8)] were prepared by hydrolysis of the phosphate triester with the hydroxo complex [[Ni([12]aneN(3))(mu-OH)](2)](PF(6))(2) or by acid-base reaction of the dialkyl or diaryl phosphoric acid and the above hydroxo complex. The acid-base reaction was also used to synthesise the phosphinate-bridged complexes [[Ni([12]aneN(3))(mu-O(2)PR(2))](2)](PF(6))(2) [[12]aneN(3)=Me(3)[12]aneN(3), R=Me (9), Ph (10); [12]aneN(3)=Me(4)[12]aneN(3), R=Me (11), Ph (12)]. The molecular structures of complexes 2, 3 and 12 were established by single crystal X-ray diffraction studies. The eight-membered rings defined by the nickel atoms and the bridging ligands show distorted twist-boat, chair and boat-boat conformations in 2, 3 and 12, respectively. The experimental susceptibility data for compounds 2, 3 and 12 were fitted by least-squares methods to the analytical expression given by Ginsberg. The best fit was obtained with values of J=-0.11 cm(-1), D=-9.5 cm(-1) and g=2.20 for 2; J=-0.97 cm(-1), D=-9.3 cm(-1) and g=2.21 for 3; and J=-0.14 cm(-1), D=-11.9 cm(-1) and g=2.195 for 12. The magnetic-exchange pathways must involve the phosphate/phosphinate bridges, because these favour antiferromagnetic interactions. The observation of a higher exchange parameter for compound 3 is a consequence of a favourable disposition of the O-P-O bridges. The kinetics for the hydrolysis of TNP (tris(4-nitrophenyl)phosphate) with the dinuclear nickel(II) hydroxo complex [[Ni(Me(3)[12]aneN(3))(mu-OH)](2)](PF(6))(2) was studied by UV-visible spectroscopy. The proposed mechanism for TNP-promoted hydrolysis can be described as one-substrate/two-product, and can be fitted to a Michaelis-Menten equation.     

Accurate intermolecular ground state potential of the Ne-HCl van der Waals complex

From an accurate ground state intermolecular potential energy surface we evaluate the rovibrational spectrum of the Ne-HCl van der Waals complex. The intermolecular potential is obtained by fitting a considerable number of interaction energies obtained at the coupled cluster singles and doubles including connected triple excitations level and with the augmented correlation consistent polarized valence quintuple zeta basis set extended with a set of 3s3p2d1f1g midbond functions. This basis set is selected after a systematic basis set study carried out at geometries close to those of the three main surface stationary points. The surface is characterized by two linear minima, i.e. Ne-ClH and Ne-HCl, with distances from the Ne atom to the HCl center of mass of 3.398 and 3.833 angstroms, respectively; and binding energies of -65.10 and -66.85 cm(-1), respectively. These results agree well with the experimental data available in contrast to previous theoretical results. The rovibrational spectra calculated for the different isotopic species are also compared to the experiments.     

The reactant state for substrate-activated turnover of acetylthiocholine by butyrylcholinesterase is a tetrahedral intermediate

Secondary beta-deuterium kinetic isotope effects have been measured as a function of substrate concentration for recombinant human butyrylcholinesterase-catalyzed hydrolysis of acetyl-L3-thiocholine (L = 1H or 2H). The isotope effect on V/K is inverse, D3V/K = 0.93 +/- 0.03, which is consistent with conversion of the sp2 hybridized carbonyl carbon of the scissile ester bond of the E + A reactant state to a quasi-tetrahedral structure in the acylation transition state. In contrast, the isotope effect on Vmax under conditions of substrate activation is markedly normal, D3(betaVmax) = 1.29 +/- 0.06, an observation that is consistent with accumulation of a tetrahedral intermediate as the reactant state for catalytic turnover. Generally, tetrahedral intermediates for nonenzymatic ester hydrolyses are high-energy steady-state intermediates. Apparently, butyrylcholinesterase displays an unusual ability to stabilize such intermediates. Hence, the catalytic power of cholinesterases can largely be understood in terms of their ability to stabilize tetrahedral intermediates in the multistep reaction mechanism.