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991.
992.
Michael Sperling Xiu-ping Yan Bernhard Welz 《Spectrochimica Acta Part B: Atomic Spectroscopy》1996,51(14):1875-1889
A fully automated procedure for the determination of ng l−1 amounts of lead has been developed using flow injection (FI) online column preconcentration coupled with electrothermal atomic absorption spectrometry (ETAAS). The proposed FI manifold and its operation make possible the introduction of the total eluate volume into the graphite atomizer, avoiding the necessity for optimization of subsampling the eluate. The interference of other heavy metal ions due to competition for active sites of the sorbent is overcome using a highly selective macrocycle immobilized on silica gel (Pb-02). Lead is adsorbed on a microcolumn (50 μl) packed with Pb-02, and after washing the column with dilute nitric acid, air is introduced to remove all solution from the column and connecting tubing. The sorbed analyte is then eluted quantitatively into the graphite tube atomizer, preheated to 100°C, with 36 μl of ETDA solution (0.035 mol l−1, pH 10.5), propelled by air in order to minimize dispersion. The collection efficiency was 77% and with a sample loading flow rate of 3 ml min−1 and a 60 s preconcentration time, the enhancement factor was 77 and the throughput was 17 samples per hour. The relative standard deviation (n = 10) at the 300 ng l−1 level was 2.7%, and the detection limit (3σ) was 0.4 ng l−1. No interference from heavy metals was observed, but ions of Ba2+, Sr2+ and K+ were found to interfere when the concentration ratios of interferent to lead exceeded values of 2000, 20 000 and 200 000, respectively. Quantitative recovery of lead was achieved from sodium, magnesium, aluminum, lanthanum and heavy metal salt solutions. The high selectivity and sensitivity, combined with extremely low blank values, make the proposed technique particularly attractive for the analysis of high-purity reagents, semiconductors and other high-purity materials. Results are presented for the determination of lead in some high-purity reagents. 相似文献
993.
Glättli A Daura X Bindschädler P Jaun B Mahajan YR Mathad RI Rueping M Seebach D van Gunsteren WF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(24):7276-7293
The influence of charged side chains on the folding-unfolding equilibrium of beta-peptides was investigated by means of molecular dynamics simulations. Four different peptides containing only negatively charged side chains, positively charged side chains, both types of charged side chains (with the ability to form stabilizing salt bridges) or no charged side chains were studied under various conditions (different simulation temperatures, starting structures and solvent environment). The NMR solution structure in methanol of one of the peptides (A) has already been published; the synthesis and NMR analysis of another peptide (B) is described here. The other peptides (C and D) studied herein have hitherto not been synthesized. All four peptides A-D are expected to adopt a left-handed 3(14)-helix in solution as well as in the simulations. The resulting ensembles of structures were analyzed in terms of conformational space sampled by the peptides, folding behavior, structural properties such as hydrogen bonding, side chain-side chain and side chain-backbone interactions and in terms of the level of agreement with the NMR data available for two of the peptides. It was found that the presence of charged side chains significantly slows down the folding process in methanol solution due to the stabilization of intermediate conformers with side chain-backbone interactions. In water, where the solvent competes with the solute-solute polar interactions, the folding process to the 3(14)-helix is faster in the simulations. 相似文献
994.
995.
Tatiane de A. Maranhão Edmar Martendal Daniel L.G. Borges Eduardo Carasek Bernhard Welz Adilson J. Curtius 《Spectrochimica Acta Part B: Atomic Spectroscopy》2007
Cloud point extraction (CPE) is proposed as a pre-concentration procedure for the determination of Pb and Cd in undigested urine by graphite furnace atomic absorption spectrometry (GF AAS). Aliquots of 0.5 mL urine were acidified with HCl and the chelating agent ammonium O,O-diethyl dithiophosphate (DDTP) was added along with the non-ionic surfactant Triton X-114 at the optimized concentrations. Phase separation was achieved by heating the mixture to 50 °C for 15 min. The surfactant-rich phase was analyzed by GF AAS, employing the optimized pyrolysis temperatures of 900 °C for Pb and 800 °C for Cd, using a graphite tube with a platform treated with 500 μg Ru as permanent modifier. The reagent concentrations for CPE (HCl, DDTP and Triton X-114) were optimized using a Box–Behnken design. The response surfaces and the optimum values were very similar for aqueous solutions and for the urine samples, demonstrating that aqueous standards submitted to CPE could be used for calibration. Detection limits of 40 and 2 ng L− 1 for Pb and Cd, respectively, were obtained along with an enhancement factor of 16 for both analytes. Three control urine samples were analyzed using this approach, and good agreement was obtained at a 95% statistical confidence level between the certified and determined values. Five real samples have also been analyzed before and after spiking with Pb and Cd, resulting in recoveries ranging from 97 to 118%. 相似文献
996.
FTIR methods for measuring optical rotatory dispersion (ORD) and circular dichroism (CD) were developed. The measurements of both phenomena are accomplished by means of various techniques. The study of ORD makes use of linearly polarized IR radiation while that of CD requires a circular polarizer (retarder) able to convert linearly polarized IR radiation into circularly polarized one. Examples of induced cholesteric solutions are shown. 相似文献
997.
Hildebert Wagner Bernhard Kreher Hermann Lotter Matthias O. Hamburger Geoffrey A. Cordell 《Helvetica chimica acta》1989,72(4):659-667
Beside the known naphthoquinones, dehydro-α-lapachone ( 17 ) and lapachol ( 20 ), four new naphtho[2,3-b]-furan-4,9-diones, i.e. the 2-acetyl-5-hydroxy. 2-acetyl-8-hydroxy. (?)-5-hydroxy-2-(1′-hydroxyethyl), and (±)-8-hydroxy-2-(1′-hydroxyethyl) derivatives 16, 15, 12 , and 13 , respectively, and the new compound benzo[b]furan-6-carboxaldehyde ( 8 ) have been isolated from a CHCl3 extract of the inner stem bark of Tabebuia avellanedae LORENTZ ex GRISEB ., together with four known naphthofurandiones, a dihydroisocoumarin derivative, (?)–6-hydroxymellein, and five benzoic-acid and three benzaldehyde derivatives which have not been reported previously from this plant. Structure determination of the isomeric 5- and 8-hydroxynaphtho[2,3?b]furan-4,9-diones was carried out unambiguously by a combination of selective-INEPT experiments and X-ray crystallographic analysis. 相似文献
998.
The bicyclic bisureas 2 react with triflic anhydride to give the dication ether salts 4 . Further transformation of the remaining carbonyl groups of 4 into bis-carbenium ether linkages was not possible. The monocyclic bisurea 8 yields the dicationic heterocycle 9 in an intramolecular fashion. The structure of 9 , which is the first dication ether salt with an endocyclic ether linkage, was established by independent synthesis from 10 as well as by its hydrolysis to give the hemi-protonated bisurea 11 . 相似文献
999.
Faizulla G. Kathawala Bernhard Prager Kapa Prasad Oljan Repi
Michael J. Shapiro Russell S. Stabler Leo Widler 《Helvetica chimica acta》1986,69(4):803-805
The reduction of δ-hydroxy-β-ketoesters 1 was investigated with three different reducing agents. In several instances, high selectivity in favor of syn-1,3-diols was observed. 相似文献
1000.
Bernhard Strauß Wolfgang Linert Viktor Gutmann Reginald F. Jameson 《Monatshefte für Chemie / Chemical Monthly》1992,123(6-7):537-546
Summary The iron(II) complexes of the nitrogen donor ligand 2,6-bis(benzimidazol-2-yl)pyridine (bzimpy) have been investigated in nonaqueous solvents using various spectrophotometric methods. Measurements in methanol at 20°C revealed thatbzimpy can act as a bi-or tridendate ligand towards iron(II) in this solvent, forming [Fe(bzimpy)]2+ (tridendate, logK=5.54), [Fe(bzimpy)2]2+ (bistridentate, logK=4.12) and [Fe(bzimpy)3]2+ (tris-bidentate, logK=3.85). The pronounced thermochromism of the compound is shown to be the result of both spin-crossover and dissociation equilibria. Furthermore, in solvents of higher donor numbers (30), partial or complete deprotonation of the complex [Fe(bzimpy)3]2+ is observed. Triethylamine added stepwise to methanolic solutions causes successive deprotonation.This paper is dedicated to Professor Dr. Wolfgang Beck on the occasion of his 60th birthday with warmest personal wishes 相似文献