Kinetic studies of nickel speciation in model solutions of a well-characterized humic acid using the competing ligand exchange method

Publications on the binding characteristics of metals with humic acid (HA) are sparse. Here we investigated the release of nickel from Ni(II)-HA complexes using model solutions of three different [Ni(II)]/[HA] mole ratios at three different pH values; we also compared the results with those of [Ni(II)]/[FA] complexes from previous work in this laboratory. Ligand exchange kinetics using the competing ligand exchange method (CLEM) were studied using two different techniques: graphite furnace atomic absorption spectrometry (GFAAS) with Chelex 100 resin as the competing ligand, and adsorptive cathodic stripping voltammetry (AdCSV) with dimethylglyoxime as the competing ligand to measure the rate of dissociation of Ni(II)-HA complexes. The results of the kinetic studies showed that as the [Ni(II)]/[HA] mole ratio was decreased, the rate of dissociation of Ni(II)-HA complexes decreased, and the proportion of free Ni²⁺ ions plus very labile nickel complexes decreased while the proportion of the less labile kinetically distinguishable components increased. Generally, the rate of dissociation of Ni(II)-HA complexes was slower than that of Ni(II)-FA complexes. Studies on the validity of the kinetic model showed that the concentrations of chemical species varied in a reasonable way with pH and the [Ni(II)]/[HA] mole ratios, indicating that the kinetically distinguishable components have chemical significance and the kinetic model is valid.     

Determination of elements by nuclear analytical methods

The working principle of nuclear analytical methods (NAMs) is not influenced by the chemical bond. Consequently, they are independent counterparts to the well-known chemical procedures. NAMs obey fundamental laws or can be described and understood thoroughly. This qualifies them as candidates for reference methods. Although following similar nuclear reaction schemes, they comprise bulk analyzing capability (neutron and photon activation analysis) as well as detection power in surface near regions of solids (ion beam techniques). Prominent features of NAMs are sensitivity, selectivity, multielement determination and linearity of the calibration function covering a concentration range of several orders of magnitude. Moreover, ion beam techniques allow depth profiling with nm-resolution in several cases while the ion microprobe additionally offers a lateral resolution in the micron-scale. As NAMs require expensive apparatus (nuclear reactor, accelerator in radioactive control areas) their availability is restricted to a small number of suitably equipped institutes. However, they are able to solve complex analytical tasks, take part in key comparisons and play an essential role in the certification of reference materials.     

On-membrane digestion of beta-casein for determination of phosphorylation sites by matrix-assisted laser desorption/ionization quadrupole/time-of-flight mass spectrometry

This article discusses the features of a newly developed matrix-assisted laser desorption/ionization quadrupole/time-of-flight (MALDI-QqTOF) mass spectrometer that is useful in the analysis of phosphorylated peptides. Aliquots of beta-casein, a commonly used phosphorylated protein standard, were digested with trypsin directly on a non-porous polyurethane membrane used as sample support in MALDI-QqTOF mass spectrometry (MS) experiments. Although a complete peptide map was obtained, it was difficult to obtain sequence information for some of the tryptic fragments, in particular T1-2, which bears four phosphate groups and is thus difficult to ionize in positive mode. This article focuses on the sequencing of this particular fragment by comparing MS/MS spectra obtained using different precursor ions. These precursors associated with T1-2 were [M + H](+), [M + H](2+), and [M + H - nH(3)PO(4)](+) ions. Typically, phosphorylated ions showed facile unimolecular losses of phosphoric acid moieties, and produced limited backbone fragmentation. The abundance of [M + H](2+) ions of T1-2 in the full mass spectrum was low relative to that of [M + H](+). [M + H - 4H(3)PO(4)](+) ions as MS/MS precursors underwent backbone fragmentations, with phosphoserine residues transformed into dehydroalanines or serines. Unusual b + 18 u fragments were observed, although only for segments with previously phosphorylated serines. These partly interfered with c-ions, and were noticeable due to overlapping isotopic envelopes. It was possible to establish the sequence of phosphorylated tryptic fragment T1-2 and the location of phosphate groups using the mass of dehydroalanine residues (69 Da) and b + 18 u fragments as markers. All MS and MS/MS spectra obtained with fully phosphorylated beta-casein were compared with spectra acquired with dephosphorylated beta-casein obtained commercially. These comparisons helped assess the spectral differences caused by the presence of phosphate groups. Also, they highlighted the potential usefulness of conducting dephosphorylation directly on the probe prior to MALDI analysis in future studies.     

Simultaneous determination of clebopride and a major metabolite N-desbenzylclebopride in plasma by capillary gas chromatography-negative-ion chemical ionization mass spectrometry

A procedure for the simultaneous assay of clebopride and its major metabolite N-desbenzylclebopride in plasma has been developed. The method utilizes capillary gas chromatography-negative-ion chemical ionization mass spectrometry with selected-ion monitoring of characteristic ions. Employing 2-ethoxy analogues as internal standards, the benzamides were extracted from basified plasma using dichloromethane. Subsequent reaction with heptafluorobutyric anhydride produced volatile mono- and diheptafluorobutyryl derivatives of clebopride and N-desbenzylclebopride, respectively. The methane negative-ion mass spectra of these derivatives exhibited intense high-mass ions ideal for specific quantitation of low levels in biological fluids. Using this procedure the recovery of the drug and metabolite from human plasma was found to be 84.4 +/- 1.5% (n = 3) and 77.4 +/- 4.7% (n = 3), respectively, at 0.5 ng/ml. Measurement of both compounds down to 0.10 ng/ml with a coefficient of variation of less than 10.5% is described. Plasma levels are reported in four volunteers up to 24 h following oral administration of 1 mg of clebopride malate salt.     

Synthesis of some benzimidazole- and benzothiazole-derived ligand systems and their precursory diacids

Procedures are described for the preparation of various bidentate and linear tetradentate benzimidazoles and benzothiazoles incorporating units such as pyridyl and thioether, and for the preparation of certain thioether dicarboxylic acids precursory to them. Condensations of ortho-functinal anilines with carboxylic acids were carried out in polyphosphoric acid or refluxing HCl solution. Syntheses are reported for: [HO₂C(CH₂)2S(CH₂)₂]₂X (X = O, S), 1,9-bis(benzimidazol-2-yl)-2,5,8-trithianonane, 1,11-bis(N-methylbenzimidazol-2-yl)-3,6,9-trithiaundecane, 1,11-bis(2-benzimidazol-2-yl)-6-oxo-3,9-dithiaundecane, 2-(2-pyridyl)benzothiazole, 2,6-bis(benzothiazol-2-yl)pyridine, 2-(2-pyridyl)-N-methylbenzimidazole, 2-(2-pyridylmethyl)benzimidazole and 2-(N-methyl-2-piperidyl)benzimidazole. The compounds were characterized, where appropriate, by their mass, uv and ¹H-nmr spectra.     

Schwefel(IV)-Verbindungen als Liganden. II. Die Kristall- und Molekülstruktur von Pentacarbonyl-(schwefeldioxid)chrom

Sulfur(IV) Compounds as Ligands. II. The Crystal and Molecular Structure of Pentacarbonyl--(sulfur dioxide)chromium The structure of pentacarbonyl(sulfurdioxide)chromium ( 1 ) has been determined from single crystal X-ray data. The compound crystallizes with eight formula units in the rhombic unit cell (space group Pbn2₁) of the dimensions a = 657.8(2) pm, b = 1245.2(4) pm, c = 2177.4(5) pm (at 180 K). The sulfur dioxide is η¹-coplanar coordinated, the Cr? S distance is 219 pm, the shortest bond reported so far between chromium(0) and sulfur. The Cr? C(ax) bond (189 pm) was found only marginally shorter than the Cr? C(eq) bonds (190 pm) providing proof of the high π-acceptor capacity of SO₂.     

Zeolite content determination in kaolinic clays via thermal effects

For application in catalysis and adsorption processes, zeolites are synthesized on the surface of spray-dried kaolin microspheres. Various thermal effects are used for the rapid characterization of the zeolite type and content. From DTA measurements, the exothermic lattice break-down peak gives information on the zeolite type and the thermal stability of the sample. Measurements of the heats of immersion by a very simple and quick method allow conclusions about the type and the sorption properties of the zeolite part of the clay. The most precise method to estimate the zeolite content was a standardized technique of desorption and readsorption of water or organic compounds by means of a thermobalance.

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Zusammenfassung Zur Anwendung in der Katalyse und bei Adsorptinsprozessen werden Zeolithe auf der Oberfläche sprüh-getrockneter Kaolin-Mikrokugeln eingesetzt. Verschiedene thermische Effekte werden zur schnellen Charakterisierung des Zeolith-Typs und -Gehalts genutzt. Der exotherme DTA-peak beim Gitterzusammenbruch liefert Informationen über Zeolith-Typ und thermische Stabilität der Probe. Eine sehr einfache Schnellmethode zur Messung der Immersionswärmen erlaubt Schlussfolgerungen über Typ und Sorptionseigenschaften des zeolithischen Anteils des Tons. Die genaueste Methode zur Bewertung des Zeolithgehalts war eine standardisierte thermogravimetrische Desorptions- und Readsorptions-Methode mit Wasser oder organiscen Verbindungen.

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A new method for the direct calculation of resonance parameters with application to the quasibound states of the H2X1Σ g + system

A new method for the direct calculation of resonance parameters is presented. It is based upon searching for poles of the scattering matrix at complex energies. This search is expedited by the use of analytic derivatives of the scattering matrix with respect to the total energy. This procedure is applied initially to a single channel problem, but is generalizable to more complicated systems. Using the most accurate available potential energy data, we calculate resonance parameters for all of the physically important quasibound states of the ground electronic state of the hydrogen molecule. Corrections to the Born-Oppenheimer potential are included and assessed. The new method has no difficulty locating resonances with widths greater than about 1×10^–7 cm^–1. It is easier to find narrow resonances by monitoring the dependence of the imaginary part of the reactance matrix on the real part of a complex energy than to monitor the dependence of the eigenphase sum on energy at real energies.     

Surface reaction dynamics and the harmonic oscillator

One of the fundamental steps in chemical reaction dynamics involves breaking reactant bonds. This is facilitated by placement of energy into the vibrational degrees of freedom associated with the bond. Here we present a model for vibrational excitation in molecule-surface collisions in which the equilibrium geometry of the (diatomic) molecule varies with distance from the surface. The special feature of this model is that the potential energy surfaces for bound nuclear motion are constructed from quadratic potentials, thus enabling analytic solutions. Comparisons are made between exact results obtained from a purely classical trajectory model and various hybrid models in which the internal vibrational modes are treated quantum mechanically in the harmonic limit.     

Depolarization and fine structure effects in half collisions of sodium-noble gas systems

Population ratios and polarization of the Na3P _J fine structure states following far wing photon excitation of Na-noble gas collision pairs are studied with respect to the underlying interaction potentials and molecular coupling schemes. For this purpose spectral profiles of these quantities, i.e. its dependencies on excitation frequency, have been measured up to ±200 cm^?1 detuning from the NaD lines for NaAr and NaHe under nearly single collision conditions. Comparing the measured population ratios with quantummechanical coupled channels calculations the Σ potential well depths differ considerably from results of model potential calculations. Large residualJ=3/2 alignment observed in the far wings disagrees with simple models assuming incoherent excitation and/or full adiabatic reorientation of the radiating dipole along straight trajectories. By comparison with Lewis model calculations, using realistic trajectories and decoupling radius, it is found instead, that realistic trajectories are constitutive for alignment after Σ-excitation (blue wing), whereas coherence between the²Π_1/2 and²Π_3/2 states determine primarily alignment after Π-excitation (red wing).