Speciation of organotin in environmental sediment samples

An optimized sample preparation procedure for organotin speciation in sediment samples has been applied to the analysis of sediments collected in the environment. The method is based on tropolone complexation of the ionic organotins, followed by extraction into a hexane-ethylacetate mixture and derivatization by NaBEt(4). The method was applied to the determination of organotin in various harbour, shipyard and dry-dock sediments in Belgium. Butyltin compounds were detected in all samples analyzed, often at high mg kg(-1) levels. A limited number of samples showed the presence of phenyltin compounds. Further, the method was adapted to the analysis of river sediments sampled from the vicinity of shipyards. Butyltin concentrations were detected at the microg kg(-1) level in the majority of samples.     

Evaluation of a purge-and-trap injection system for capillary gas chromatography-microwave induced plasma-atomic emission spectrometry for the determination of volatile selenium compounds in water

A method is presented for the selective determination of the volatile selenium species dimethylselenide and dimethyldiselenide, using a commercially available purge-and-trap injection system coupled to capillary gas chromatography-microwave induced plasma-atomic emission spectrometry. The efficiency of the purging step was evaluated and the parameters affecting the purge and trap processes were optimized. The method was applied to the determination of volatile selenium compounds in lake water. Relative detection limits of 2ng/l for dimethylselenide and dimethyldiselenide, corresponding to an absolute detection limit of 10 pg, were achieved.     

Rapid sensitive speciation analysis of butyl- and phenyltin compounds in water by capillary gas chromatography atomic emission spectrometry (GC-AES) after in-situ ethylation and in-liner preconcentration

Summary A rapid, simple and sensitive method has been developed for the simultaneous determination of butyl- and phenyltin species in environmental waters. The ionic organotin compounds are ethylated in the aqueous phase using sodium tetraethylborate (NaBEt₄) and extracted with hexane. A 25 l aliquot of the extract is injected at a low temperature into a Tenax filled liner. After solvent venting the analytes are transferred onto the capillary column using programmed temperature vaporization (PTV) injection. Detection is done by means of a microwave induced plasma atomic emission detector (MIP AED). The method allows the determination of butyl- and phenyltin compounds in water samples down to the level of 0.1 ng/l (as Sn) while 50 ml of sample is sufficient for analysis. The accuracy of the method was confirmed by GC-AAS after chelation and Grignard derivatization.     

Recognition of chiral catechols using oxo-titanium phthalocyanine

Oxo-titanium phthalocyanine (TiOPc) derivatives of catechin and hematoxylin (natural ortho-diol type chiral compounds) have been prepared and characterized by spectral and chromatographic techniques. It is demonstrated that the TiOPc unit is an excellent template for chiral recognition through its isolated Q-transitions. The formation of a helical dimeric complex with hematoxylin induces strong CD-activity in the Q-band region. Ab initio geometry optimizations were combined with a Kuhn-Kirkwood coupled-oscillator mechanism to obtain the absolute configuration of hematoxylin. In addition, it is shown that the described chiroptical recognition method is sensitive to slight conformational changes.     

Definitive assignments of the visible-near-IR bands of porphyrin-naphthalocyanine rare-earth sandwich double- and triple-decker compounds by magnetic circular dichroism spectroscopy

A series of heteroleptic rare-earth sandwich complexes [M(Nc)(OEP)] (M = La, Nd, Eu, Dy, and Lu; Nc = 2,3-naphthalocyaninate; OEP = octaethylporphyrinate) have been investigated by electronic absorption and magnetic circular dichroism (MCD) spectroscopy. The electronic absorption spectra of the neutral forms showed two characteristic transitions (bands I and II) in the near-IR region, both of which were systematically shifted depending on the size of their central metal. In the MCD spectra, a relatively intense Faraday A term and a significantly weak Faraday B term have been observed corresponding to bands II and I, respectively. The spectral features were successfully interpreted using a simple MO model by considering the relevant interactions of Gouterman's four orbitals of the constituent chromophores. The model succeeded in assigning the MCD spectra of the related compounds, the oxidized and reduced forms of the dimer ([M(Nc)(OEP)]+ and [M(Nc)(OEP)]-), and neutral forms of the triple-decker compounds (M2(Nc)(OEP)2, M = Nd, Eu). DFT calculations of the dimers supported the validity of this model.     

Interferences in ultratrace speciation of organolead and organotin by gas chromatography with atomic spectrometric detection

Interferences in trace and ultratrace speciation analysis of organotin and organolead compounds in various samples by gas chromatography coupled with atomic absorption spectrometry (AAS) and/or microwave induced plasma atomic emission spectrometry (MIP AES) are investigated. Particular attention is given to the effects of matrix co-extractives and reagents impurities introduced during sample preparation. Their influence on the detection limits is discussed in terms of baseline noise level, blank value, formation of artefacts and signal suppression. Loss of column resolution during the analysis of some matrices is observed.     

Assessment of organolead species in the Austrian Danube-basin using GC-MIP-AED

In order to evaluate the effectiveness of recent restrictions of Austrian government for the consumption of leaded gasoline, ionic organolead species were determined in 13 sampling sites of the Austrian and Slovakian Danube-basin in 4 bimonthyl sampling campaigns. Speciation analysis was performed using a rapid and sensitive method based on Grignard derivatization of the ionic organolead species and a GC-MIP-AED coupling for separation and detection. The operational variables were optimized for chromatographic resolution and detection limits. 100 ml of sample were used for the analyses and the detection limits for trimethyllead were 0.5 ng Pb l^–1 and for triethyllead 0.85 ng Pb l^–1. In general, absence of organolead induced pollution has been observed for most of the sampling locations and campaigns. Only the part of the river Danube between Vienna (Austria) and Bratislava (Slovakia), which is loaded by various intensive anthropogenic sources, showed in two campaigns the occurrence of very low trimethyl- and triethyllead concentrations, ranging between 1.2–12.0 ng Pb l^–1 and 1.6–5.8 ng Pb l^–1, respectively. Received: 21 March 1996 / Revised: 6 May 1996 / Accepted: 9 May 1996     

Mechanism of a strongly anisotropic MoIII-CN-MnII spin-spin coupling in molecular magnets based on the [Mo(CN)(7)](4-) heptacyanometalate: a new strategy for single-molecule magnets with high blocking temperatures

Unusual spin coupling between Mo(III) and Mn(II) cyano-bridged ions in bimetallic molecular magnets based on the [Mo(III)(CN)(7)](4-) heptacyanometalate is analyzed in terms of the superexchange theory. Due to the orbital degeneracy and strong spin-orbit coupling on Mo(III), the ground state of the pentagonal-bipyramidal [Mo(III)(CN)(7)](4-) complex corresponds to an anisotropic Kramers doublet. Using a specially adapted kinetic exchange model we have shown that the Mo(III)-CN-Mn(II) superexchange interaction is extremely anisotropic: it is described by an Ising-like spin Hamiltonian JS(z)(Mo) S(z)(Mn) for the apical pairs and by the J(z)S(z)(Mo) S(z)(Mn) + J(xy)(Sx(Mo) Sx(Mn) + Sy(Mo) Sy(Mn)) spin Hamiltonian for the equatorial pairs (in the latter case J(z) and J(xy) can have opposite signs). This anisotropy resulted from an interplay of several Ising-like (Sz(Mo) Sz(Mn)) and isotropic (S(Mo)S(Mn)) ferro- and antiferromagnetic contributions originating from metal-to-metal electron transfers through the pi and sigma orbitals of the cyano bridges. The Mo(III)-CN-Mn(II) exchange anisotropy is distinct from the anisotropy of the g-tensor of [Mo(III)(CN)(7)](4-); moreover, there is no correlation between the exchange anisotropy and g-tensor anisotropy. We indicate that highly anisotropic spin-spin couplings (such as the Ising-like JS(z)(Mo) S(z)(Mn)) combined with large exchange parameters represent a very important source of the global magnetic anisotropy of polyatomic molecular magnetic clusters. Since the total spin of such clusters is no longer a good quantum number, the spin spectrum pattern can differ considerably from the conventional scheme described by the zero-field splitting of the isotropic spin of the ground state. As a result, the spin reorientation barrier of the magnetic cluster may be considerably larger. This finding opens a new way in the strategy of designing single-molecule magnets (SMM) with unusually high blocking temperatures. The use of orbitally degenerate complexes with a strong spin-orbit coupling (such as [Mo(III)(CN)(7)](4-) or its 5d analogues) as building blocks is therefore very promising for these purposes.     

Thermodynamics of binding of 2-methoxy-3-isopropylpyrazine and 2-methoxy-3-isobutylpyrazine to the major urinary protein

In the present study we examine the thermodynamics of binding of two related pyrazine-derived ligands to the major urinary protein, MUP-I, using a combination of isothermal titration calorimetry (ITC), X-ray crystallography, and NMR backbone (15)N and methyl side-chain (2)H relaxation measurements. Global thermodynamics data derived from ITC indicate that binding is driven by favorable enthalpic contributions, rather than the classical entropy-driven hydrophobic effect. Unfavorable entropic contributions from the protein backbone and side-chain residues in the vicinity of the binding pocket are partially offset by favorable entropic contributions at adjacent positions, suggesting a "conformational relay" mechanism whereby increased rigidity of residues on ligand binding are accompanied by increased conformational freedom of side chains in adjacent positions. The principal driving force governing ligand affinity and specificity can be attributed to solvent-driven enthalpic effects from desolvation of the protein binding pocket.     

Oriented 2-cell embeddings of a graph and their symmetry classification: generating algorithms and case study of the möbius-kantor graph

We discuss a method to derive all symmetry-distinct oriented 2-cell embeddings of a given graph and classify them based on their symmetry. As an example, we apply the algorithm to the highly symmetrical trivalent M?bius-Kantor graph. Considering the derived 2-cell embeddings as carbon networks leads to some interesting negative curvature carbon allotropes.