The distribution of nickel ions among octahedral and tetrahedral sites in NiAl2O4MgAl2O4 solid solutions

The cation distribution in NiAl₂O₄ and in the solid solutions Ni_xMg_1?xAl₂O₄ with x ranging from 0.01 to 0.55 has been studied by X-ray analysis, magnetic susceptibility and reflectance spectroscopy.The relative X-ray intensities of various reflections for each compound were measured and compared with intensities calculated for various models of cation distribution. Lattice parameters, magnetic moments of Ni²⁺, and the position of nickel absorption bands have also been measured. The results show that all specimens have a predominantly octahedral distribution for their nickel ions, and that there is a small change towards a random distribution when the quenching temperature is increased from 1273 to 1673°K.Both X-ray and magnetic results show that the fraction, α, of Ni²⁺ ions on tetrahedral sites varies with nickel content. For samples quenched from 1273°K, NiAl₂O₄ is 78% inverse, but between x = 1.0 and x = 0.25 the value of α decreases from 0.22 towards 0.1. Thus, the fraction of nickel on octahedral sites in the solid solutions is higher than in NiAl₂O₄. The effect is linked with an increase in Dq for the Ni²⁺ ion. Below x = 0.25, there is tentative evidence that α may increase. In nickel-dilute solutions the matrix as a whole is nearly normal, and it is suggested that Madelung energy may then be more effective in counteracting the octahedral stabilization of nickel. The same trend is observed in the specimens quenched from 1673°K.     

Effect of included guest molecules on the normal state conductivity and superconductivity of beta''-(ET)(4)[(H(3)O)Ga(C(2)O(4))(3)].G (G = pyridine,nitrobenzene)

Normal state conductivity and superconductivity together with bulk magnetic susceptibility and magnetization measurements have been measured for two molecular charge-transfer salts: beta' '-(ET)4[(H3O)Ga(C2O4)3]G (ET = bis(ethylenedithio)tetrathiafulvalene, G = pyridine for compound I and nitrobenzene for compound II). With the exception of the included guest molecules (G) the crystal structures are almost identical. Both show minima in their electrical transport at 130 K for I and at 160 K for II, but at lower temperatures their behaviors differ markedly. The resistance of I reaches a maximum at 50 K with a further small peak at 2 K and possible superconductivity only below 2 K, whereas that of II increases continuously down to 7.5 K, where an abrupt transition to a superconducting state occurs.     

The preparation,phase relationships,and Eu-151 Mössbauer spectroscopy of europium tungsten bronzes and related phases

Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze Eu_xWO₃ is stable up to 1000°C when x ? 0.125 and in the region 0.085 ? x ? 0.125 the symmetry is cubic. A tetragonal bronze exists at x = 0.05, and an orthorhombic bronze with a structure closely related to the orthorhombic form of WO₃ exists below x = 0.01. Mössbauer spectra at room temperature and at 80 K indicate that in all these phases the europium is highly ionized as Eu(III) with no electron localization to give (EuII) even at low values for x. The decomposition products of the bronzes have been established, and the Mössbauer parameters for the highly nonstoichiometric tungstates Eu_xWO₄ were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for Eu_xWO₄ was found to fit the data best. In conformity with the foregoing data, a sample of composition “Eu₂W₂O₇” was found not be be a pyrochlore but to comprise a mixture of Eu₆WO₁₂, Eu_xWO₄, and W. The phase relationships for the europium bronze system Eu_xWO₃ are compared with those of other ionic bronzes Na_xWO₃, Li_xWO₃, and Al_xWO₃.     

A combinatorial approach to obtain affinity media with binding properties towards the aflatoxins

In our work we performed a combinatorial solid-phase synthesis in aqueous medium to prepare peptide libraries from which to select an amino acid sequence with binding properties towards aflatoxins.We used polystyrene beads, functionalised with carboxylic groups as solid support and eight amino acids as monomers. During the first step 64 different sequences of two amino acids were prepared by exploiting the principles of combinatorial chemistry; then the binding properties of all sequences towards aflatoxin B(1 )were checked. We determined binding constants towards aflatoxin B(1) and towards aflatoxins B(2), G(1) and G(2). Results were promising, so we prepared a new library by using the selected dipeptide as the starting solid phase. After selecting the best tetrapeptide sequences, we determined binding constants towards the quoted aflatoxins. We obtained binding constants ( K>10(4) M(-1)) similar to those shown by human serum albumin for similar compounds. Preliminary studies on an extraction column were promising for the development of an SPE system and for its application in food matrices.     

π‐Facial Selectivity in the Formation of Tricarbonylchromium(0) Complexes of Estra‐1, 3, 5(10), 6‐tetraenes

The preparation of two η⁶‐estra‐1, 3, 5(10), 6‐tetraene tricarbonylchromium complexes 4 and 6 are described. In both cases only one stereoisomer can be isolated, in contrast to other estrane‐tricarbonylchromium complexes, where complexations are non‐stereoselective. X‐ray crystal structural analysis of 4 discloses that only the more sterically hindered β‐facial isomer is formed. It is assumed that the 6, 7‐olefinic moiety exerts a directive influence on the complexation.     

Mediated amperometric biosensors for d-galactose,glycolate and l-amino acids based on a ferrocene-modified carbon paste electrode

Direct-current cyclic voltammetry is used to investigate the suitability of a ferrocene derivative as a mediator with galactose, glycolate and l-amino acid oxidases. The three enzymes coupled catalytically to ferrocene monocarboxylic acid exhibiting homogeneous second-order rate constants in the range 0.4 × 10⁵ to 8.5 × 10⁵ l mol^?1 s^?1. Enzyme electrodes which responded to d-galactose, glycolate or l-amino acids were constructed. The appropriate oxidase was retained behind a dialysis membrane at a carbon paste electrode containing the poorly soluble derivative 1,1′-dimethylferrocene. All the electrodes responded rapidly to millimolar concentrations of their respective substrates producing 95% of the steady-state current response in <2 min. This general method of biosensor construction should be widely applicable to oxidases and other oxidoreductase enzymes.     

Positive ion chemistry of esters of carboxylic acids in air plasma at atmospheric pressure

Ionization of esters of carboxylic acids RCOOR' (R = H, alkyl; R' = alkyl) within the air plasma of the Atmospheric Pressure Chemical Ionization (APCI) source occurs largely via H(+)-transfer and, to a minor extent, via NO(+) association. The protonated ester MH(+) is normally observed as M(2)H(+) and as higher aggregates (M(3)H(+), M(3)H(+)(H(2)O)) also at high source temperature. The behavior of M(2)H(+) upon collisional activation is consistent with the reported dissociation of proton-bound dimers to MH(+) species that, in turn, fragment according to the known paths of lowest energy. In addition, other important product ions form within the plasma, some in very high relative abundance, which are attributed to ion-molecule condensation reactions between neutral M and either MH(+) or M(2)H(+) resulting in the elimination of CO, R'OH, alkene from the alkoxy moiety of the ester and HCOOH. A general scheme is proposed to account for the experimental observations, which suggest that the encounter complex formed between MH(+) and M or between M(2)H(+) and M may either collisionally relax to the protonated dimer or trimer, respectively, or react via covalent bond forming and cleaving steps to eliminate stable neutral molecules. The proposed scheme is supported by both the observed concentration dependence and the temperature dependence of the products relative abundances within the plasma. Such reactions can be the dominant process, as in the case of formate esters. A second significant ionization route involves addition of NO(+) to form M(n)NO(+) (n = 1, 2, 3). An additional product corresponding to [M(2)NO(+) - CO(2)] is also observed with iso- and n-butyl formate esters.     

Application of amperometric sol-gel biosensor to flow injection determination of glucose

A glucose biosensor with enzyme immobilised by sol–gel technology was constructed and evaluated. The glucose biosensor reported is based on encapsulated GOX within a sol–gel glass, prepared with 3-aminopropyltriethoxy silane, 2-(3,4-epoxycyclohexyl)-ethyltrimetoxy silane and HCl. A flow system incorporating the amperometric biosensor constructed was developed for the determination of glucose in the 1×10⁻⁴–5×10⁻³ mol l⁻¹ range with a precision of 1.5%. The results obtained for the analysis of electrolytic solution for iv administration and human serum samples showed good agreement between the proposed method and the reference procedure, with relative error <5%.     

Photoelectron energy analysis in a strong magnetic field: The role of elastic scattering

A new photoelectron spectroscopic method is described directed towards the measurement of the photoelectron spectra of trapped ions. An electrostatic retarding field electron energy analyser incorporating a channel plate is included with an ion trap in a strong magnetic field. The form of photoelectron spectrum is shown to be strongly influenced by a elastic scattering effects which can be enhanced by using an electron trapping potential well. The resultant spectra then have a particularly simple and useful form.     

Radiative lifetimes of ions measured with the photoelectron spectrometer

Using a modified photoelectron spectrometer the radiative lifetimes of some electronically excited diatomic and triatomic molecular ions have been determined. The method is based on measuring the time delay between the ejection of a photoelectron from a molecule and the emission of a photon from the resulting ion.