Stopped-flow injection analysis. The influence of diffusion on dispersion of normal and reverse flow injection

A relationship is derived to enable the comparison of the dispersion heights of normal and reverse flow injection analysis (FIA). A single channel flow system is employed in the absence of a chemical reaction. The stopped-flow injection method is used to probe the influence of molecular diffusion on the overall dispersion of normal and reverse FIA, which appeared to demonstrate fundamentally different diffusion behaviors. Small discrepancies are observed between the dispersion heights, which are enhanced by the stopped-flow period, especially when unmatched matrix ionic compositions of the indicator and counter solutions were involved. For these conditions, the diffusion flux rate is enhanced considerably, displaying a peak, in addition to the transient, for both methods. The influence of diffusion on the dispersion characteristics of normal and reverse FIA is discussed theoretically. Diffusion in the proposed model is postulated to oppose dispersion by convection. The latter initiates concentration gradients in the injection zone and propagates it with flow time over the dispersion zone profile. The diffusion flux then reacts in order to confine the indicator dispersion for normal FIA and to enhance it for reverse FIA. This model is consistent with the experimental results and accounts for most of the phenomena encountered. Probably owing to the influence of secondary flow phenomena, the use of coiled tubes has suppressed the effects of diffusion on the overall dispersion behavior.Part of the experimental work was performed at IMI Institute for Research and Development, Haifa, Israel.     

Studies on the [2,3]-Stevens rearrangement of aziridinium ions

The aziridinium ylide generated by the intramolecular reaction of a metal carbenoid tethered to a vinylaziridine undergoes [2,3]-Stevens rearrangement to furnish the indolizidine skeleton. It is essential that the correct nitrogen invertomer is used or a competing [1,5]-hydrogen shift predominates. During the preparation of a second system a `one-pot' acylation-[3,3]-Claisen rearrangement was observed, delivering a seven-membered lactam.     

Determination of some trace elements in human tooth enamel

Determination of seven elements (Cu, Fe, Mg, Mn, Pb, Sr and Zn) in whole enamel and surface layers of extracted non-carious human teeth by FAAS, ETA AAS, ICP-AES and ICP-MS (Pb) is demonstrated. Techniques are described for obtaining whole enamel and its acid dissolution. Fifty microm width enamel layers from outer enamel surface to a 200 microm depth were successively etched in 1 mL of 3 M HClO4 for 3 min dissolution periods. Enamel samples were analyzed for populations under and over 20 years of age and enamel from Bronze Age teeth. Concentrations of microelements in the whole enamel and in the first surface layer (50 microm depth) were compared. With exception of Sr and Mg, all elements show significantly higher concentrations in the first layer than in whole enamel and higher concentration in teeth of individuals over 20 years, which demonstrate the cumulative effect of these elements. The Cu, Fe, Mn, Pb and Zn concentrations in four layers of erupted and non-erupted teeth decreased while Mg and Sr concentrations increased toward enamel-dentine junction. The concentrations of most elements were almost constant as they approached the 150 microm layer. This concentration gradient may result from interaction between saliva and teeth and supports the hypothesis that the surface de- and re-mineralization process is effective at most to 150 microm from the enamel surface.     

Determination of lead concentration and lead isotope ratios in calcium supplements by inductively coupled plasma mass spectrometry after high pressure, high temperature digestion

The presence of lead as a contaminant in calcium supplements has aroused considerable public health interest in recent years. In this investigation lead and lead isotope ratios were determined by ICP-MS in ten brands of calcium supplements after high pressure/temperature digestion. Calcium supplements (200 to 250 mg) were digested in 2 mL of nitric acid at 230?°C and at a pressure of 1770 psi (1.2 × 10⁴ kPa). Lead concentrations were determined by matrix-matched lead standards prepared in a high-purity calcium carbonate matrix. Good recoveries of lead and calcium were obtained for certified animal bone reference material. High levels of Pb (8 to 28 μg Pb per g of calcium) were found in calcium supplements that contain dolomite or bone meal. Chelated and refined calcium supplements had lower Pb levels (0.8 to 0.9 μg Pb/g Ca). Application of lead isotope ratios to distinguish the origin of calcium sources was also explored.     

Infrared and Raman matrix isolation studies of methylamine

Infrared (4000-50 cm^?1) and Raman spectra are reported of methylamine, methylamine-d₁ and methylamine-d₂ trapped in argon and nitrogen matrices at 4–20 K. An anomalous intensity variation was found for the NH₂ wagging mode of methylamine isolated in nitrogen matrices, while in argon matrices the NH₂ wagging absorption exhibited a complex structure due to matrix site effects. A normal coordinate analysis was carried out using a new assignment of the NHD twisting frequency. Barriers to internal rotation in argon and nitrogen matrices, calculated from the observed torsional frequencies, are compared with the gas phase value.     

The relationship between the linear (oscillatory) and nonlinear (steady-state) flow properties of a series of polymer and colloidal systems

The Cox-Merz empirical relationship between the linear (oscillatory) and nonlinear (steady-state) viscosities has been shown to be valid for many polymeric systems. Here, we present an equivalent expression to relate the linear (G) and nonlinear (N ₁) elastic properties of viscoelastic systems. Like the Cox-Merz relationship, it uses a combination of elastic and viscous parameters. The modified form of the storage modulus is then equivalent to the Cox-Merz complex viscosity. It can be used to correlate with (half) the normal force at numerically equal circular frequency and shear rate, respectively.This new expression and the Cox-Merz rule are tested for a range of polymeric and colloidal systems. It is found that both expression work for the polymeric systems considered, but fail for the colloidal systems. In the latter, the steady state values of viscosity and elasticity are consistently low, and replacing them by the complex viscosity and our new elastic expression only makes matters worse.For polymer systems, we suggest this is a general but not universal observation, since we are aware of exceptions to the rule that polymeric systems obey the Cox-Merz rule for viscosity and our rule for elasticity. For colloidal systems we find that neither rule is obeyed for any of our systems.     

Characterization of supramolecular (H2O)18 water morphology and water-methanol (H2O)15(CH3OH)3 clusters in a novel phosphorus functionalized trimeric amino acid host

Phosphorus functionalized trimeric alanine compounds (l)- and (d)-P(CH(2)NHCH(CH(3))COOH)(3) 2 are prepared in 90% yields by the Mannich reaction of Tris(hydroxymethyl)phosphine 1 with (l)- or (d)- Alanine in aqueous media. The hydration properties of (l)-2 and (d)-2 in water and water-methanol mixtures are described. The crystal structure analysis of (l)-2.4H(2)O, reveals that the alanine molecules pack to form two-dimensional bilayers running parallel to (001). The layered structural motif depicts two closely packed monolayers of 2 each oriented with its phosphorus atoms projected at the center of the bilayer and adjacent monolayers are held together by hydrogen bonds between amine and carboxylate groups. The water bilayers are juxtaposed with the H-bonded alanine trimers leading to 18-membered (H(2)O)(18) water rings. Exposure of aqueous solution of (l)-2 and (d)-2 to methanol vapors resulted in closely packed (l)-2 and (d)-2 solvated with mixed water-methanol (H(2)O)(15)(CH(3)OH)(3) clusters. The O-O distances in the mixed methanol-water clusters of (l)-2.3H(2)O.CH(3)OH and (d)-2.3H(2)O.CH(3)OH (O-O(average) = 2.857 A) are nearly identical to the O-O distance observed in the supramolecular (H(2)O)(18) water structure (O-O(average) = 2.859 A) implying the retention of the hydrogen bonded structure in water despite the accommodation of hydrophobic methanol groups within the supramolecular (H(2)O)(15)(CH(3)OH)(3) framework. The O-O distances in (l)-2.3H(2)O.CH(3)OH and (d)-2.3H(2)O.CH(3)OH and in (H(2)O)(18) are very close to the O-O distance reported for liquid water (2.85 A).     

Mass spectrometric methods for the analysis of chlorinated and nitrated isoflavonoids: a novel class of biological metabolites

Electrospray ionization combined with tandem mass spectrometry was applied to a study of some representative chlorinated and nitrated isoflavones-potential metabolites of isoflavones in inflammatory cells. Upon collision-induced dissociation of deprotonated [M - H](-) ions of these compounds, a number of structurally characteristic product ions were produced. The product ion analysis of 3'- and 8-chlorodaidzein in the tandom mass spectra led to ready differentiation of these isomers. 3-Nitro derivatives of both genistein and daidzein have product ions due to the losses of HNO(2) and two OH groups. Chlorinated derivatives of isoflavones were detected in cell-based experiments and their structures were proposed by comparing the tandem mass spectra of their product ions with those of standards. This work provides a suitable analytical basis to aid the characterization of chlorinated and nitrated metabolites in studies in vivo and in vitro.     

Degradation of deoxycholic acid by Pseudomonas Sp. NCIB 10590

The microbial degradation of deoxycholic acid 1 by Pseudomonas NCIB 10590 has been studied and two major products have been isolated and identified as 12β-hydroxyandrosta-1,4-dien-3,17-dione 2 and 12α-hydroxypregna-1,4-dien-3-one-20-carboxylic acid 9. Three minor products were isolated and evidence is given for the following structures: 12α-hydroxyandrosta-1,4-dien-3,17-dione 4, 12β-hydroxyandrosta-4-en-3,17-dione 7 and 12?, 17?-dihydroxyandrosta-1,4-dien-3-one 8.