Validation of the uncertainty evaluation for the determination of metals in solid samples by atomic spectrometry

 Every analytical result should be expressed with some indication of its quality. The uncertainty as defined by Eurachem ("parameter associated with the result of a measurement that characterises the dispersion of the values that could reasonably be attributed to the, . . ., quantity subjected to measurement") is a good tool to accomplish this goal in quantitative analysis. Eurachem has produced a guide to the estimation of the uncertainty attached to an analytical result. Indeed, the estimation of the total uncertainty by using uncertainty propagation laws is components-dependent. The estimation of some of those components is based on subjective criteria. The identification of the uncertainty sources and of their importance, for the same method, can vary from analyst to analyst. It is important to develop tools which will support each choice and approximation. In this work, the comparison of an estimated uncertainty with an experimentally assessed one, through a variance test, is performed. This approach is applied to the determination by atomic absorption of manganese in digested samples of lettuce leaves. The total uncertainty estimation is calculated assuming 100% digestion efficiency with negligible uncertainty. This assumption was tested. Received: 3 November 1997 · Accepted: 2 January 1998     

Equilibrium in Saturated Ca(OH)2 Solutions: Parameters and Dissociation Constants

A new computer program has been developed for the calculation of pH, pOH, hydroxide ion concentration m _OH, species distribution coefficients _i, ionic activity coefficients _i ionic strength I, buffer capacity , solubility product K _s0, and the two dissociation constants, K _b1 and K _b2, corresponding, respectively, to first and second dissociation steps of Ca(OH)₂ in aqueous solution. Previously developed methodology, for the calculation of pH, _i, _i I, and parameters of pH buffer solutions, starting from the corresponding acidity constants, has been adapted for the case of aqueous Ca(OH)₂ solutions, for which the pertinent stoichiometric relationships are different from those applicable to mixtures of acids and their salts. The results show that, contrary to what is currently assumed, the first dissociation is far from complete. Values are given for the concentrations and activities of species Ca(OH)₂(aq), Ca(OH)⁺(aq), and Ca²⁺ (aq) in saturated calcium hydroxide solutions at 25°C.     

EFFECT OF THYMINE DIMER INTRODUCTION IN A 21 BASE PAIR OLIGONUCLEOTIDE

Abstract— It is well known that the pyrimidine dimers are the main damage produced by UV radiation on the DNA structure. However, while studies on the photoproduct structure have been carried out extensively, uncertainties still exist on the implication that a single damaging event has on the overall conformation. In particular, the extension of the damage influence on the polynucleotide chain is a matter of debate. This problem is especially important to understanding some steps of the repair mechanisms. In this study we performed a chemical-physical characterization of 21 base pair oligonucleotides containing a single thymine dimer in one strand. Thermodynamic parameters were determined by means of thermal denaturation experiments, and static fluorescence measurements were performed to unequivocally define the primary structure-conformation relationship in this specific case. We used hydroxyl radicals, produced by means of γ-irradiation of the sample solution, to detect fine structure changes. Our data show that the introduction of a single thymine dimer might cause only a slight distortion of the helix geometry, as judged by the evaluation of the enthalpic and the entropic terms and by the small changes observed in the binding of ethidium bromide to DNA. The modifications in the sugar phosphate backbone subsequent to the damaging event are especially evident, near the thymine dimer, toward the 5'-end direction in the strand containing the dimer.     

The Effects of Cosolvents on the Complexation of α-Cyclodextrin with Alkylated Substances. Calorimetric Studies at 25 °C

The formation of complexes of -cyclodextrin with 1,2-alkanediols, ,-alkanediols and some cycloalkanols has been studied calorimetrically at 25 °C in water, in 7 mol kg^-1 aqueous urea and in 3 mol kg^-1 aqueous glucose. When a complex is formed, calorimetry enables the calculation of both the enthalpy and the association constant, from which the free energy and the entropy of the process can be obtained. The forces involved in the association process are discussed in the light of the signs and values of the thermodynamic parameters obtained. The effect of the variation of the aqueous medium on the hydration of the interacting substances and the consequent changes in the association parameters have been investigated. As respect to water, complexes are less stable in urea and more stable in glucose. The analysis of the data shows that this is the result of a different enthalpy-entropy balance in the two solvent media. Deaquation of the interacting substances plays a major role in determining the stability of the inclusion complexes.     

Phase behavior of C60 by computer simulation using ab‐initio interaction potential

A first‐principles intermolecular potential recently proposed by Pacheco and Ramalho [Phys Rev Lett 1997, 79, 3873–3876] has been used with the Gibbs ensemble and Gibbs–Duhem integration Monte Carlo methods to simulate the vapor–liquid and fluid–solid coexistence properties of C₆₀. The critical properties were calculated by fitting the results to the laws of rectilinear diameters and order parameter scaling. The triple‐point properties were determined from the limiting behavior of the Gibbs ensemble vapor–liquid simulations at the lowest temperature range. A stable liquid phase is predicted for temperatures between 1570±20 and 2006±27 K and densities between 0.444±0.003 and 1.05±0.01 nm^?3. The estimated critical and triple‐point pressures are, respectively, 35±6 and 5±16 bars. We show for the first time, to our knowledge, that it is possible, strictly by computer simulation, to estimate a triple point for C₆₀ in accordance with the predictions of theoretical methods and the basic concepts of thermodynamics. The liquid and fluid radial distribution functions indicate the presence of solid or glasslike features. This may support the suggestion of a more cooperative interaction of clusters in C₆₀. A comparison of our results with the data obtained by other authors is presented and discussed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem 84: 375–387, 2001     

Symmetry of positive solutions of an almost-critical problem in an annulus

We consider the subcritical problem & 0 & \qquad\textrm{in} \; A\\ u & = & 0 & \qquad\textrm{on} \; \delta A\\ \end{array} \right.$$" align="middle" border="0"> where A is an annulus in , , is the critical Sobolev exponent and 0$" align="middle" border="0"> is a small parameter. We prove that solutions of (I) which concentrate at one or two points are axially symmetric.Received: 7 July 2003, Accepted: 10 May 2004, Published online: 16 July 2004Filomena Pacella: Research supported by MIUR, project Variational Methods and Nonlinear Differential Equations.     

On an eigenvalue problem related to the critical exponent

In this paper we study the eigenvalue problemwhere is a smooth bounded domain, and u is a positive solution of the problemsuch thatwhere S is the best Sobolev constant for the embedding of H¹₀() into L²*(), We prove several estimates for the eigenvalues _i, of (I), i=2,..,N+2 and some qualitative properties of the corresponding eigenfunctions.Supported by M.I.U.R., project Variational methods and nonlinear differential equations.     

Chemical composition and phytotoxic effects of essential oils from four Teucrium species

The essential oils of four Teucrium species were studied and 131 components, in all, were identified. All oils were rich in sesquiterpenes (50.0-61.9%). Caryophyllene and caryophyllene oxide were the main components of Teucrium arduini; germacrene D, delta-cadinene and gamma-cadinene predominated in Teucrium maghrebinum. Carvacrol and caryophyllene predominated in Teucrium polium ssp. capitatum, while carvacrol, caryophyllene oxide and caryophyllene were the most abundant components in Teucrium montbretii ssp. heliotropiifolium. The germination of radish and garden cress was less sensitive to the four essential oils. The radicle elongation, above all, of radish was significantly inhibited by all oils, in particular by the essential oil of T. arduini, at the highest doses tested. Among the main components of the oils, monoterpenes resulted the more active compounds.