Changes in the Transfer Function of a flow microcalorimeter

Flow microcalorimeters are used to determine thermodynamic properties of liquid mixtures, the accuracy of these measures depends on the right calibration of the instrument. In this work the system is identified by means of the transfer functions of the two poles, it is proven that the first time constant and the sensitivity change with the value of rc _p f of the injected liquids (r - density, c _p - heat capacity, f - injection flow), and that the sensitivities obtained in the electrical and chemical calibrations are different for the same value of rc _p f because the dissipation in each case does not occur in the same place. As a summary of the calibration carried out, it is proposed a sensitivity value of 313±4 mV W^-1 for rc _p f<15 mW K^-1 that permits to make thermal measures with an uncertainty of 3%. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transition probabilities and average cross sections for the Na2 B1pi(u) --> X1sigma(g)+ system using as excitation the 4880 A Ar+ laser line

The fluorescence spectrum of Na2 induced by the 4879.86 A line of an Argon ion laser has been analyzed with special emphasis on determination of accurate relative intensities. We have observed nineteen fluorescence series for the B1pi(u) --> X1sigma(g)+ band system. Some series are reported for the first time. The radiative transition probabilities for the observed fluorescence series were calculated using hybrid potential energy curves for the B1pi(u) and X1sigma(g)+ states constructed up to dissociation and a B-X transition dipole moment function. Radiative lifetimes for the rovibrational levels of the upper states pumped by the laser line have also been calculated. The transition probabilities and lifetimes agree with the corresponding observed measurements usually within the experimental uncertainty. From the rotational satellite structure with deltaJ' = +/- 1, +/- 2...+/- 20, for some nu'-bands of the most intense fluorescence series induced by emission from the vibrational-rotational levels: nu' = 6, J' = 43 and v' = 9, J' = 56, collision-induced transition rates and average cross sections have been obtained.     

Structural Models of Dense Gels

The structural models of gels typically reported concentrate either on the solid space or the pore space of the system. The models described in this paper present a connection between grain and pore spaces, applicable to dense gels with uniform particulate microstructure. The gel structure is depicted as a hierarchy at several levels by means of models built up using the Monte-Carlo technique, on the basis of random close packing (RCP) premises. The pore volume distributions are calculated from the largest sphere radius inscribed within the interstices. These distributions are compared to the pore volume distributions of various RCP classic models and to the pore volume distributions of a series of xerogels measured by means of the Brunauer-Emmett-Teller method. Data on the pore volumes associated with different hierarchical levels (e.g., micro-, meso-, or macropores), the local density of the i-th aggregation level and packing of the successive levels are obtained.     

Bonding of atomic phosphorus to polycyclic hydrocarbons and curved graphitic surfaces

We present a theoretical study of the bonding of atomic phosphorus to planar hydrocarbons and to curved graphite-like surfaces. We find that bonding of phosphorus to planar polycyclic hydrocarbons induces curvature away from the phosphorus atom, as defined by the pyramidalization angle. Similarly, bonding of atomic phosphorus to the [5,5] fulvalene-circulene semifullerene and buckminsterfullerene is only possible on the convex side of the carbon surface. On the other hand, we find the interaction of atomic phosphorus with the concave side of fullerene-like surfaces to be nonbonding for both quartet and doublet spin states. We find the prerequisite for stable epoxy-type bonds within these systems is the ability of the carbon atoms to maintain or induce curvature away from the P.C=C bond.     

Determination of triazine herbicides in natural waters by solid-phase extraction and non-aqueous capillary zone electrophoresis

Capillary zone electrophoresis (CZE) in an organic medium was used to analyse triazines at sub-ppb concentration levels in natural waters after a preconcentration step using conventional C18 cartridges and new Oasis HLB devices. With both sorbents, satisfactory results were obtained on analysing deionized water. However, on analysing natural waters, both sorbents showed very different types of behaviour. The different variables affecting the elution of both sorbents were studied, resulting in the choice of Oasis HLB as the most suitable for later separation by CZE in non-aqueous medium. Combination of a preconcentration step with electrokinetic injection revealed that capillary electrophoresis with simple UV detection can also be used satisfactorily for the quantification of micropollutants in natural waters. The detection limits obtained varied between 0.01 and 0.05 microg l(-1), depending on the type of matrix analysed. The day-to-day precision varied between 0.9% and 2.3%, expressed as the relative standard deviation.     

MEPSIM: A computational package for analysis and comparison of molecular electrostatic potentials

Summary MEPSIM is a computational system which allows an integrated computation, analysis, and comparison of molecular electrostatic potential (MEP) distributions. It includes several modules. Module MEPPLA supplies MEP values for the points of a grid defined on a plane which is specified by a set of three points. The results of this program can easily be converted into MEP maps using third-parties graphical software. Module MEPMIN allows to find automatically the MEP minima of a molecular system. It supplies the cartesian coordinates of these minima, their values, and all the geometrical relationships between them (distances, angles, and dihedral angles). Module MEPCOMP computes a similarity coefficient between the MEP distributions of two molecules and finds their relative position that maximizes the similarity. Module MEPCONF performs the same process as MEPCOMP, considering not only the relative position of both molecules but also a conformational degree of freedom of one of them. The most recently developed module, MEPPAR, is another modification of MEPCOMP in order to compute the MEP similarity between two molecules, but only taking into account a particular plane. The latter module is particularly useful to compare MEP distributions generated by systems of aromatic rings. MEPSIM can use several wavefunction computation approaches to obtain MEP distributions. MEPSIM has a menu type interface to simplify the following tasks: creation of input files from output files of external programs (GAUSSIAN and AMPAC/MOPAC), setting the parameters for the current computation, and submitting jobs to the batch queues of the computer. MEPSIM has been coded in FORTRAN and its current version runs on VMS/VAX computers.     

Liquid chromatographic-mass spectrometric determination of post-harvest fungicides in citrus fruits

Liquid chromatography (LC)-atmospheric pressure ionisation (API)-mass spectrometry (MS) has been used to determine residues of five fungicides in oranges with a minimum sample cleanup. Atmospheric pressure chemical ionisation (APCI) and electrospray (ES) were compared and both gave similar results in terms of sensitivity and structural information. The main ions were [M+H]+ for carbendazim, imazalil, thiophanate methyl and thiabendazole, and [M+H-C4H9NHCO]+ for benomyl. Samples were extracted with sodium sulphate and ethyl acetate. Although benomyl and thiophanate methyl were transformed through the extraction procedure to carbendazim, the method showed good precision (<13%) and recovery (>70%), except for thiophanate methyl (50%), whilst also yielding limits of detection (<0.03 mg kg(-1)) that are adequate for the determination of the studied fungicides in oranges.     

Sb(III) and Sb(V) separation and analytical speciation by a continuous tandem on-line separation device in connection with inductively coupled plasma atomic emission spectrometry

A sensitive, precise and automated non-chromatographic method for Sb(III) and Sb(V) analytical speciation based on a continuous tandem on-line separation device in connection with inductively coupled plasma-atomic emission (ICP-AES) detection is proposed. Two on-line successive separation steps are included into this method: a continuous liquid-liquid extraction of Sb(III) with ammonium pyrrolidine dithiocarbamate (APDC) into methylisobuthylketone (MIBK), followed by direct stibine generation from the organic phase. Both separation steps are carried out in a continuous mode and on-line with the ICP-AES detector. Optimization of experimental conditions for the tandem separation and ICP-AES detection are investigated in detail. Detection limits for Sb(III) were 3 ng.mL(-1) and for Sb(V) 8 ng.mL(-1). Precisions observed are in the range +/- 5%. The proposed methodology has been applied to Sb(III) and Sb(V) speciation in sea-water samples.     

Fenton’s reagent-mediated degradation of residual Kraft black liquor

In this work, the effect of Fento’s reagent on the degradation of residual Kraft black liquor was investigated. The effect of Fenton’s reagent on the black liquor degradation was dependent on the concentration of H₂O₂. At low concentrations (5 and 15 mM) of H₂O₂, Fenton’s reagent caused the degradation of phenolic groups (6.8 and 44.8%, respectively), the reduction of reaction medium pH (18.2%), and the polymerization of black liquor lignin. At a high concentration (60 mM) of H₂O₂, Fenton’s reagent induced an extensive degradation of lignin (95–100%) and discoloration of the black liquor. In the presence of traces of iron, the addition of H₂O₂ alone induced mainly lignin fragmentation. In conclusion, Fenton’s reagent and H₂O₂ alone can degrade residual Kraft black liquor under acidic conditions at room temperature.     

1H and 13C NMR spectral assignments and conformational analysis of 14 19-nor-neoclerodane diterpenoids

Unambiguous and complete assignments of 1H and 13C NMR chemical shifts for 14 19-nor-neoclerodane diterpenoids, nine of them isolated from natural sources and five other synthetic derivatives, are presented. The assignments are based on 2D shift-correlated (1H,1H-COSY, 1H,13C-gHSQC and 1H,13C-gHMBC) and NOE experiments. The conformations of rings A and B of these compounds are supported by the 3J(H,H) values and they agree with the low-energy conformations obtained by semi-empirical calculations. Moreover, the data obtained in this work for 2-acetoxyteucvidin and a semisynthetic 18-aldehyde derivative indicate that the configuration at C-2 of the former and at C-10 of the latter must be reversed with respect to those reported previously.