3-D structural modeling of humic acids through experimental characterization,computer assisted structure elucidation and atomistic simulations. 1. Chelsea soil humic acid

This paper describes an integrated experimental and computational framework for developing 3-D structural models for humic acids (HAs). This approach combines experimental characterization, computer assisted structure elucidation (CASE), and atomistic simulations to generate all 3-D structural models or a representative sample of these models consistent with the analytical data and bulk thermodynamic/structural properties of HAs. To illustrate this methodology, structural data derived from elemental analysis, diffuse reflectance FT-IR spectroscopy, 1-D/2-D 1H and 13C solution NMR spectroscopy, and electrospray ionization quadrupole time-of-flight mass spectrometry (ESI QqTOF MS) are employed as input to the CASE program SIGNATURE to generate all 3-D structural models for Chelsea soil humic acid (HA). These models are subsequently used as starting 3-D structures to carry out constant temperature-constant pressure molecular dynamics simulations to estimate their bulk densities and Hildebrand solubility parameters. Surprisingly, only a few model isomers are found to exhibit molecular compositions and bulk thermodynamic properties consistent with the experimental data. The simulated 13C NMR spectrum of an equimolar mixture of these model isomers compares favorably with the measured spectrum of Chelsea soil HA.     

Chromatographic and non‐chromatographic characterization of poly‐α‐olefins

Both chromatographic and non‐chromatographic techniques were used in the characterization and identification of poly‐α‐olefins. Synthetic base oils, although produced from the same raw material, exhibit different physico‐chemical properties. Their mutual miscibility and behaviour in final engine oils may be predicted from data collected through classical physico‐chemical analysis, simulated distillation chromatography as well as inverse gas chromatography. Copyright © 2007 John Wiley & Sons, Ltd.     

Study of mass transfer and determination of drop size distribution in a pulsed extraction column

A better understanding of pulsed liquid-liquid extraction columns was obtained by using an online digital image analysis system to characterize emulsion drop. The mass transfer of acetic acid from dispersed phase (ethyl acetate) to continuous phase (water) was studied under standard conditions. The system enabled drop size distribution (DSD) to be measured as a function of physical and thermodynamic parameters. The surface tension was investigated by static contact angle measurement. Mass transfer and energy transfer, characterized by the surface/volume ratio of the droplets were compared with the working parameters in order to interpret restrictive phenomena such as hold up and column efficiency. The experimental values of Sauter diameter (d₃₂) and those predicted by a correlation proposed in the literature are in good agreement. However, the adhesion work between liquid and PTFE plate surface indicates that interface property variations, as a function of solute concentration, must be taken into account in the theoretical correlations. It was found that hold up and separation efficiency depend mainly on the stirring rate. These results show that online image analysis can be used as a process control of a liquid-liquid extraction column in order to optimize the mixing phenomena and the DSD, the key parameter of extraction efficiency.     

Depth resolution and multiexponential lifetime analyses of reflectance-based time-domain fluorescence data

Time-domain fluorescence imaging is a powerful new technique that adds a rich amount of information to conventional fluorescence imaging. Specifically, time-domain fluorescence can be used to remove autofluorescence from signals, resolve multiple fluorophore concentrations, provide information about tissue microenvironments, and, for reflectance-based imaging systems, resolve inclusion depth. The present study provides the theory behind an improved method of analyzing reflectance-based time-domain data that is capable of accurately recovering mixed concentration ratios of multiple fluorescent agents while also recovering the depth of the inclusion. The utility of the approach was demonstrated in a number of simulations and in tissuelike phantom experiments using a short source-detector separation system. The major findings of this study were (1) both depth of an inclusion and accurate ratios of two-fluorophore concentrations can be recovered accurately up to depths of approximately 1 cm with only the optical properties of the medium as prior knowledge, (2) resolving the depth and accounting for the dispersion effects on fluorescent lifetimes is crucial to the accuracy of recovered ratios, and (3) ratios of three-fluorophore concentrations can be resolved at depth but only if the lifetimes of the three fluorophores are used as prior knowledge. By accurately resolving the concentration ratios of two to three fluorophores, it may be possible to remove autofluorescence or carry out quantitative techniques, such as reference tracer kinetic modeling or ratiometric approaches, to determine receptor binding or microenvironment parameters in point-based time-domain fluorescence applications.     

Reduction of diagnostic window by new fourth-generation human immunodeficiency virus screening assays

In order to reduce the diagnostic window between the time of human immunodeficiency virus (HIV) infection and laboratory diagnosis, new screening enzyme-linked immunosorbent assays (ELISAs) which permit the simultaneous detection of HIV antigen and antibody have been developed. Two fourth-generation assays, HIV DUO (Biomérieux) and HIV Combi (Boehringer Mannheim), for the combined detection of HIV antigen and antibody, were compared with a third-generation assay (HIV-1/HIV-2 3rd Generation Plus enzyme immunoassay [EIA]; Abbott) and a p24 antigen test (HIV-1 Ag monoclonal; Abbott). A total of 17 seroconversion panels, 15 cell culture supernatants infected with different HIV type 1 (HIV-1) subtypes, and 255 potentially cross-reactive serum samples were tested. Ten seroconversions were detected an average of 8.1 days earlier with HIV DUO and 7.5 days earlier with HIV Combi than with the third-generation ELISA. Overall, in the 17 seroconversion panels tested, HIV DUO detected HIV-1 infection an average of 4.8 days and HIV Combi detected infection an average of 4.4 days earlier than HIV-1/HIV-2 3rd Generation Plus EIA. HIV antigen was detected with HIV DUO and HIV Combi in all of the 15 cell culture supernatants infected with different HIV-1 subtypes, including subtype O. With fourth-generation assays, considerably fewer false-positive results (n = 4 to 6) were obtained, in comparison with the third-generation EIA (n = 18). Fourth-generation assays permit an earlier diagnosis of HIV infection than third-generation antibody screening assays through the detection of p24 antigen, which may be present in serum samples from individuals with recent HIV infection prior to seroconversion.