Home About us Contact
|
Home About us Contact | ||
|
|
||
Latter Technique (latter + technique)
Selected AbstractsLaser desorption/ionization techniques in the characterization of high molecular weight oil fractions.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 9 2006Part 1: asphaltenes Abstract The molecular weight distribution of the asphaltene fractions of two types of crude oils from two different Italian fields (samples 1 and 2) was investigated. The analytical tools used to perform these analyses were matrix assisted laser desorption ionization (MALDI) and laser desorption ionization (LDI) mass spectrometry. After observing that the use of the matrix (as well as the addition of Ag+) did not improve the quality of the data compared to that obtained in LDI conditions, all further measurements were performed with the latter technique. Operating under usual conditions of laser power and delay time, a very low resolution was observed, showing only macroscopic differences between the two samples in the molecular weight distribution of the different components. An accurate study on the possible reasons of this undesirable behavior indicates that it can originate from space charge phenomena occurring either in the ion source region or during the flight. A valid parameterization of the delay time and the laser power allowed higher quality spectra to be obtained. Surface-enhanced laser desorption ionization (SELDI) measurements were also performed using normal phase (silica) as the sample holder surface. Under these conditions, better results are obtained, proving that the sample,surface interaction is important to achieve, by means of laser irradiation, a homogeneous set of product ions. Both asphaltene samples were fractionated in five subfractions by gel-permeation chromatography (GPC) to obtain a better separation of the molecular weight distributions; the related spectra confirmed these findings. By using different approaches, relevant and reproducible differences between the asphaltene fractions of the two oil samples were observed. Copyright © 2006 John Wiley & Sons, Ltd. [source] A Streamflow Forecasting Framework using Multiple Climate and Hydrological Models,JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 4 2009Paul J. Block Abstract:, Water resources planning and management efficacy is subject to capturing inherent uncertainties stemming from climatic and hydrological inputs and models. Streamflow forecasts, critical in reservoir operation and water allocation decision making, fundamentally contain uncertainties arising from assumed initial conditions, model structure, and modeled processes. Accounting for these propagating uncertainties remains a formidable challenge. Recent enhancements in climate forecasting skill and hydrological modeling serve as an impetus for further pursuing models and model combinations capable of delivering improved streamflow forecasts. However, little consideration has been given to methodologies that include coupling both multiple climate and multiple hydrological models, increasing the pool of streamflow forecast ensemble members and accounting for cumulative sources of uncertainty. The framework presented here proposes integration and offline coupling of global climate models (GCMs), multiple regional climate models, and numerous water balance models to improve streamflow forecasting through generation of ensemble forecasts. For demonstration purposes, the framework is imposed on the Jaguaribe basin in northeastern Brazil for a hindcast of 1974-1996 monthly streamflow. The ECHAM 4.5 and the NCEP/MRF9 GCMs and regional models, including dynamical and statistical models, are integrated with the ABCD and Soil Moisture Accounting Procedure water balance models. Precipitation hindcasts from the GCMs are downscaled via the regional models and fed into the water balance models, producing streamflow hindcasts. Multi-model ensemble combination techniques include pooling, linear regression weighting, and a kernel density estimator to evaluate streamflow hindcasts; the latter technique exhibits superior skill compared with any single coupled model ensemble hindcast. [source] Voltammetric determination of parthenolide in spiked human plasma and urinePHYTOCHEMICAL ANALYSIS, Issue 4 2001F. Belal Abstract The voltammetric behaviour of parthenolide, a biologically active sesquiterpene lactone, was studied using direct current (DCt), alternating current and differential-pulse polarography (DPP). Parthenolide developed well-defined cathodic waves over the whole pH range in Britton,Robinson buffers. At pH 10 the diffusion current constant was 3.54,±,0.08 (± standard deviation; n,=,8). The current vs concentration plots were rectilinear over the range 4,36 and 1,28,µg/mL in the DCt and DPP modes, respectively, with a minimum detectability of 0.06,µg/mL (about 1,×,10,7,M) using the latter technique. The waves were characterised as being diffusion controlled, although adsorption phenomenon played a limited role in the electrode process. The described analytical method was applied to the determination of parthenolide in spiked human urine and plasma; the percentage recoveries were 95.72,±,0.22 and 94.0,±,0.13 (± standard deviation; n,=,9), respectively. Copyright © 2001 John Wiley & Sons, Ltd. [source] The crystallinity of ancient bone and dentine: new insights by transmission electron microscopyARCHAEOMETRY, Issue 3 2002I. Reiche We studied various archaeological and palaeontological bones and dentines from different burial environments by Fourier transform infrared spectroscopy (FT,IR), X,ray diffraction (XRD) and transmission electron microscopy (TEM), in the framework of a general study of diagenesis. FT,IR and XRD were used to evaluate the global preservation state of the bone and dentine mineral phase by determining a splitting factor (SF) or a crystallinity index (CI), respectively. These data can be combined with studies on the nanometer scale made with TEM. This latter technique,coupled with electron microdiffraction, provides determination of dimensions and shapes of individual bone and dentine apatite nanocrystals as well as of secondary minerals formed during diagenesis. It enables us to distinguish between heat,induced recrystallization processes and crystal growth in solution occurring during diagenesis. [source] | ||||||||||