Search results for "Llar"
showing 10 items of 3592 documents
Fast determination of prominent carotenoids in tomato fruits by CEC using methacrylate ester-based monolithic columns.
2007
In this study, the major carotenoids (beta-carotene and lycopene) present in tomato fruits were analyzed by CEC with a methacrylate ester-based monolithic column. The effects of the porogenic solvent ratio, and the hydrophobicity of bulk monomer employed were examined on carotenoids separations. A fast separation of these analytes was achieved in less than 5.0 min in a mobile phase containing 35% THF, 30% ACN, 30% methanol, and 5% of a 5 mM Tris aqueous buffer, pH 8, with lauryl methacrylate-based monoliths. The CEC method was evaluated in terms of detection limit and reproducibility (retention time, area, and column preparation) with values below 1.6 microg/mL and 7.2%, respectively. The p…
Determination of alkylphenol ethoxylates by micellar electrokinetic chromatography with bile salts
2001
Octyl- and nonylphenol ethoxylates (OPEs and NPEs) with different numbers of ethoxy units (average values: n = 10 and N = 40 for OPEs, and n = 10 for NPEs) were separated by micellar electrokinetic chromatography under positive polarity using an 80 mM borate buffer of pH 8.5 containing sodium deoxycholate (SDC) or sodium cholate (SC). When sodium dodecyl sulfate (SDS) was added to the background electrolyte (BGE) in the absence of the bile salt, a single peak at a migration time longer than that of the EOF was obtained. Substituting the SDS by a bile salt, the homologues were resolved. At the same bile salt concentration, resolution between the homologues was higher with SDC than using SC. …
In-tube solid-phase microextraction coupled by in valve mode to capillary LC-DAD: Improving detectability to multiresidue organic pollutants analysis…
2009
Abstract A simple and fast capillary chromatographic method has been developed to identify and quantify organic pollutants at sub-ppb levels in real water samples. The major groups of pesticides (organic halogens, organic phosphorous, and organic nitrogen compounds), some hydrocarbons (polycyclic aromatic hydrocarbons), phthalates and some phenols such as phenol and bisphenol A (endocrine disruptors) were included in this study. The procedure was based on coupling, in-tube solid-phase microextraction (IT-SPME) by using a conventional GC capillary column (95% methyl–5% phenyl substituted backbone, 80 cm × 0.32 mm i.d., 3 μm film thickness) in the injection valve to capillary liquid chromatog…
Direct evaluation of the electron density correlation function of partially crystalline polymers
1980
A discussion of the general properties of the one-dimensional electron density correlation function K(z) of a partially crystalline polymer with lamellar structure shows that application of a graphical extrapolation procedure permits direct determination of the crystallinity, the specific inner surface, and the electron density difference ηc − ηa. The procedure is based upon the occurrence of a straight section in the “self-correlation” range of K(z). Curved and nonparallel lamellae do not invalidate the concept. In the case of heterogeneous samples composed of partially crystalline and totally amorphous regions, some of the parameters of the experimentally obtained correlation function, as…
Chemical degradation products of lignin and humic substances part I Synthesis, structure verification and gas chromatographic separation of chlorinat…
1993
Abstract Chlorinated vanillins (4-hydroxy-3-methoxybenzaldehydes) and syringaldehydes (3,5-dimethoxy-4-hydroxy-benzaldehydes) were synthesized and their purities and structures were examined by quartz capillary gas chromatography and mass spectrometry. The detailed procedures for the synthesis, gas chromatographic separation and mass spectroscopic features are presented.
Lauroyl peroxide as thermal initiator of lauryl methacrylate monolithic columns for CEC
2008
The preparation of lauryl methacrylate (LMA)-based monolithic columns for CEC using lauroyl peroxide (LPO) as thermal initiator of polymerization has been investigated. The influence of initiator amount and composition of porogenic solvent on the physical and electrochromatographic properties of the resulting LMA-based monoliths was evaluated. A comparison with LMA-based columns thermally polymerized with AIBN was performed. At a given porogenic solvent composition, LMA stationary phases initiated with LPO showed higher permeabilities and better efficiency values than those prepared using AIBN as initiator. The optimum polymerization mixture found for LPO initiator provided a minimum plate …
Determination of major carotenoids in vegetables by capillary electrochromatography.
2006
A simple and rapid method for the isocratic separation and determination of carotenoids (carotenes and xanthophylls) in vegetables by CEC is described. The capillary column (100 microm ID, 25 cm effective length) was packed with 3 microm Hypersil ODS particles. The optimized mobile phase contained 60% ACN, 35% THF and 5% of a 5 mM Tris aqueous buffer of pH 8. beta-Carotene, lycopene and lutein were separated with efficiencies of 66 000-128 000 plates/m within a short time (less than 12 min for the last peak eluted, 13/13'-cis-beta-carotene). An excellent resolution of the three carotenoids, as well as partial resolution of their geometrical isomers, was achieved. Application to the determin…
Quantitative analysis of broad molecular weight distributions obtained by matrix-assisted laser desorption ionisation-time-of-flight mass spectrometry
1996
In order to quantify the error of matrix-assisted laser desorption ionisation (MALDI) time-of-flight (TOF) mass spectrometry in the determination of broad molecular weight distributions, different mixtures by weight of two poly(methyl methacrylate) standards were prepared. These mixtures, with well-defined bimodal molecular weight distributions were analysed by MALDI-TOF mass spectrometry using different matrices (2,4,6-trihydroxyacetophenone and 2,5-dihydroxybenzoic acid) and different cations (Li+, Na+, K+, Rb+ and Cs+) for doping the analyte. From the MALDI-TOF mass spectrometric data, the weight fractions of the two polymers of all mixtures were determined and compared to the values mea…
The chemistry of death--Adipocere degradation in modern graveyards.
2015
The formation of adipocere slows further decomposition and preserves corpses for decades or even centuries. This resistance to degradation is a serious problem, especially with regard to the reuse of graves after regular resting times. We present results from an exhumation series in modern graveyards where coffins from water-saturated earth graves contained adipocere embedded in black humic material after resting times of about 30 years. Based on the assumption that this humic material resulted from in situ degradation of adipocere, its presence contradicts the commonly held opinion that adipocere decomposition only occurs under aerobic conditions. To test our hypothesis, we collected black…
A study of the enolization of some alkyl ketones in lyomesophases formed by alkylammonium surfactants
1988
The kinetics of enolization of acetone, 3-pentanone, and 6-undecanone have been studied in lyotropic systems in the isotropic, nematic, and lamellar phases. The data, analysed in terms of spontaneous and acid-catalyzed rate constants, do not show any relevant influence of the physical nature of the phase. Effects are instead observed in relation to the length of the alkyl chains of the ketones and the type of the amphiphilic molecule. 6-undecanone has a surprising effect in stabilizing ordered mesophases.