Search results for "LIMIT"

Electrosynthesis and electrochemistry

2015

Since the pioneering work of Kolbe, electrochemistry and electrosynthetic methods have been a part of the repertoire of the organic synthesis toolbox [1–2]. In general, only electrons are employed as reagents or the reagents are electrochemically regenerated. Consequently, waste can be avoided, and limited resources can be used in a careful and economic manner. Because alternative reaction pathways are employed by electrosynthetic methods, scarce and toxic elements can be replaced or are not required at all [3]. Moreover, in the foreseeable future regenerative sources of electricity, for example, photovoltaics and wind power, will provide a surplus of electricity as the current unsteady sup…

On the tautomerization process of glycine in aqueous solution

2000

The experimental activation energy for the tautomerization of glycine zwitterion neutral form has been reported to be 14.6 kcalrmol. It has been generally assumed that this energy barrier is needed for proton transfer to occur. However, previous theoretical results do not support this interpretation. In the present work, we examine this question using density functional calculations, extended basis sets and a polarizable continuum solvent model. Our results suggest that the limiting step for the tautomerization process corresponds basically to H-atom reorientation in the -COOH group. This could be a general feature in the tautomerization of amino acids. q 2000 Elsevier Science B.V. All righ…

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…

2017

Abstract. Further development and optimisation of a previously described ion trap aerosol mass spectrometer (IT-AMS) are presented, which resulted in more reproducible and robust operation and allowed for the instrument's first field deployment. Results from this 11-day-long measurement indicate that the instrument is capable of providing quantitative information on organics, nitrate, and sulfate mass concentrations with reasonable detection limits (0.5–1.4 µg m−3 for 1 h averages) and that results obtained with the IT-AMS can directly be related to those from Aerodyne aerosol mass spectrometers. The capability of the IT-AMS to elucidate the structure of fragment ions is demonstrated via an…

The catalytic-thermometric determination of molybdenum in steels based on the hydrogen peroxide-iodide indicator reaction

1990

Abstract The Mo VI -catalysed hydrogen peroxide-iodide reaction has been optimised for thermometry in an HCl medium using the initial slope method. The substitution of sodium perborate by H 2 O 2 and the conversion of the reaction to a process showing a Landolt effect are considered and discussed. The linear range extended from 0.2 μg Mo VI ml −1 to 1.2 μg Mo VI ml −1 with a detection limit of 0.06 μg Mo VI ml −1 . Interferences were also studied. The optimised procedure was applied to the evaluation of molybdenum in steels after extraction with α-benzoinoxime in chloroform.

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. …

Inductively coupled plasma atomic emission spectrometric determination of arsenic in mussel products

1993

A precise, accurate procedure is proposed for the determination of arsenic in mussel products by inductively coupled plasma atomic emission spectrometry. Organic matter is destroyed by dry ashing. Experimental conditions for the determination of arsenic were selected and an interference study was carried out. The methodology developed has a detection limit of 0.1 μg g−1, a relative standard deviation of 3%, and a recovery percentage of 98±2%. The accuracy of the method was checked by analysis of a certified sample of NIST oyster tissue (certified 14.0±1.2 μg g−1; found 14.1±1.1 μg g−1). The proposed procedure was used to analyze real samples of mussel products.

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…

Method for determination of formaldehyde in air in the pptv-range by HPLC after extraction as 2,4-dinitrophenyl-hydrazone

1982

The derivatization of formaldehyde with 2,4-dinitrophenylhydrazine and the extraction of the product formalhydrazone is studied. The determination is carried out by HPLC. Applying a cryogenic sampling technique and a sampling volume of 300l of air the detection limit of formaldehyde in air is 30 pptv and this is suitable for measurements of formaldehyde in the atmosphere. (IFU)

Gaschromatographisch-massenspektrometrische identifizierung der durch umsetzung von aminen mit isocyanaten gebildeten harnstoffderivate

1977

Abstract Gas chromatographic-mass spectrometric identification of urea derivatives formed by the reaction of amines with isocyanates N,N′-Di- and N,N′,N′-trisubstituted ureas constitute suitable derivatives for the gas chromatographic (GC) determination of primary and secondary amines. The GC identification of N- tert. -butyl-,N′,N′-alkyl urea derivatives presented in the first communication has been succesfully confirmed by mass spectrometry (MS) and mass fragmentometry. The reproduced mass spectra of six N- tert. -butyl-,N′,N′-alkyl ureas were interpreted according to the decomposition mechanisms of less complicated alkyl ureas known from the literature. Using combined GC-MS the detection…