Search results for "SAMPLE"
showing 10 items of 2270 documents
Multi-class determination of antimicrobials in meat by pressurized liquid extraction and liquid chromatography–tandem mass spectrometry
2008
A multi-residue method using pressurized liquid extraction (PLE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) has been developed for determining trace levels of 31 antimicrobials, including beta-lactams, lincosamides, macrolides, quinolones, sulfonamides, tetracyclines, nitroimidazoles and trimethoprim. The extraction method required pre-homogeneization of the meat with EDTA-washed sand and subsequent one-static-cycle extraction for 10 min with 40 ml of water at 1500 psi and 70 degrees C. The effect of operation temperature, pressure, flush volume, and static cycles on PLE performance was studied. Average recoveries ranged from 75 to 99% with relative standard deviations <1…
Analysis of thiabendazole and procymidone in fruits and vegetables by capillary electrophoresis–electrospray mass spectrometry
2002
Abstract A capillary electrophoresis–mass spectrometry method for determining procymidone and thiabendazole in apples, grapes, oranges, pears, strawberries and tomatoes is described. Separation is achieved using a buffer of formic acid–ammonium formate at pH 3.5 with 2% of methanol. Fungicide residues present in the sample are preconcentrated by both solid-phase extraction and injection of large sample volumes into the capillary by a stacking technique, to obtain lower detection limits. Ionization is performed at atmospheric pressure in an electrospray type source and detection is carried out using positive ionization and selected ion monitoring modes. The quantitation limits are 0.005 and …
Comparison of different sorbents for on-line liquid-solid extraction followed by high-performance liquid chromatographic determination of nitrogen-co…
1998
Abstract LiChrolut EN [poly(styrene-divinylbenzene), PSDVB], Carbograph (graphitized carbon black, GCB), Isolute CN (cyanopropylsilica), Isolute C 2 (ethylsilica), and LiChrospher RP18 (octadecylsilica) were studied for on-line solid-phase extraction of twelve nitrogen containing pesticides from water. Determination (UV 210 nm) was performed with a Spherisorb C 8 analytical column and an acetonitrile-water gradient. The mean recoveries from 50 ml ranged from 83% for RP18 to 44% for GCB, and decreased in the following order: RP18, CN, LiChrolut EN, C 2 , and GCB. GCB showed poor recoveries due to incomplete desorption with the acetonitrile-water gradient used, and memory effects were detecte…
Determination of glyphosate and aminomethylphosphonic acid in natural water using the capillary electrophoresis combined with enrichment step
2005
A previously elaborated capillary electrophoresis (CE) method used for the determination of glyphosate and aminomethylphosphonic acid (AMPA) was slightly modified in order to improve the sensitivity. However, detection limits attained (5 μg mL−1 for glyphosate and 4 μg mL−1 for AMPA) were still not satisfactory for analytical purposes, thus the addition of a preconcentration step before the CE analysis was proposed. AMBERLITE®IRA-900, a strong anion-exchange resin, was used to preconcentrate both analytes in environmental aqueous samples. The experimental conditions optimised in a previous work were readapted, by decreasing the eluent concentration due to CE limitations. Satisfactory result…
Non-chromatographic speciation of toxic arsenic in fish
2004
Abstract A rapid, sensitive and economic method has been developed for the direct determination of toxic species of arsenic present in fish and mussel samples. As(III), As(V), dimethylarsinic acid (DMA), and monomethylarsonic acid (MMA) were determined by hydride generation–atomic fluorescence spectrometry using a series of proportional equations without the need of a chromatographic previous separation. The method is based on the extraction of arsenic species from fish through sonication with HNO3 3 mol l−1 and 0.1% (m/v) Triton and washing of the solid phase with 0.1% (m/v) EDTA, followed by direct measurement of the corresponding hydrides in four different experimental conditions. The li…
A validated and fast procedure for FTIR determination of Cypermethrin and Chlorpyrifos.
2005
Abstract A FTIR methodology has been developed for the simultaneous determination of Cypermethrin and Chlorpyrifos in pesticide commercially available formulations. The method involves the extraction of both active principles with CHCl3 and direct measurement of the peak area values between 1747 and 1737 cm−1 corrected with a baseline defined at 2000 cm−1 for Cypermethrin and peak height values established at 1549 cm−1 corrected using a baseline situated at 1650 cm−1 for Chlorpyrifos. The limits of detection achieved were of the order of 0.7 and 0.4% (w/w), and the relative standard deviation 0.4 and 0.2% for Cypermethrin and Chlorpyrifos, respectively. The developed procedure provided stat…
Direct FIA—AS determination of potassium and magnesium in cement samples by use of the slurries approach
1992
A direct procedure has been developed for the flame atomic determination of potassium and magnesium in cement samples. A 50-mg sample is dispersed in 25 ml of 0.13M nitric acid; 100 mul of this slurry is injected in a double channel FIA manifold simultaneously with 100 mul of a 10% (w/v) lanthanum solution. This procedure allows the rapid extraction of potassium and magnesium by leaching of the sample; nitric acid is not necessary if only potassium must be determined and the sample can be diluted with only distilled water. Aqueous standards are used. The manifold employed includes a well-stirred mixing chamber, which provides an adequate on-line dilution of the sample, in order to obtain em…
Atomic fluorescence determination of inorganic arsenic in soils after microwave-assisted distillation
2000
Abstract An inexpensive microwave-assisted distillation procedure has been developed for quantitative determination of inorganic arsenic in soils by atomic fluorescence spectrometry (AFS). After reduction of As(V) to its trivalent state with potassium iodide, inorganic arsenic was distilled as AsCl 3 that was finally determined by atomic fluorescence after hydride generation with NaBH 4 in HCl medium. The different parameters that control the distillation: concentration and volume of HCl, time of distillation, sample weight, and oven load, were studied. The methodology developed has a detection limit of 0.015 μg l −1 , which corresponds to a concentration of 0.006 μg of As per gram of soil,…
The Effective Sample Size
2013
Model selection procedures often depend explicitly on the sample size n of the experiment. One example is the Bayesian information criterion (BIC) criterion and another is the use of Zellner–Siow priors in Bayesian model selection. Sample size is well-defined if one has i.i.d real observations, but is not well-defined for vector observations or in non-i.i.d. settings; extensions of critera such as BIC to such settings thus requires a definition of effective sample size that applies also in such cases. A definition of effective sample size that applies to fairly general linear models is proposed and illustrated in a variety of situations. The definition is also used to propose a suitable ‘sc…
Las representaciones del saber académico: Aportaciones desde la Geografía Escolar
2019
When we talk with basic education´s teachers there is an impression in the case of learning Social Sciences, and Geography in particular that these sciences are not useful for the explanation of socio-environmental and daily problems. In this research, improvement proposals are sought so that students are motivated to learn about these problems. Based on two case studies such as flood risks (and their relationship with climate change) and the problems of the rural areas, the possibilities of combining motivation for learning close to personal emotions and with their own rigor have been explored of the conceptual explanation. To validate these results, quantitative and qualitative research t…