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  • Author or Editor: J. Mitchell x
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Abstract  

The detection limits of many nonnuclear methods for ultratrace (≤0.1 μg/g) elemental determinations lie well below the level at which precise and accurate practical analysis can be executed routinely. Advances in analytical methodologies which are rapidly eroding such disparities are reviewed. The applications of X-ray fluorescence, atomic absorption, stable isotope dilution mass spectrometry, and laser intracavity absorption spectrophotometry to ultratrace analyses not amenable to solution by nuclear methods are discussed.

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Abstract  

Gamma-ray spectrometric survey analyses were used to screen raw materials available during the infancy of the optical waveguide research program. Examinations of γ-ray spectra and semiquantitative survey analyses showed most samples to be insufficiently pure for waveguide applications. Highly pure samples have been prepared successfully under carefully controlled conditions when the purification process is adequately monitored by analyses. Radioisotope techniques and neutron activation analyses were vital in the development of successful procedures for purifying various reagents from which glass could be subsequently fabricated by melting techniques. Contamination sources during fiber production have also been detected by neutron activation analyses.

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Abstract  

The distribution of nitrogen in plasma deposited silicon nitride films and in commercially produced, hot-pressed bulk material has been determined by the nuclear (proton) track image analysis technique. The nuclear track technique is shown to have the unique capability of sampling large areas (cm2) while providing distribution information on the micro scale (100 m2). Nitrogen over the range of 2 to 40% is determined quantitatively. Spatial distribution and topographical maps are plotted. The overall composition of the material is established by 14 MeV NAA through the determinations of silicon, nitrogen, and oxygen. An application in the micro electronic industry is described.

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Abstract  

Un-hydrated Portland cement consists of several anhydrous and reactive phases, that when mixed with water react to form hydrates. The main hydration product of Portland cement is calcium silicate hydrate (C–S–H). It is the main binding phase in a concrete system, hence is important to construction chemists. The concrete engineer measures the compressive strength of concrete after prescribed hydration periods, typically 1, 3, 7, 28 days. It is often convenient to mimic these intervals by stopping the hydration reaction at the same times. Several techniques can be employed to stop this hydration reaction. One of which is solvent-based and involves mixing a polar solvent such as acetone or isopropyl alcohol, with the hydrated cement. This mixing should be vigorous enough to blend the free water, in the partially hydrated cement system, with the polar solvent without altering the cement system’s matrix. The solvent-water mixture has a much lower boiling point and the mixture quickly evaporates out of the system. This achieves two goals. It stops the hydration reaction at the moment of solvent mixing, and it removes free water to prevent further hydration from occurring. This procedure theoretically leaves behind a dry, chemically unaltered, partially hydrated cement paste. In this way, pastes can be analyzed after the prescribed 1, 3, 7 or 28 days of hydration. This paper uses thermogravimetric analysis (TG) results to investigate the assumption that solvents have no thermodynamic or chemical effect on the hydrated cement paste phases.

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Résumé  

On mesure le cuivre 64 et le manganèse 56 dans du carbonate de sodium ultra pur par spectroscopie du rayonnement gamma après élimination du24Na par passage sur colonnes de pentoxide d'antimoine hydraté (PAH); On a aussi déterminé le cobalt, le chrome et le fer par détection spectrométrique non-destructive du rayonnement gamma du60Co,59Fe et51Cr. Dans le cas de CaCo3, après irradiation et dissolution, on a déterminé simultanément le64Cu et le56Mn, par une séparation radiochimique sélective à pH 3 à 4 avec un mélange de dithizone (H2D) et d'acide pynolidinedithiocarbamique (HPDC) dans CHCL3. L'interférence radioactive de47Sc et47Ca produits pendant 100 heures d'irradiation de CaCO3, étaient supprimées par extraction sélective de ce dernier en milieu HCl 5,0 M par l'oxyde de Tri-n-octylphosphine (TOPO) 0,1 M dans du cyclohexane. On a utilisé en mélange de 0,1 M Thenoyltrifluoroacetone (HTTA) 0,1 M et de TOPO 0,1 M dans du cyclohexane pour enlever le47Ca à pH≥8.0. Après avoir éliminé l'activité des isotopes de la matrice, on mesure les pics photoélectriques de59Fe.60Co et51Cr pour doser ces impuretés.

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Abstract  

A highly precise and selective method is described for the determination of traces of gold by substoichiometric extraction from hydrochloric acid with tri-n-octylphosphine sulfide in cyclohexane following thermal neutron activation. Fundamental aspects of the extraction system are discussed and results are reported for the determination of gold in an effluent from a recovery process.

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Abstract  

Stoichiometric, stable fluorides for comparison standards in neutron activation analysis were prepared by horizontal zone-refining or sublimation in HF atmospheres. The fluorides of Na, Mg and Al were shown by thermogravimetry to be anhydrous and to remain water-free after exposure to room air. Fluorine in the purified samples was separated by the pyrohydrolysis method and subsequently measured by acid-base titrimetry or selective ion electrodes. Deviations from theoretical values were ±0.1, ±0.9 and ±2.8% respectively for fluorides of Na, Mg and Al.

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Abstract  

Bulk and surface nitrogen levels of compression-molded samples of polyacrylonitrile, poly (styrene-co-acrylonitrile), poly(methacrylonitrile), polycaprolactam, polyimide, and a cured epoxy resin (EPON 828 — JEFFAMINE T-403, 100:50) were determined, respectively, by 14 MeV NAA (14N(n,2n)13N) and by analysis of proton tracks registered in cellulose nitrate detectors (CN85) as a result of their exposure to protons from the termal neutron induced reaction,14N(n,p)14C. Except for a few polymers, agreement of results obtained for nitrogen levels in the bulk and surface regions is within expected limits. Intrinsic limitations of the method for practical determinations of nitrogen are covered. The spatial homogeneity of materials consisting of nitrogen and non-nitrogen containing segments is easily determined by image analysis of proton track densities. These data are then used to construct nitrogen distribution plots and topographical maps. The potential ability of the proton track image analysis technique for distinguishing phase separation or immiscibility of nitrogen containing polymers is discussed.

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Abstract  

Chromium, iron and copper were determined in niobium by radiochemical proton activation analysis (PAA). The main steps of the technique involved the irradiation of the samples with 13 MeV protons, the post-irradiation decontamination of the sample surface, a two-step separation procedure based on anion exchange from HF and HCl medium, and counting the separated indicator radionuclides with a well-type NaI detector. For a 5-hr irradiation, limits of detection for chromium, iron, and copper were 0.2, 5.0, and 15 ppb respectively. The results obtained by this technique are compared with data obtained by radiochemical neutron activation analysis (NAA) and atomic absorption spectrometry (AAS).

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Summary A novel method for the determination of the point of micellar saturation has been developed. To exemplify the theory a model system was considered, this being the saturation of two aqueous micellar solvents with dimethyl phthalate ester (DMP). Upon addition of a hydrophobic compound to an aqueous micellar system partitioning will occur. On further addition, the inner hydrophobic regions will eventually be unable to accommodate any more DMP and, at this specific concentration, the micelle is saturated. With a comparatively large enthalpy change upon partitioning the point of saturation can be determined by a corresponding significant reduction in enthalpy change.

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