Uranium, fluorescence

Hexavalent uranium fluoresces brilliant green, peaking between 500 and 540 nm in most glass hosts. The brilliance of uranium fluorescence far exceeds that of the most efficient organic dyes. The fluorescence emission spectrum of hexavalent uranium-doped silica gel-glass is presented in Fig. 7. The peak maximum is at 533 nm [35]. 

It should be noted that the intensity of fluorescence is dependent on the melting conditions to a considerable extent. Care should be taken that these are closely reproducible. The specified melting temperature and the melting period must not be exceeded since otherwise platinum dissolves out from the crucibles and may interfere with the uranium fluorescence. 

Uranium fluoresces and therefore can be analyzed by XRF. This allows direct analysis of uranium in solid matrices and overcomes the challenges of digestion and separation. 

In general, the absorption bands of the actinide ions are some ten times more intense than those of the lanthanide ions. Fluorescence, for example, is observed in the trichlorides of uranium, neptunium, americium, and curium, diluted with lanthanum chloride (15). 

The uranium ores from which this new radiation was discovered were fluorescent, and x-ray tubes fluoresced thus, an early hypothesis was that the visible fluorescence and the new penetrating radiation were related and would occur together. Becquerel did a series of careful experiments showing that penetrating radiation also came from some materials that did not fluoresce. 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

The discovery and detailed investigations of the phenomenon of fluorescence is generally considered the main contribution of Edmond Becquerel. It had the further impact of leading later to the discovery of radioactivity by his son Henri, as Henri continued th ese studies, including among the substances examined salts of uranium. 

Umbelliferone, pH-dependent change of fluorescence color 44 Universal reagents 4,46,195,376,402,405, 412, 430, 434 Uracil derivatives 44,45 Uranium cations 144 Uranyl acetate reagent 44 Urea... 

There are methods available to quantify the total mass of americium in environmental samples. Mass spectrometric methods provide total mass measurements of americium isotopes (Dacheux and Aupiais 1997, 1998 Halverson 1984 Harvey et al. 1993) however, these detection methods have not gained the same popularity as is found for the radiochemical detection methods. This may relate to the higher purchase price of a MS system, the increased knowledge required to operate the equipment, and the selection by EPA of a-spectrometry for use in its standard analytical methods. Fluorimetric methods, which are commonly used to determine the total mass of uranium and curium in environmental samples, have limited utility to quantify americium, due to the low quantum yield of fluorescence for americium (Thouvenout et al. 1993). 

Leung et al. [ 104] and Kim and Zeitlin [105] described a method for the separation and determination of uranium in seawater. Thoric hydroxide (Th(OH)4) was used as a collector. The final uranium concentration was measured via the fluorescence (at 575 nm) of its Rhodamine B complex. The detection limit was about 200 jLg/l. 

Numerous substances that produced fluorescence were examined by Stokes plant extracts (e.g., chestnut rind, chlorophyll in water), glass, paper, animal material, uranium compounds, etc., and he pointed out that the rays produced by the fluorescence process were much more refrangible than the rays initiating them. ... 

Carl Zeiss, Inc. also describes a spectrofluorometer system for process monitoring," but it does not currently appear as a standard marketed product on their web site. HORIBA Jobin Yvon also markets a fluorescent process analyzer, but it is a laser-induced time-domain based measurement system tailored for uranium or equivalent analysis." Finally, while numerous miniature spectrofluorometers are also available (Carl Zeiss, StellarNet Inc., Ocean Optics and Avantes), they are not packaged and configured for process applications. Although there is an established need and continued growing interest in realtime process spectrofluorometry, relative to conventional process spectroscopic instruments such as NIR, UV-vis and Raman, commercial process spectrofluorometers are currently available on a very limited basis. 

Danesi et al.96 applied SIMS, in addition to X-ray fluorescence imaging, by using a microbeam (p-XRF) and scanning electron microscope equipped with an energy dispersive X-ray fluorescence analyzer (SEM-EDXRF) to characterize soil samples and to identify small DU particles collected in Kosovo locations where depleted uranium (DU) ammunition was employed during the 1999 Balkan conflict. Knowledge of DU particles is needed as a basis for the assessment of the potential environmental and health impacts of military use of DU, since it provides information on possible resuspension and inhalation. The measurements indicated spots where hundreds of thousands of particles may be present in a few mg of contaminated soil. The particle size distribution showed that most of the DU particles were < 5 pm in diameter and more than 50 % of the particles had a diameter of < 1.5 p.m.96... 

V. Vigneron, A. C. Simon, R. Junca and J. M. Martinez, Neural techniques applied to analysis of X-ray fluorescence spectra. Example of determination of uranium, Analusis, 24(9-10), 1996, 37-41. 

AUTUNITE. This mineral is a hydrous phosphate of calcium and uranium, crystallizing in the tetragonal system, usually in thin tabular crystals. Good basal cleavage hardness. 2-2.5 specific gravity, 3.1 luster, subadamantine to pearly on the base color, lemon yellow streak, yellow transparent to translucent strongly fluorescent. 

IJmnate. Sodium uranate, uranium yellow, Na2U04, yellow solid, insoluble, formed by reaction of soluble uranyl salt solution and excess sodium carbonate solution. Used (1) in the manufacture of yellowish-green fluorescent glass, (2) in ceramic enamels, (3) as a source of uranium for chemical reactions. 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

Parsa [228] has described a sequential radiochemical method for the determination of thorium (and uranium) in soils. Mukhtar et al. [229] have described a laser fluorometric method for the determination of thorium (and uranium) in soils. Steam digestion has been employed in the preparation of soil samples for the determination of thorium (and uranium) [230]. Thorium (and uranium) were determined by X-ray fluorescence using a germanium planar detector and by chemometric techniques. No sample preparation was required in this method [231]. 

Earlier methods for the determination of uranium in soils employed spectrophotometry of the chromophore produced with arsenic(III) at 655 nm [237 ] and neutron activation analysis [238]. More recently, laser fluorescence [239] and in situ laser ablation-inductively coupled plasma atomic emission spectrometry [240] have been employed to determine uranium in soil. D Silva et al. [241] compared the use of hydrogen chloride gas for the remote dissolution of uranium in soil with microwave digestion. 

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