Separation radiochemical

For matrices other than silicon, such as GaAs, InSb, AlGaAs, and InP, it is difficult to measure trace elements because the activity from the matrix is intense and long-lived. In these cases, laborious radiochemical separation techniques are employed to measure impurities. 

The NAA measurements on the paper samples were made at the Breazeale Nuclear Reactor Facility at the Pennsylvania State University with a TRIGA Mark III reactor at a flux of about 1013 n/cm2-sec. Samples were irradiated from 2 to 20 min and counted for 2000 sec, after a 90 min decay time for Ba and a 60 hr decay for Sb, Analyses were performed instrumentally, without radiochemical separation, using a 35cm3 coaxial Ge-Li detector and a 4096-channel pulse height analyzer. With these procedures, detection limits for Ba and Sb were 0.02ug and 0.001 ug, respectively. These sensitivities are comparable to those obtained by GA s radiochemical separation procedure, and are made possible by the use of the higher neutron output from the more powerful reactor and in combination with the higher resolution solid state detector... 

In cases where the induced radionuclides of trace elements are masked by matrix activity, radiochemical separation provides interference-free detection limits close to... 

Of the four strategies given above, the best condition for obtaining independent data for quality control (QC) are satisfied when INAA and RNAA results are compared, because the use of RNAA dramatically improves the selectivity of signal measurement and eliminates or greafiy reduces the measurement uncertainty sotuces, such as spectral interferences. A variety of radiochemical separations and... 

Pietra, R., Fortaner, S., and Sabbioni, E. (1993). Use of Chelex 100 resin in preconcentration and radiochemical separation neutron activation analysis applied to environmental toxicology and biomedical research. /. Trace Microprobe Tech. 11, 235-250. 

The sources of uncertainty in NAA analysis are well understood, and can be derived in advance, modelled and assessed experimentally. There are two main kinds of interferences in the calculation of trace-element concentrations by INAA. The first one is formation of the same radionuclide from two different elements. Another kind of interference is from two radionuclides having very close y lines. Whenever interferences occur, the radionuclide of interest can be carried through a post-irradiation radiochemical separation without the danger of contamination. 

Photoactivation analysis has also been used to determine fluoride in seawater [73]. In this method a sample and simulated seawater standards containing known amounts of fluoride are freeze-dried, and then irradiated simultaneously and identically, for 20 min, with high-energy photons. The half-life of 18F (110 min) allows sufficient time for radiochemical separation from the seawater matrix before counting. The specific activities of sample and standards being the same, the amount of fluoride in the unknown may be calculated. The limit of detection is 7 ng fluoride, and the precision is sufficient to permit detection of variations in the fluoride content of oceans. The method can be adapted for the simultaneous determination of fluorine, bromine, and iodine. 

Keith-Roach et al. [114] has described a radiochemical separation and ICPS protocol for determining "technetium in seawater. 

Hiraide et al. [68] used continuous flow coprecipitation-floatation for the radiochemical separation of cobalt from seawater. The 60cobalt activity was measured by liquid scintillation counting with greater than 90% yield and a detection limit of 5 fCi/1 seawater. 

Livingstone et al. [87] carried out double tracer studies to optimise conditions for the radiochemical separation of plutonium from large volumes of seawater. 

Contents Introduction. - Experimental Techniques Production of Energetic Atoms. Radiochemical Separation Techniques. Special Physical Techniques. - Characteristics of Hot Atom Reactions Gas Phase Hot Atom Reactions. Liquid Phase Hot Atom Reactions. Solid Phase Hot Atom Reactions. - Applications of Hot Atom Chemistry and Related Topics Applications in Inorganic, Analytical and Geochemistry. Applications in Physical Chemistry. Applications in Biochemistry and Nuclear Medicine. Hot Atom Chemistry in Energy-Related Research. Current Topics Related to Hot Atom Chemistry and Future Scope. - Subject Index. 

The Sr-82 used in these studies was produced by spallation of a molybdenum target with 800 MeV protons at the Los Alamos Meson Physics Facility (LAMPF) and radiochemically separated by the Nuclear Chemistry Group at Los Alamos Scientific Laboratory (LASL) (22). The major radionuclidic contaminant in the Sr-82 is Sr-85 which is present in at least 1 1 ratio relative to Sr-82. The actual ratio depends upon the length of time after the production of radioactive strontium. Because of the 65 day half life of Sr-85 and the 25 day half life of Sr-82, the Sr-85 Sr-82 ratio increases with time. Other radionuclides found by the Hammersmith group in the processed Sr-82/85 shipment were Sr-89 ( 1%), Sr-90 ( 0.01%), Co-58 ( 1%) and Rb-84 ( 1%) from (17). 

Our method for measuring leach rates is thought superior to other methods currently in use. Meaningful leach rate data can be obtained using relatively simple laboratory scale equipment coupled with standard NAA techniques. More detailed information can be procured by applying radiochemical separations and more sophisticated counting methods. The experimental technique described here is applicable to the measurement of leach rates for the elements of interest, from any solid waste form, in any potential storage environment. 

Trace Elements in Coal by Neutron Activation Analysis with Radiochemical Separations... 

Procedures for the determination of 11 elements in coal—Sb, As, Br, Cd, Cs, Ga, Hg, Rb, Se, U, and Zn—by neutron activation analysis with radiochemical separation are summarized. Separation techniques include direct combustion, distillation, precipitation, ion exchange, and solvent extraction. The evaluation of the radiochemical neutron activation analysis for the determination of mercury in coal used by the Bureau of Mines in its mercury round-robin program is discussed. Neutron activation analysis has played an important role in recent programs to evaluate and test analysis methods and to develop standards for trace elements in coal carried out by the National Bureau of Standards and the Environmental Protection Agency. 

Radiochemical separations are necessary for many elements when only a Nal detector is available. Even with a Ge(Li) detector, a radiochemical separation increases the sensitivity and accuracy and permits the determination of some elements whose radioactivities are masked by stronger activities in the multi-element spectrum of a coal sample. For example, mercury, selenium, gallium, and zinc in most coals are below the limit of detection instrumentally even with the resolution of a Ge(Li) crystal (7), but can be determined after radiochemical separations as is described later. 

The increase in accuracy afforded by a radiochemical separation is absolutely necessary in the determination by NAA of trace elements in the coals selected as standards. The fact that interferences from the coal matrix are removed by a radiochemical separation is the advantage of this method of analysis over such instrumental methods as x-ray fluorescence and emission spectroscopy. 

This article presents a comprehensive view of the present state-of-the-art of radiochemical separations for the following trace elements in coal Hg, Rb, Cs, Se, Ga, As, Sb, Br, Zn, Cd, and U. Most of the work on the determination of trace elements in coal is very recent. The accuracy of the analysis methods, nearly all newly developed, has been open to question because of the lack of standards and lack of knowledge of the range of concentrations for many trace elements in coal. Federal government laboratories have taken the lead in evaluating methods of analysis and in developing standards. By a round-... 

For a trace element concentration to be certified by NBS, it must be determined by at least two independent methods, the results of which must agree within a small experimental error range of 1% to 10%, depending on the nature of the sample and the concentration level of the element. Such accuracy in determining some trace elements for certification of coal SRM is achieved most easily by NAA with radiochemical separation. Scientists at NBS have extensively tested a neutron activation method that involves a combustion separation procedure on coal as well as on several other matrices to be certified as standard reference materials. The procedures they have thus developed to determine mercury (12), selenium (13), and arsenic, zinc, and cadmium (14) are outlined in a following section on methods for determining specific elements in coal. 

After the coal or coal ash sample has been brought into solution, radiochemical separations may be made by any of several techniques, such as distillation, precipitation, solvent-extraction, ion exchange, etc. 

Common Features of NAA Procedures. In all of the procedures discussed in this article, irradiations are made in a high thermal neutron flux (1011 to 1013 neutrons cm"2 sec 1) simultaneously with the samples and standard(s) sealed in polyethylene containers for a short irradiation or in silica containers for a long irradiation. The standard is a known amount, or solution of known concentration, of a pure compound of the element to be determined. The concentration of the element in the sample is determined by comparing its radioactivity with that of the standard, which is either subjected to the same radiochemical separation as the sample with an inactive matrix or diluted. The radioactivity is counted directly if the sample is measured in solution. The radiochemical yield of precipitated samples is determined directly by weighing and that of solutions of samples by aliquot re-irradiation. 

Weaver and von Lehmden (20), under sponsorship of the EPA, evaluated two instrumental NAA methods and one with radiochemical separation for determining mercury in coal. 

The flameless atomic absorption method has a reproducibility of about 2% or better for homogeneous specimens. Checks (3) between AA and NAA (with radiochemical separation after irradiation) and isotope dilution spark source mass spectroscopy on thoroughly homogenized tuna fish and Bureau of Mines round-robin coal specimens indicate good agreement between the methods. (0.425 0.9%, 0.45 3.5%, and 0.45 4.4% for tuna by AA, NAA, and SSMS, respectively, and 1.004 is the average ratio of NAA to AA results for five coal samples.) The similar results indicate that the technique used in sample preparation... 

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